• Experimental Demonstration of the Presence of Three Sets of Gustatory Organs in the Mandibles, p. 7. Method of stimulating the organs, p. 7. Description of the reflexes caused by stimulation of the same, pp. 7—9. Location of the organs—(1) By local stimulation, p. 10. (2) By amputation of mandibles, p. 10. (3) By shaving off of spines, p. 10. Temporary suspension of reflexes, pp. 11, 12. General remarks.

  • Structure of Gustatory Cells of the Mandibular Spines, p. 12. Chitinous tubules, p. 13. The spindle, the axial, and external nerves, p. 14. Gustatory cells, p. 15. Gigantic ganglion-cells, p. 15. Double nature of gustatory cells, p. 15. Cuticular canals, p. 16.

  • Experimental Demonstration of the Presence of Gustatory and Temperature Organs in the Chelæ, p. 16. Stimulation with clam, ammonia, and warm air ; reflexes, p. 16. Experiments on amputated chelæ—description of the organs, p. 17.

A. Structure of the Olfactory Organ : external appearance, general topography, cuticula, air in pore canals, spines, p. 17. Olfactory buds,p. 18. Shape, structure; central ganglion-cells; non-glandular nature of the buds; lumen of the buds; chitinous tubule; cuticular canals; nervesupply, p. 22. Lateral olfactory nerves; origin, course, distribution, structure ; lateral olfactory ganglion. Median olfactory nerve, p. 22. Structure, distribution, termination in buds. Problematical organs.

B. Physiology of the Olfactory Organ, p. 26. Absence of reflexes on stimulation with food, ammonia, &c. ; stimulation with electricity ; nature of reflex in the males; same in females; function of the organ; effect on males on excising the organ.

C. The Development of the Olfactory Organ, p. 29. Primitive olfactory thickenings; origin of the ganglion in the lateral olfactory nerve ; origin of definitive olfactory organ, p. 30. Origin of median olfactory nerve ; origin of olfactory lobes.

D. Relation of Primary Olfactory Organ to segmental sense organs ; to lateral eyes of scorpions ; to frontal organ of Phyllopods ; cause of change of function, p. 32.

Position and structure of those in the inner mandibles; effect of various reagents.

Experimental demonstration of the same, p. 34.

  1. Experiments showing course of temperature impulses; section of tegumentary nerves ; of spinal cord ; longitudinal median section through hind brain and vagus region ; section of left crus ; of both crura ; temperature centre in the fore-brain ; supplementary centres in legs, p. 36.

  2. Position of Gustatory Centre, p. 39.

  3. Structure of Temperature Organ, p. 39; probably much the same as the gustatory buds.

  4. Use of temperature sense, p. 39.

Structure and nature of sense organs, p. 40; innervation; multiplication by division; comparison with Gephyreans, Galeodes, Mutilla. Very wide distribution of the sense buds, their specialisation as isolated organs, and their fusion to form complex aggregates.

Part II.—The Morphology of the Arthropod Brain

A. Structure of Cephalic Lobes: fore-brain; mid-brain: their different origins, p. 46.

B. The Stomodæal Nerves, p. 46. The labrum, p. 47. The anterior pons stomodæi ; the posterior pons stomodæi ; the anterior stomodæal ganglion; the lateral stomodæal ganglia; the lateral and the median sympathetic nerves of the trunk.

C. The Convex Eyes and their Ganglia, p. 48. They do not belong to the cephalic lobes, but to the mid-brain neuromeres; absence in scorpions ; origin from the cheliceral segment in Limulus ; is the organ of Tômôsvary in Myriapods the “anlage” of the convex eye ? Conversion of the larval optic ganglion in Acilius into that of the adult ; relation of the frontal ocelli to the larval ones.

A. The Cephalic Lobes, p. 50. Semicircular lobes, p. 50. Formation of primitive cerebral vesicle, p. 51. Homology of the eyes of spiders and scorpions, p. 51.

B. The Mid-brain, p. 53. Homology of fore- and mid-brain of scorpions with those of insects, p. 53. Stomodæal nerves.

C. Development of the lateral stomodæal nerves, p. 54.

D. Comparison of cephalic lobes of Arthropods and those of Annelids, p. 55.

E. Are the stomodæal nerves derived from the circumoral nerves of a Cœlenterate? p. 56.

A. The Cephalic Lobes of Limulus, p. 57. Ganglionic invaginations; origin of the cerebral hemispheres ; fusion of the two invaginations of the second segment to form the parietal eye-tube, or epiphysis.

B. The Mid-brain, p. 60. Stomodæal nerves.

C. Later Modifications of the Brain: fate of the invaginations; formation of medullary folds ; fate of the anterior neuropore, p. 61.

Development of the Cerebral Hemispheres: the posterior lobe; the anterior lateral lobe; the internal median, or corpus striatum. The semicircular lobes, p. 66. Their relation to nerves of parietal eye; the optic ganglia ; modification and origin of the various brain-vesicles.

D. Comparison with Vertebrates, p. 67. The semicircular lobes and the infundibulum of Vertebrates, p. 69. Changes in the position of the parietal eye; peduncles to the parietal eye of mammals. The third ventricle; the lateral eyes; explanation of choroid fissure in Vertebrates, p. 72. The “iter the fifth ventricle ; the fourth ventricle ; summary, p. 73.

Endo- and ecto-parietal eyes ; origin ; four nerves to the same ; the epiphysis; the stalk of parietal eye-nerve; the transverse tube; the bloodvessel; comparison with Vertebrates; nature of the epiphysis; white pigment of the endoparietal eye ; comparison with white pigment in the eye of Pe-tromyzon ; Gaskell’s comparison of parietal eyes without sufficient data.

Comparison with olfactory organ in Vertebrates ; its four nerves correspond to the four olfactory nerves of Amphibians. Relation in Limulus and Vertebrates of the olfactory nerves to the optic thalami ; comparison of the olfactory organs of Vertebrates and Limulus with the supra-branchial or mandibular sense organs. The function, distribution, and multiplication of the mandibular sense organs of Limulus are like those of the supra-branchial organs of Vertebrates. The mandibles probably represent the rudimentary endopodites of the thoracic appendages.

Comparison of the fore-, mid-, hind, and accessory brains and their nerves with the corresponding regions in Vertebrates, p. 83. Concluding remarks, p. 85.

SOME two years ago I published a short paper in this Journal calling attention to many striking resemblances between Arachnids and Vertebrates. I maintained that the Vertebrates are descended from a great group of Arthropods, in which I included the Arachnids, Trilobites, and Mero-stomata ; and that the remarkable palæozoic fishes Pterichthys, Bothriolepis, and Cephalaspis resemble merostomatous Arthropods like Pterygotus, Eurypterus, &c. Now, the internal structure of the Merostomata and Trilobites cannot differ greatly from that of Limulus, judging from their resemblance in external characters ; therefore, although Limulus itself is not in the main line of Vertebrate descent, a study of its structure will best enable us to understand that of the Merostomata and of the primitive Vertebrates.

In my preliminary paper the lines were indicated along which I had found evidence of relationship between Vertebrates and Arachnids. It was shown—and this first drew my attention to the subject—that the invaginations, which in insects give rise to the optic ganglia, in scorpions and Limulus become so extensive as to enfold not only the optic ganglia, but the eyes and the fore-brain as well. A cerebral vesicle is thus formed, from the floor of which arise the fore-brain and the optic ganglia, and from the roof a tubular outgrowth, at the end of which lie the inverted retinas of the parietal eye. Such a condition is found only in Arachnids and Vertebrates; and in my judgment this fact, when all its bearings are considered, affords as trustworthy evidence of relationship as the presence of a notochord or of gill-slits.

It was further shown (1) that the lateral eyes of Limulus could be compared with the lateral eyes of Vertebrates; (2) that there is in Arachnids a cartilaginous endocranium similar in position, shape, and development to the primordial cranium of Vertebrates ; (3) that there is in scorpions and in other Arthropods a subneural rod similar to the notochord of Vertebrates; (4) that in the Arachnids and in the Vertebrates the brain contains approximately the same number of neuromeres ; it is divided into the same number of regions, i. e. fore-brain, mid-brain, hind brain, and accessory brain ; and while in each region there is a different number of neuromeres, i. e. 3, 1, 5, 2 to 4, the number in the corresponding regions in Vertebrates and Arachnids is very nearly the same; (5) the nerves of each brain region in both Vertebrates and Arachnids show in a general way the same relation to sense organs, the same ganglia, and the same distribution and fusion with one another; (6) finally, there is a striking similarity between the cephalo-thoracic shields of Arthropods and the cephalic bucklers of the earliest fishes, such as Zenaspis, Both-riolepis, Pteraspis, Auchenaspis, &c. The shape and microscopic structure of the shields, and the arrangement of the eyes upon them, are practically the same in both groups. The three median and two lateral eyes of Cephalaspis Campbelltonensis as figured by Whiteaves (′Trans. Roy. Soc. Canada/vol. vi, 1888, pl. x) have exactly the same arrangement as those of Limulus. I have carefully revised the palæontological aspect of the subject, and I hope in a separate paper to give it the careful consideration it deserves.

Attention was also called to resemblances of a more general character between Vertebrates and Arachnids, such as, for example, in the structure of the muscles, the nerves, and the liver; in the position and the net-like structure of the sexual organs ; in the origin of the ova and the spermatozoa; in the origin of the germ-layers, and in the general formation of the embryos.

To this formidable array of evidence I can now add the following :—(1) It is possible to identify nearly all the important lobes and cavities characteristic of the Vertebrate fore-brain in the fore-brain of Limulus. (2) The coxal sense organs are shown by conclusive experiments to be gustatory, and to correspond to the supra-branchial sense organs of Vertebrates. (3) An extraordinary organ has been discovered in Limulus, having all the characteristic morphological features of the olfactory organ in Vertebrates. It is united with olfactory lobes that arise as outgrowths from the fore-brain. It consists of an upright layer of epithelium containing olfactory buds similar to those in the coxal (supra-branchial) sense organs. It is supplied by four nerves, the median ones resembling, in histological structure, those of Vertebrates. One pair of these nerves arises from the cerebral hemispheres, the other from the optic thalami. I long ago looked for something answering to the olfactory organ in Vertebrates, but finally gave up the search because that part of the cephalic lobes where, as I supposed, they ought to appear was invaginated, consequently any sense organ derived from that region must also be invaginated, and could not be homologous with the olfactory organ of Vertebrates.

Most of the physiological results were obtained during the summer of 1892 at the United States Fish Commission Laboratory at Wood’s Holl, Mass., the facilities of which were generously placed at my disposal by Commissioner MacDonald.

The descriptive part of this paper deals mainly with Limulus, but incidental references are made to scorpions and other Arthropods. As I aim to point out resemblances between Vertebrates and Arachnids, I shall not enter into histological details that might obscure the meaning of the broad facts I wish to present. However, I shall describe the morphology and physiology of the olfactory organ, and of the gustatory and temperature organs on the appendages and elsewhere in detail. In order to justify the comparisons instituted between these sense organs and those of Vertebrates I have given a general account of the structure and development of the brain and median eyes.

I do not hesitate to say that I believe the results herewith presented prove beyond any reasonable doubt that the Vertebrates are descended from the Arachnids.

A. Experiments on the Gustatory Organs of the Mandibles.—If an adult horseshoe crab be placed on its back with its abdomen hanging over the edge of the table, it makes fruitless movements of the legs and abdomen to recover its natural position. The muscles, however, soon relax, and the animal usually becomes perfectly quiet, except that after long intervals the gills are raised and lowered a few times, and then held up motionless a few seconds till every part, expanded to its full extent, is thoroughly aërated ; they then sink slowly back to their original position. If, while in the quiescent condition, the jaw-like spurs or mandibles (Pl. 1, fig. 3,o. mid) at the base of the legs are gently rubbed with some hard object, such as a piece of wood, glass, or iron ; or if water or air, the temperature of the surrounding medium, be gently poured over them, or if the animal be vigorously fanned, or loud noises be made near it, only slight aimless movements of the legs or abdomen are produced, usually none at all. But if a very small piece of clam or other edible substance be rubbed ever so gently over the stout spines that arm the mandibles, very characteristic chewing movements are immediately produced. If all the mandibles are touched in this way, or even moistened with a few drops of water in which pieces of clam have been soaked,the chelicerae snap and work alternately back and forth, as though tucking something into the mouth ; at the same time the metastoma are moved forward and backward. But the most constant feature is the following movement of the second to the fifth pair of appendages ; the second and fourth pairs of mandibles move in unison inward toward the median plane, and downward toward the mouth ; then back again in the reverse order. When they are farthest from the mouth the corresponding legs (except the second pair in both males and females) are quickly raised, flexed, and the tips carried toward the mouth, where they remain an instant, and then fall back on to the under side of the carapace ; the corresponding jaw movement then begins again. The third and fifth pairs of appendages and the corresponding jaws work in unison in the same manner, but they alternate with those of the second and fourth. At intervals these movements cease, the abdomen is raised, and the stout crushing mandibles on the sixth pair of appendages, which have heretofore remained motionless, are slowly closed with great force, as though to crush some object too large to be swallowed whole, or to kill some struggling prey. These powerful jaws then slowly relax their convulsive grasp, and the chewing movements are resumed. All these movements go on with the greatest precision and regularity, so that any food placed on the jaws is forced into the mouth, and gradually disappears down the oesophagus. These chewing movements are produced when drops of soluble food, or almost any bit of animal matter, or wads of blotting-paper wet in nutritive animal fluids, come in contact with the mandibles. Drops of water from the interior of a clam will set the whole complicated mechanism to working in exactly the same manner as during the actual process of eating. Again, chewing movements of the mandibles are produced whenever ammonia vapour, ether, or chloroform is blown over them with a medicinedropper, or when they are stimulated by a weak interrupted current ; but in such cases these movements are rarely accompanied by the leg movement. If the irritation with ammonia or acids has been rather great the mandibles work apparently as in eating, but the cheliceræ move rapidly back and forth, making frantic snapping movements toward the mouth, as though to pick away some disagreeable object. These movements usually last a long time. If wads of blotting-paper wet in ammonia or picric acid are used the chewing movements are reversed, and the offensive object is sometimes snapped up by the cheliceræ and rejected.

Holding strong-smelling food as close as possible to the mouth or to the jaws produces no effect, although chewing movements are instantly produced when the jaws are touched by it.

When a very small piece of clam or of pecten is touched to the surface—say of the third mandible on the left side—care being taken not to touch any other parts, that leg will be promptly raised and the tip bent toward the mouth ; it soon falls back on to the cephalo-thorax again, and then its mandible moves back and forth, alternating with the leg movement, as in eating; but all the other mandibles and appendages remain quiet. One may start in this way one appendage after the other (except the cheliceræ, which have no mandibles), until all of them, first on one side and then on the other, are working in perfect rhythm.

If the mandibles of the post-oral appendages on one side are stimulated, the chelicera of that side, although not stimulated itself, soon has its tip extended straight backward as far as it can reach, and may remain some time in this rigid, unnatural position ; or else it begins those characteristic back and forward movements, snapping its chela from time to time, as though to seize something and lift it up, or else thrusting the chela down the mouth as though forcing some piece of food into the oesophagus. If the jaws on the opposite side are now stimulated, the chelicera of that side begins to work also. It is a curious fact that the cheliceræ rarely move when the mandibles of the second or third appendages are stimulated ; not till the last one or two pairs of mandibles are set in motion do they begin their characteristic movements. It is evident that the chewing movement produced in these various ways is a reflex act caused by the stimulation of gustatory organs about the mouth.

The following experiments show that there are three kinds of these gustatory organs, and that they are situated in the inner and outer mandibles of the second to the fifth pair of appendages. The organs of the first kind are located in the mandibular spines, the second on the smooth concave margin of the inner mandible (fig. 3, g. b.), and the third are scattered over the surface of the mandibles between the spines.

(1) If the outer surface of one of the mandibles—say the second or third on the right side—be very gently touched with a piece of clam, as small even as the head of a pin, the characteristic chewing movements of that mandible and the corresponding leg are alone produced. With care all the appendages on one side, or any number of them, can be made in this way to go through the chewing movements, while the other legs and mandibles remain perfectly quiet.

(2) If any one mandible, or any combination of them, be amputated, then stimulation of the mouth region with food produces chewing movement of the remaining mandibles and of the corresponding legs, but those legs from which the mandibles have been removed remain perfectly quiet, even though food be rubbed on the mouth, the rostrum, the soft skin about the base of the legs, or on the proximal part of the leg itself. This proves that the gustatory organs are located in the mandibles alone, and not in or around the mouth, in the rostrum, or in the base of the leg.

(3) If the stiff spines be shaved off of one or more mandibles and a piece of clam rubbed over the outer anterior surface of the shaved mandibles, either no effect at all is produced, or else feeble movements of the mandibles only, without the leg movement. But the least contact of the clam against the unshaved mandibles produces at once the characteristic mandible and leg movements ; it makes no difference whether the shaved mandibles are all on one side or not, or what combination may be selected. This proves that a large proportion of the gustatory organs are situated in the mandibular spines; it also shows that the reflex in each leg and mandible is due solely to the stimulation of its own gustatory organs, and that it is entirely independent of thé reflex in the adjacent appendages, either of the same or opposite side: this, however, does not apply to the cheliceræ.

(4) If the shaved mandibles be rubbed on their outer anterior surface, movements are rarely produced ; but they are fairly well marked whenever a piece of clam is touched against the smooth concave surface of the inner mandible (fig. 3,g. b.). In performing this experiment, the shaved or unshaved mandible, it is immaterial which, is gently raised with a pair of forceps, care being taken not to touch the animal with the fingers, and a very small piece of clam rubbed on the smooth surface in question. Immediately the inner mandible is retracted by the muscle shown in fig. 3, m. i. m,, and then the whole mandible begins its rhythmic movements. This proves that, besides those organs in the spines, the smooth inner surface of the inner mandible contains a second set of gustatory organs, which when stimulated produce reflex chewing movements.

(5)The results of the following experiments may be stated in this way :—Destroying a certain number of gustatory organs in one or more mandibles suspends the reflex in the mutilated appendages. The reflex may be partially restored by destroying a corresponding number of sense organs in each of the remaining mandibles.

The following experiments were performed successively on the same individual :—(a) A healthy crab that was known to be very sensitive to gustatory stimulation was deprived of a part of its gustatory organs by shaving off all the gustatory spines on the mandibles of the right side. Ten days after, on equal stimulation of both sides, the shaved appendages remained perfectly motionless, while the unshaved ones began the normal chewing movements. But the shaved mandibles could be made to act when vigorously stimulated. (b) When the mandibles on the left side also were shaved, the reflexes were impaired for some time. However, a week or ten days after, vigorous movements on both sides were produced by rubbing pieces of clam well over the mandibles. As might be supposed from experiment No. 4, the movements were especially well marked when the clam was rubbed over the under side of the inner mandibles, but they could be produced when only the outer part of the mandibles was touched. After the reflex had been once restored it required but little more stimulation to start the reflex in the shaved mandibles than it did before in the unshaved ones, (c) Now if we cut off the inner mandible of the right side the reflex on that side will again be suspended, but it can be once more partially restored by (d) cutting off the inner mandibles of the other side. The restored powers are each time feebler than before, but nothing short of amputating both inner and outer mandibles will completely and permanently destroy the reflex. The feeble movements caused by stimulation after the spines and the inner mandibles have been removed are produced by scattered gustatory buds distributed between the spines over the anterior face of the mandibles. The reflex chewing movements in one appendage, therefore, are not lost in proportion to the destruction of sense organs in it, but depend rather on the relation between the number of sense organs retained in it and those in the other appendages. Hence if the reflex in one appendage is suspended by destruction of a certain number of sense organs, it may be partially restored by reducing the number of sense organs in the other appendages in a like degree. In other words, the reflex impulse enters the widest door.

The reflex flows more readily along the most recently used lines, as shown by the following experiment :—If mandible A be slightly stimulated with food, whether enough to produce reflex movements or not, subsequent (say five minutes after) stimulation of all the mandibles to the same extent will produce reflex movement in mandible A first, and afterwards feebler movements in all the others.

B. Structure of the Gustatory Organs of the Man-dibular Spines.—It is not difficult to find the three sets of gustatory organs, the existence of which is demonstrated above. Examination under a low magnifying power shows that the mandibular spines are thickly covered on their sides nearest the mouth with minute pores arranged in from eight to ten interrupted vertical lines (fig. 2). Each line consists of several subordinate groups composed of from two to twelve or more pores. Longitudinal sections of one of the spines (fig. 1) show that the cuticula is perforated by parallel canals, in each of which is a delicate chitinous tubule (ch. i.) ; the latter terminates at the outer opening of the canal flush with the surface ; at the opposite end it bends nearly at right angles towards the base of the spine, where it soon expands into a clear, spindle-shaped body (sp.) ; beyond the spindle it is continued as a very long slender process that constitutes the body of the gustatory cell, the nucleated end of which unites with other cells to form great ganglion-like masses (gsc.).

The spindle, when more highly magnified (fig. 5), seems to be merely an inflation of the cell-wall, and contains, besides a watery fluid, a number of fibrillæ, arranged in a single layer along its inner wall, and continuous with fibrillæ in the stalk of the cell. The proximal half of the spindle is usually stained a little darker than the rest, and in it each fibril expands into a fusiform thickening that stains deeply in acetic acid carmine. The fibrillæ converge toward the distal half of the spindle to form an axial bundle that can be followed nearly to the free outer end of the chitinous tubule. From two to eight slender bipolar cells surround each spindle, and send their fibrous processes outward along the outer surface of the chitinous tubule (figs. 5, 6,g. c.).

It is difficult to tell just where the tubule begins. A short distance beyond the spindle (figs. 5 and 6) it appears to be continuous with the cell-wall, which there becomes rather distinct owing to its separation from the cell contents. It is slightly indented in some places, and is apparently enclosed by a second membrane, probably the outward prolongation of the nerve-like cells surrounding the spindle, for when thoroughly macerated these cells fall off, and the outer membrane is then absent. The sharp inner wall, the investing membrane, and the axial nerve produce a picture very much like that of a Vertebrate medullated nerve.

In macerated preparations the cell usually breaks just beyond the spindle, as in figs. 5—7, but in some instances nearly the whole tubule is isolated. When the tubule breaks near the spindle, the axial nerve usually projects a long distance from its open end, either as a single fibre (figs. 5, 6) or as a delicate brush of fibrillæ (fig. 7). Beyond this region it breaks with a clean fracture, as though made of glass, and a protruding axial fibre is rarely seen. Towards its outermost end the axial nerve cannot be seen under any circumstances, but this is due to the fact, I believe, that the canal in the tubule is so small that it is completely filled by the nerve. The tubule is thickest where it enters the cuticular canal (fig. 1, s. ch. t.) ; but it becomes smaller and smaller, as well as the canal in which it lies, until, at the surface, the tubule just fills the canal. In surface views (fig. 2) the black dot is the pore of the canal, completely filled by the tubule ; the clear halo surrounding it is cuticula more transparent than the rest.

The stalk of the gustatory cells, just below the spindle, is very finely striated, and when macerated and broken, as in fig. 5, the striation is seen to be due to the presence of excessively fine fibrillæ, much more numerous than those in the spindles. There are no nuclei to be seen on the long nerve-like stalk of the cell, between the spindle and the nucleus. On the distal side of the nucleated part of the cell are usually a few yellowish-brown pigment granules, that in some cases are large and very numerous, as in fig. 4.

The nucleated ends of the gustatory cells are arranged in long clusters along roughly parallel lines, each band of cells corresponding to a line of pores. Judging from the number of cells it contains, the cluster at gs. c., fig. 1, is probably connected with a row of pores running the whole length of the spine, although this is difficult to determine with certainty.

There are a few very large ganglion-cells, with coarse, anastomosing, plasmodia-like processes, that run at pretty regular intervals one above the other around the inside of the spines (fig. 1). One may often isolate great masses of this finely fibrillate reticulum without finding a nucleus in it. It usually overlies and unites large bundles of the slender stalks of the gustatory cells, but the latter appear to pass through the plasmodium without change.

The body of the gustatory cells resembles that of the double retinal cells of Molluscs and Arthropods, described by me in Area, Pecten, Acilius, Lycosa, and others, in that the large nucleus is nearly always excentric, and a spiral partition separates the cell into two more or less distinct portions (fig. 12). In one lies the main nucleus, n1; in the other a small unstained body that I regard as the aborted nucleus of the second cell, n2. The proximal end of the cell is sometimes forked, each branch being continued into a nerve-fibre. The interesting fact is thus established that double Cells are not confined to the retina.

The cuticular canal of the gustatory organ can be readily distinguished from other canals by the presence of a peculiar swollen knee, or bend, near its outer third, the surrounding cuticula of which stains more deeply in borax carmine than elsewhere (fig. 1). Under favorable conditions one can see in the “knee” a kind of spiral thread, caused by what in some cases appears to be a spiral ridge on the wall of the canal, in others by a spiral nerve-like fibril that seems to surround more or less loosely the chitinous tubule (fig; 13). A similar spiral thread is sometimes seen on the isolated chitinous tubule (fig. 6, a) ; when treated with dilute potash the tubule swells a little, and a thin, finely granular layer is seen about it, together with what appears to be an extremely delicate fibril, wound spirally about the tubule and its sheath (fig. 14).

In some of the cuticular canals there are a few slender nerve-like cells loosely surrounding the chitinous tubule (fig. 1) ; in other cases these cells seem to be absent. That the tubules of the gustatory organs and of the others described below are chitinous is shown by their resistance to caustic potash, and by the fact that they are shed during ecdysis. On examining cast-off shells one can see the perfectly preserved tubule projecting out of the inner ends of the cuticular canals.

Each spine on the jaws is crowded with the organs just described, and contains also a large blood-vessel. There can be no doubt that they are the gustatory organs, which, when stimulated, produce the reflex chewing movements described above.

c. Experiments on the Gustatory and the Temperature Organs of the Chelæ.—Two varieties of organs, having nearly the same histological structure and arrangement as those on the mandibular spines, are found on the last two joints, or chelæ, of the first to sixth appendage. One kind is a gustatory, the other, in all probability, a temperature organ.

The presence of the gustatory organs may be demonstrated by the following experiments :—Place a crab on its back and allow it to become quiet ; then if the chelæ, which are usually lightly closed, are rubbed with a small piece of clam, they will open wide, and remaining so, move about rather vaguely, as though trying to grasp something. They are specially sensitive at the tips and along the cutting edge. Ammonia vapour will produce the same result, but it cannot be produced by any purely mechanical stimulus.

The temperature organs betray their presence by an entirely different action, for if one breathes very gently, or blows warm air on the chelæ, they suddenly close and open once, and will repeat the action without variation as often as they are stimulated in this way.

A very curious fact is the following :—If the chelæ are amputated at the penultimate joint, they remain perfectly quiet if left alone. But for five or ten minutes after the operation they will snap once, i. e. close and immediately open again, every time they are gently breathed upon. But stimulation with food or ammonia produces no effect whatever. These facts seem to indicate that there is a reflex centre in the chelæ for the temperature impulses, but none for the gustatory ones. A rather hurried examination, however, failed to show the presence of any centre there, unless the few scattered tripolar ganglionic cells found everywhere in the subdermal nerveplexus can be regarded as such.

Description of the Organs.—As one might expect after the above experiments, surface views of the chelæ show the presence of two kinds of organs. Those of one kind appear as small pores surrounded by a clear halo, and resembling, in their arrangement in lines and in every other particular, the gustatory organs on the mandibular spines (figs. 10 and 11, g. o.). See section C. The others are less numerous than the first, but larger, and over each canal there is a saucer-shaped depression, from which projects a short blunt spine. A chitinous tubule passes up the wide cuticular canal, and terminates in the spine. The same kind of organ is also found about the bases of the larger mandibular spines. As the first organs are just like those in the mandibular spines, and as no other organs are found near the tips of the chelæ, they are without doubt the gustatory organs. The second kind must, in all probability, be the temperature ones. Sections of the chelæ, blackened in osmic acid to show the distribution of the gustatory canals (Pl. 3, fig. 44), show that they are very abundant along the cutting edge of the chelæ, and even more numerous at the flattened apex of the fingers—in other words, just where they are most sensitive to taste, and where they would be most likely to come in contact with foreign bodies.

The pedal nerve in the propodite, or the next to the last joint, divides into four branches which run along the anterior and posterior margin respectively of each arm of the chelæ. Whether this division of the nerve is due to a′ separation of fibres into nerves, going some to temperature organs and others to gustatory ones, could not be determined.

A. Structure of the Olfactory Organ.—

The olfactory organ is visible from the exterior as an irregular yellowish-brown, wart-like thickening of the cuticula, from 5 to 8 mm. broad, and situated about 25 or 30 mm. in front of the mouth. In specimens from 2 to 4 inches long it is usually raised into a beak-like projection directed backwards.

Directly beneath the ectoderm are a great many—at a rough estimate from 1500 to 2000—clear, flask-shaped sense buds, each of which is connected by a narrow neck with a cuticular canal. The distribution of the olfactory buds, as I shall call them, is fairly well shown by surface views of the olfactory region (Pl. 2, fig. 19). In this preparation, which is probably from an adult male, there are two unusually well-defined median elevations containing many more canals than elsewhere. The olfactory buds underlying this portion are supplied by a large median nerve (fig. 18, m. ol. n.); and this fact, together with the method of development, shows that it constitutes a distinct part of the olfactory organ. The lateral portions are clearer and smoother, and contain comparatively few canals ; this is specially the case on the posterior lateral borders immediately over the bulb-like termination of the lateral olfactory nerve (fig. 19). In young individuals, 2—4 inches long, the cuticula of this part is more transparent than elsewhere, and looks like a small lens. This fact, together with the presence of pigment there in the early stages, was what led me, in my paper on the “Origin of Vertebrates,” to regard this organ as a degenerate pair of eyes.

In the adult the cuticula over the olfactory organ often appears a dirty silvery white in reflected light, and black by transmitted light, owing to the inclusions of air in the extremely minute “pore canals.” These “pore canals” are found equally abundant elsewhere, but they do not contain air. The cuticula in the median part of the olfactory organ often contains irregular cavities (fig. 19), as though some animal had eaten into it, or it may contain a network of membranous canals, evidently the tubes of some Annelid ; they are usually most abundant in the median portion of the olfactory organ, and either lie on the surface or are buried in the cuticula.

Scattered over the olfactory organ are many blunt back-wardly curved spines. The olfactory cuticula can be easily peeled off in successive layers, but it adheres strongly around the pores leading to these spines. When it does come away large tufts of pigment-cells are seen projecting from the spine pores, and the outer surface of the inner layer of cuticle projects in a crater-like collar around the pore. There are similar spines on the cuticle surrounding the olfactory organ, but they do not act like the ones just described. For this reason I supposed at first that the olfactory spines were perhaps the true sense organs supplied by the olfactory nerves, but I can find but little evidence in support of this view. The large pores leading up into the spines over the olfactory organ are lined with thin cells and crowded with pigmented tissue, and in some cases contain a transparent, fragmented coagulum. A chitinous tubule, similar to that of the gustatory cells, usually runs the whole length of the canal, and becomes continuous with a minute canal extending from base to summit of the spine. The latter is pinnately striated in section, as though its central canal were connected with the exterior by innumerable radiating canals. Under favorable conditions a rather large nerve may be seen to enter the base of the spine canal. The spines are suspended in sockets, and are moveable. They are undoubtedly of a sensory nature, but they seem to play only a very subordinate part in the olfactory region, and, contrary to what I at first surmised, cannot be compared with the large gustatory spines on the mandible.

A section through the olfactory organ (fig. 21) shows the larger branches of the nerve-plexus arising, in the main, from the median olfactory nerve ; also the densely pigmented layer of ectoderm confined to the olfactory region, the clear olfactory buds (pl. b.), the small clusters of dark cells looking like ganglia, and numerous branching blood-vessels. One layer of cuticula has been peeled off, so that below where the tooth-like spines should be there are large pores with projecting craterlike summits (5. sp.), containing many pigmented cells.

The olfactory buds vary a good deal in size and form. They are usually spherical or pear-shaped, and composed of a varying number of large pyramidal cells, the apices of which sometimes surround a perfectly clear spherical lumen. The appearance of the buds varies greatly according to the method of preparation, and other causes not clearly understood. In some they appear perfectly empty, so that the organs look like so many blank spaces in the tissue, with only a few cell outlines visible ; or, in organs isolated by maceration in Bela Haller’s fluid, a few cells may contain a very delicate spongy reticulum (fig. 9), while others in the same organ may be densely crowded with refractive globules, so that they resemble certain gland-like cells that I have found associated with sensory cells on the tentacles and mantle edge of Molluscs, such as Area, Pecten, and Lima (‘Eyes of Molluscs and Arthropods,’ p. 722). A dark multipolar cell with a large nucleus can usually be seen in the interior of the organ ; it looks like a ganglion-cell with two or more fibrous ends, the course of which cannot be followed very far in sections. It is undoubtedly the same dark ganglion-like cell so conspicuous in the young stages of these organs (fig. 23).

The clear lumen seen in the younger specimens, the chitinous, duct-like tubule, and the whole appearance of these remarkable structures point toward their glandular nature. On the other hand, their extraordinarily rich nerve-supply, and the unquestionable derivation of the whole group of organs from a primitive segmental sense organ, seem to show equally clearly that they are sense buds.

Not till I was able to demonstrate experimentally that similar organs in the inner mandibles were gustatory organs did I feel satisfied that the “olfactory buds” were of a sensory nature. I then studied them anew by macerating fresh material, paying special attention to the central ganglion-cell and its relation to the chitinous tubule. I did not succeed in getting such perfect isolation as with the gustatory cells, and there still remain some points of importance unanswered. But the cardinal point at issue, whether the buds are glandular or sensory, was settled beyond doubt, for I was able to demonstrate that there is rarely a lumen in the fully formed buds, and that when it does occur it does not communicate with the exterior. Moreover I proved that the chitinous tubule cannot be a duct to the gland, since it is in reality a direct prolongation of the central ganglion-cell, and may be compared with the tubule in the distal end of the gustatory cells. Thus every reason for regarding these sense buds as glands disappears.

The lumen of the buds varies greatly in its appearance. It is most commonly present in the newly formed buds, where it is spherical and sharply circumscribed (fig. 23). In the adults it seldom has this appearance, and may be entirely absent, or it may be reduced to a small irregular space between the cells. Although I have looked carefully for them I have never seen any of the clear globules of the gland-like cells in the lumen of the gustatory buds. The lumen appears to be something like a much-retarded invagination cavity, although, as nearly as I can make out, the organs arise by a solid ingrowth from the ectoderm.

The tubule can be followed in sections from near the centre of the bud, through the cuticular canals, to a point very near the outer surface ; here it becomes very faint or disappears. It may be either straight, very much coiled, broken at intervals as though it were very brittle, or may have one or more spindleshaped swellings. The tubule is undoubtedly composed of chitin, for, as with the gustatory tubules, they can still be seen in the cast-off shells of immature specimens and in the fresh shells cleaned with potash.

The cuticular canals of the olfactory buds are easily distinguished from the gustatory ones by their shape. Each canal in the adult has a nearly uniform diameter, except that near the outer surface it suddenly narrows and communicates with the exterior by a transverse slit (figs. 9 and 12, C and D).In the younger specimens they are slightly expanded near the top, and a flange-like rim is formed by the projection of the cuticula into the outer end of the canal (fig. 23).

The canals contain, besides the chitinous tubule, a varying number of fibres, with here and there a few minute nuclei (fig. 9, n. c.). They can be traced a short distance only toward the outer end of the canal ; in the opposite direction they seem to run either over the outer surface of the olfactory bud, or apparently between its cells toward the interior. I have never been able to trace them with certainty up to nerve-fibres, although they appear to have such connections.

The tubules isolated by maceration in Haller’s fluid stain deeply in methyl green and in acetic acid carmine, resembling the chitinous tubules of the gustatory organs. They are usually collapsed, and appear to be perfectly empty. Although I have examined them in many ways, paying special attention to their broken ends for protruding fibres, I have never seen a trace of the axial fibres so conspicuous in the other gustatory tubules. Isolated tubules from the gustatory buds on the inner mandible sometimes show, when treated with potash, a protoplasmic-like envelope with spiral markings (fig. 14) ; others have two or more coarse refractive fibres, often thrown into numerous irregular folds, extending along their outer surfaces (fig. 16, a). The tubules in the olfactory organ are very rarely convoluted, and they never have the two refractive fibres just mentioned. When the olfactory buds are isolated and examined whole, spindleshaped cells are often seen adhering to their outer surface, also a few scattered nuclei that appear to belong to the delicate membranous investment of each organ.

The nerves that supply the olfactory buds are small strands arising from an extensive anastomosing plexus found everywhere beneath and around the olfactory buds. The plexus itself arises from three large nerves, that I shall call the lateral and the median olfactory nerves (Pl. 2, fig. 18 ; see also Pl. 4, figs. 48 and 49).

The lateral olfactory nerves arise apparently from the anterior part of the brain, but in sections one can follow their roots on to the ventral surface into the optic ganglia (fig. 49). In the adult the proximal ends of the nerves consist of coarse, transparent nerve-tubes, while their distal extremities contain many large ganglion-cells, which form an elongated swelling at the tip of the nerve : the latter terminates abruptly just beneath the cuticle on the lateral edge of the olfactory organ (fig. 49). The lateral olfactory nerve is accompanied by a large blood-vessel that divides into numerous branches, supplying the tissues in front of the olfactory organ (fig. 18, bl. y.) ; small nerve-filaments accompany some of these blood-vessels, and probably supply the ectoderm of the same region.

Several larger nerve branches leave the median border of the lateral nerves a little distance back of the olfactory organ, and mingle with the plexus formed by the median nerve (fig. 18). Some of these branches terminate in small, rounded, ommatidia-like clusters of large cells, which contain irregular refractive plates like those seen on the borders of the young lateral eyes. The lateral olfactory ganglion contains a great many of these clusters of cells. A fair idea of their appearance may be had from fig. 20, which represents some of them at the root of the lateral nerve in an immature specimen. Some near the tip of the lateral nerve, in a still younger specimen, are shown in fig. 22 (g. ol. n.). The lateral ganglion and the isolated clusters of cells are the remnants of a primitive sense organ1 derived originally from the margin of the brain, but which is now converted into these ganglion-like cells. They are retinal cells that have lost their pigment, and now have all the appearances of ganglion-cells, except that they still retain their rods. In other words, we have caught a sense organ in the act of being transformed into a ganglion—the only case of this nature on record, so far as I know.

It is an extraordinary fact that the lateral nerve terminates abruptly in this great mass of ganglion-cells, which are apparently neither connected with surface sense organs nor themselves in a position to receive stimuli from without.

The median olfactory nerve is of an entirely different nature from the one just described. If differs greatly in size and complexity in different individuals, and if the supposition shortly to be advanced is right, it is better developed in the males than in the females. It arises long after the lateral nerves (after the third larval moult) as an outgrowth from the anterior wall of the median eye tube (Pl. 3, fig. 43). In the adult it is a solid nerve composed of two portions, a distal and a proximal one. The latter is composed of a mixture of nervefibres and ganglion-cells. The nerve-fibres are not the apparently hollow nerve-tubes seen in the lateral olfactory nerves and elsewhere, but appear to be more solid and refractive, with a yellowish cast, and nuclei here and there. At intervals throughout the proximal portion of the nerve there are spindle-shaped ganglia composed of small densely crowded and deeply stained nuclei, exactly like those in the fore-brain. They vary in number and size, and may extend directly into the brain-tissue at one end of the nerve, or up to the olfactory organ at the other (fig. 18).

The distal end of the median nerve divides into many diverging branches, which can be followed by means of a hand lens to. the posterior edge of the olfactory organ ; they there begin to anastomose, and form a dense plexus underlying the olfactory region, but a considerable number of fibres extend beyond the olfactory region to the neighbouring ectoderm. Here and there the larger strands of the plexus contain small groups of the dark-coloured nuclei, similar to those in the brain; or the smaller strands may contain a single large tripolar ganglion-cell, with granular protoplasm and a large clear nucleus (fig. 9,g. c.).

A large blood-vessel accompanies the median nerve ; under the olfactory organ it breaks up into numerous branches, some of which are crammed full of blood-corpuscles that stain dark red, and under a low power might be mistaken for ganglia (fig. 18, b. v.).

Termination of the Nerves.—

On the terminal joints of the exopodites to the abdominal appendages sense buds like those in the olfactory organ are very numerous ; and as there is little surrounding tissue, one can peel off the cuticle organs, together with the after maceration in nitric acid. When such a preparation is placed in dilute glycerine and examined from the inner surface, the nerve-plexus underlying the organs can readily be seen. Branches like those shown in fig. 9 are seen anastomosing with one another in a most intricate manner. In the olfactory organs one cannot obtain such instructive surface preparations, owing to the crowding together of the sense buds and the abundance of connective tissue and blood-vessels ; but sections and isolated sense buds show clearly that the plexus is much the same as in the abdominal appendages, only a little denser. In the olfactory region the larger branches of the plexus usually lie a little below the organs, but they may lie directly on their inner surfaces, or may be squeezed in between adjacent buds. In some cases large branches seem to penetrate into the interior of the organ, but such appearances may be deceptive owing to the crowding of the organs.

The nerves actually connected with the organ are delicate, transparent, nucleated filaments, easily overlooked ; they spring from the coarse strands and spread over the surface of the buds, as shown in fig. 9. I could discover no uniformity in number, or any of that drawing out of the cells often seen where nerves unite with sense organs. They can be followed a short distance as very faint, but rather wide fibrillated bands, some of which appear to dip down between the cells toward the central cavity. Occasionally one sees an irregular, poorly defined body containing several nuclei, from which arise a few nerve strands that spread out over the surface of the bud (fig. 9, g. c.). They sometimes contain a single large multipolar ganglion-cell, with dark granular protoplasm and a clear nucleus. The exact method in which the nerve strands terminate is very difficult to determine.

About the neck of the buds are numerous fibrous strands which extend outwards over the surface of the buds into the large pore canals (fig. 9, n. c.).

There are some remarkable organs scattered about in the subdermal tissue of the olfactory region of the adult(fig. 21,g.). They are irregular, usually oblong, spindle-shaped, or branched masses of small cells, whose nuclei stain deeply. They are consequently very conspicuous, and I at first took them for ganglia connected with the nerve-plexus; but I have found them in other parts of the body where there seemed to be no plexus, so their nature is doubtful. When examined under a high power the nuclei appear to be surrounded by concentric layers of protoplasm, giving the whole mass a very characteristic appearance. In some places the cells are so loosely packed that they fail to touch one another ; they then lose their concentric striations, and appear like masses of bloodcorpuscles. Usually a blood-vessel passes through the centre or along the side of the bodies in question. In cross sections the central blood-channel is seen to be either empty, filled with a dark coagulum, or crammed with blood-corpuscles, which in some cases are difficult to distinguish from the cells composing the surrounding tissue. The nuclei of the cells usually arrange themselves concentrically about the bloodvessel. In other cases these problematical organs contain a central canal, or irregular space, through which passes one or more nerve-strands. The nerve-strands may run over or through these organs, dividing into several branches on the way, but emerging at the opposite end without any apparent diminution in size, and without any intimate connection with the organ.

B. Physiology of the Olfactory Organ.—

The anatomical features of the olfactory organs are sufficiently remarkable to make any physiological experiments as to their function of great interest and importance. Their similarity to the gustatory organs on the inner mandibles might lead one to expect that stimulating them with food would produce reflex chewing movements. But although I have made repeated attempts to stimulate the olfactory organ with various kinds of food, with acids and ammonia, I have never succeeded in producing any characteristic reflex movements in that way. Even drops of strong hydrochloric acid or ammonia seem to have no more effect than when applied to other parts of the body; they cause a slight start, nothing more.

In order to remove all doubt as to its glandular nature, I have excised the olfactory organ, wiped its outer surface dry, and then stimulated with electricity the attached nerves, but have never seen any traces of a secretion, such as might be formed provided it were a gland.

Although these negative results are a little surprising, they do not render the sensory nature of the olfactory organ any less probable ; for we cannot expect every sense organ to give on stimulation such beautifully “diagrammatic reflexes” as those on the mandibles. However, I finally discovered that electrical stimulation of the olfactory region produced at once very remarkable leg movements, such as are never seen under any other circumstances.1 The experiment is not always successful, but when it is, the moment the electrodes are applied to the olfactory organs of the male, rapid chewing movements of the mandibles are produced, accompanied by vigorous snapping of the chelicerae, which may finally become rigid and stretched out backward at full length. At the same time the second pair of legs, which during all our preceding experiments on the gustatory organs have remained motionless, are now quickly and repeatedly flexed, as though trying to hug or grasp some object and force it toward the mouth; all the other legs remain motionless. Stimulation of the region about the olfactory organ, or along the median line between the olfactory organ and the brain, or above the brain, may produce the same effect. It is a remarkable fact, for which I can give no explanation, that one does not always get the same results on stimulating the olfactory organ, although there can be no doubt about the character of the response when it does occur. For example, some males would never respond, although repeatedly stimulated at different times. Others would respond immediately and at almost every stimulation. There was one male upon which stimulation had at first no effect, but which responded beautifully a short time after its mandibles had been stimulated into action by rubbing clam upon them. Another would not respond at first, but did after it had lain on its back in the air for two or three hours. Again, it might happen that repeated stimulation produced at first no effect, when suddenly the characteristic movements of the second pair of legs and the cheliceræ would begin of themselves. Subsequent stimulation, however, failed to reproduce this response, although the animal would start whenever the electrodes were applied, showing that the current had passed through the cuticula into the underlying tissue.

It is also a curious fact that persistent stimulation of both sides will sometimes produce the characteristic leg movements on one side only ; then suddenly both sides, or perhaps the opposite side alone, respond. When the mandibles were shaved or amputated on one side, the opposite side usually responds first on stimulating the olfactory organ.

In one female, in which all the mandibles of the right side were amputated, stimulation of the olfactory organs produced sudden raising of the second to the fifth legs on the left side, and forcing of their tips toward the mouth ; the movements of the second leg were most marked. The legs on the right side remained motionless. This is the nearest approach I have seen in the female to the movements so characteristic of the male under the same circumstances. In every other case that came under my observation (at least twenty) stimulation of the olfactory organ of the female produced only slight starts of the legs and abdomen.

These experiments point toward a double function of the olfactory organ. The reflex chewing movements indicate its association either as a tasting or a smelling organ with the process of eating. But it is very difficult to explain why these movements are not produced by direct stimulation with food. On the other hand, the extraordinary hugging and grasping movements of the second pair of legs in the males clearly show that they are in some way functionally associated with the olfactory organs. Now it is well known that these legs in the male are specially modified for grasping the female during copulation ; and, as I have shown that they are the only legs not involved in the reflex chewing movements caused by stimulating the mandibles, I can conceive no other explanation for these facts than that the olfactory organ is used by the males in detecting the females. This is, moreover, a function for which its position is well suited. That an organ for this purpose must be present seems certain, for the males during the breeding season hunt out the females and attach themselves to them with great precision. It is hardly probable that this could be done by means of touch or vision. While I can find no important difference between the structure of the olfactory organ in the adult males and females, in the young I find that the median olfactory nerve has an enormous ganglionic enlargement in some individuals and a much smaller one in others, and I suspect that the former are males and the latter are females. Again, when sound males and females are put in the same aquaria, the males usually attach themselves to the abdomen of the females, but I have never seen a male whose olfactory organ has been cut out (and I have had, at different times, half a dozen such specimens) attached to a female. This experiment might be conclusive if it were performed on a larger scale. Unfortunately I did not have sufficient material to do this.

I have tried several times to arouse movements of the second pair of legs in the male by rubbing the olfactory organ with fresh ova, and with the secretions of the oviduct, but without success. Renewed experiments are necessary here, for I have not given this aspect of the question the attention it deserves.

C. The Development of the Olfactory Organs.—

The olfactory organs first appear in surface views as a pair of oval ectodermic thickenings, the “primary olfactory organs” on the lateral margin of the brain, just in front of the optic ganglion (figs. 24, 25, 46, 47, 49, and 61). Each organ soon separates from the brain and grows forward, leaving behind long, thick strands of ganglion-cells, which constitute the lateral olfactory nerves (fig. 25). As soon as the primary olfactory thickenings are separated from the brain they become filled with white pigment ; at the same time branched cells filled with white pigment leave the thickening, and, extending under the ectoderm in all directions, form a gradually widening pigmented plexus; that in the adult maybe several square inches in extent. In the young larvæ the pigmented or choroid plexus is attached to the anterior edge of the primary olfactory thickening by a stout stalk (figs. 46 and 47, p. st.). Each plexus lies beneath the ectoderm and a little in front of the brain, which even at this stage it more than equals in size. At a little later stage the two plexi become completely united (fig. 45). In fig. 22, w. p., a part of the plexus is shown in section under a higher power.

The primitive olfactory thickenings soon unite in the median line to form an apparently unpaired organ lying some distance in front of the brain. Before this takes place the cells constituting the thickening arrange themselves in irregular clusters, and their walls develop those peculiar cuticula-like thickenings that look so much like groups of visual rods in the ommatidia. Some of these cell-clusters soon leave the main thickening, and lie scattered about under the ectoderm, but connected with the distal portion of the lateral olfactory nerve by branching nerve-bundles (fig. 18, x) ; others remain in the distal end of the nerve to form the terminal ganglionic swelling. In specimens about two inches long there is nothing left of the primitive olfactory thickening but an irregular mass of large cells, mostly pear-shaped, in which the lateral olfactory nerves terminate. There is nothing in the final position, shape, or structure of these cells to indicate that they now communicate with the exterior, or could function as sensory cells, although there is every reason to believe that they are derived from what were once sensory cells, like those in the eyes.

The definitive olfactory organ does not appear till the changes just described are nearly finished, i. e. in the larvae from one half to one inch in length. During this period buds begin to appear in the ectoderm between what is left of the primitive olfactory thickenings, or between what now constitute the swollen ends of the lateral olfactory nerves. At about the same time the median olfactory nerve arises from the median eye-tube, near where it joins the brain as an outgrowth from its anterior wall. Now this part of the tube may be considered morphologically as a part of the anterior neural wall or roof of the brain, just as a corresponding part of the stalk of the pineal eye in Vertebrates or in scorpions might be regarded as a part of the brain roof (compare Pl. 3, figs. 41 and 42). Moreover, as in their earliest stages the median olfactory nerves contain numerous small ganglioncells which soon develop into two large irregular botryoidal lobes, having the identical histological structure as the cerebral hemispheres; and as the nerve soon unites directly with the cerebral hemispheres, from which it then appears to be a direct outgrowth, I think we are justified in regarding the median olfactory nerves and olfactory lobes as outgrowths from the anterior wall or roof of the cerebral hemispheres.

Of course the fully developed lobes, as seen in fig. 17, do not grow Out from the cerebral hemispheres, but as the few cells from which they arise do so, it amounts to the same thing. It is evident that if the growth of the lobes and their separation from the brain took place at the same time the olfactory lobes would, as in most Vertebrates, make their appearance as massive outgrowths of the cerebral hemispheres.

The olfactory lobes vary greatly in size in different individuals. They are relatively largest in immature forms from about 3 to 6 inches long. They may consist of two very distinct botryoidal masses composed of irregular lobes of small, densely packed, and deeply stained nuclei, each surrounding a central mass of medullary substance (fig. 17). The whole appearance of the lobes is very much like that of the convoluted parts of the cerebral hemispheres (fig. 52). In some cases there is only one large lobe, evidently formed by the more or less complete fusion of two like those in fig. 17. The stalk of the lobes in this stage is a solid column of small nuclei, and passes without any perceptible change into the cerebral hemispheres. The fusion is so complete that immediately after reaching the brain all distinction of olfactory nerve and cerebral substance is lost, for there is no trace of any medullary strands or strings of cells that can be regarded as roots to the nerve. After the young crabs reach a length of from 5 to 8 inches the lobes are less conspicuous, breaking up into spindle-shaped masses scattered along the middle third of the nerve. The distal third breaks up into many fine strands, that form a plexus beneath the olfactory buds, from which the buds are supplied. The proximal third shows a more or less clearly marked division into two main strands, corresponding with the olfactory lobes, and each strand goes to its respective cerebral hemisphere. Thus, although I have called this nerve a median nerve, it is in reality a paired one.

D. Nature of Olfactory Organs.—

The primary olfactory thickenings are undoubtedly segmental sense organs serially homologous with the eyes, as first stated in my paper “On the Origin of Vertebrates from Arachnids,” p. 337. They correspond exactly in position with the lateral eyes of scorpions (Pl. 5, figs. 58 and 59), and in their histological structure they show traces of ommatidia and retinal rods, like those in the lateral and median eyes of Limulus. The degeneration of this pair of eyes in Limulus was due probably to their being retained on the under side of a broad shield-like carapace, where they could be of little or no use. Their gradual degeneration and conversion into ganglion-cells, and their subsequent incorporation with a new set of sense organs with an entirely different function, can be followed in great detail, and furnishes a most remarkable instance of change of function and structure.

A fact, however, of great morphological significance is the striking resemblance between the structure and mode of development of the olfactory organ in Limulus and that of the so-called “frontal Sinnesorgan” of Phyllopods, as described by Leydig, Claus, and others. Claus’s description of this organ in Branchipus will serve equally well for Limulus. The peculiar “kolbenformige” cells described by him as originating from the brain ; their position beneath the ectoderm in ommatidia-like clusters, and containing the refractive “zinkage” needles; their position in relation to the median eye, as well as their relatively late appearance, are much the same as in Limulus. It seems to me there can be but little doubt that the frontal organ of Branchipus, with its ganglioncell masses arising from the median part of the brain, corresponds to the median nerve and median olfactory region of Limulus; while the “Kolbenzellen” organ, with its lateral ganglionic nerves, corresponds to the lateral nerve and primitive sense organs of Limulus. Of course at first sight the appearance of the organ in Limulus is different from that in Branchipus, but its fundamental structure and relations to the brain are the same. These facts point conclusively to a much closer genetic relationship between Limulus and the Phyllopods than has been recently supposed to exist, and this supposition has further support in the similarity in the development of the trifold median eyes. If a more careful comparative study of the frontal organ in other Phyllopods—Apus, for example—should confirm the above comparison, it would settle once for all the vexed question of the relation of Limulus to the Crustacea, and would furnish very strong evidence of the common ancestry of Crustacea and the Arachnids in the trilobites.

It will be remembered that our physiological experiments demonstrated the existence of special gustatory organs on the under side of the inner mandibles (4, p. 11). On examining this place (fig. 3, g. b.) one can readily detect just beneath the smooth cuticle a yellowish granular mass, 7 or 8 mm. long and 2 or 3 mm. deep. Sections show that the cuticle is perforated with an immense number of canals, something like those in the olfactory region. The canals contain chitinous tubules extending into the yellowish mass, which consists of innumerable gustatory buds, apparently exactly like those in the olfactory organ, only much more numerous, and densely packed together many rows deep. The chitinous tubules are coarser than in the olfactory organ, and many of them are thrown into complicated folds, as in fig. 16, b. I have wiped off the surface over these organs very carefully, and stimulated the organs and their nerves with electricity, but have never seen a trace of any secretion appear there.

When treated with chromo-acetic osmic acid, and stained in haematoxylin, most of the organs are darkened around the base of the chitinous tubule, assuming the colour and appearance of sensory tissues when treated with this reagent, while the periphery stains a bright blue. It is possible that the peripheral gland-like cells secrete a substance having special powers to absorb certain chemical substances in the surrounding media, and in this way the stimulation of the centrally placed ganglion-cell is increased. But how the stimulus can reach the organ through these long tubules, Which in some cases are much coiled, is not easily understood. Some of the older and larger organs seem to be quite empty and dead ; others stain a dense blue-black in haematoxylin; while still others, apparently young ones, show very little of this blue colour, but stain dark brown in osmic acid, like ordinary sense organs.

The same kind of buds, but isolated, are found thinly scattered over the surface of the outer jaws, between the bases of the spines. It is probably these organs which, after the spines and the inner jaws have been removed, produce the faint reflex chewing movements referred to in our description of the physiological experiments (d, p. 12).

The whole body of Limulus is very sensitive to changes of temperature. This may be easily demonstrated in the following manner :—If a crab be placed on its back and allowed to become perfectly quiet, one may grasp the appendages or mandibles with forceps and gently move them about without arousing the animal ; or one may touch the upper or lower surface of the carapace, or the gills, with any object the same temperature as the air, but the instant one touches any of these parts with the fingers, or drops water on them warmer or colder than the surrounding air, the animal at once becomes more or less agitated, and moves the appendages and abdomen about in vain efforts to regain its normal position. There is no way in which we can make the quiescent animals start more quickly or violently than by very gently breathing on the gills and under surface of the body, although quite violent fanning may produce no effect at all.

If, holding the head within about a foot of a crab, one blows little puffs of warm air on the parts about the mouth, the cheliceræ will snap with every puff. If the puffs of warm air are made a little stronger the chelaria are brought forward, and the chelæ of the first and third pairs of appendages close at every puff; the chelæ of the second pair meantime, in both males and females, remain motionless.

Whenever the sides of the cephalo-thorax of a quiescent crab are touched with the fingers prompt movements of the appendages follow, the legs opposite to the point of contact, and on the same side that was touched, beginning first.

The gentle warmth of the hands held within two or three inches of the sides of the cephalo-thorax, or from the face or body when watching closely the experiments, usually produces uneasy movements in crabs that were before perfectly quiet.

The temperature sense is very acute on the lateral margins of the thorax and abdomen, and on the tips of the legs, and of the abdominal appendages, being apparently most acute in the last-mentioned organs. The flat triangular area on the anterior margin of the under side of the cephalo-thorax is unusually blunt to temperature changes.

It is a remarkable fact that the regions so sensitive to slight temperature changes can be touched with small wires hot enough to singe the cuticula without producing any movement. But if a rather large iron, about 2 or 3 mm. in diameter, be held for a quarter of a minute on an abdominal appendage, movements are produced, but they are evidently due to irritation of organs situated more deeply than those stimulated by gentle breathing.

A. Course of Temperature Impulses.—

The following experiments show the course of the temperature impulses to a temperature centre, located somewhere in the fore-brain region.

Experiment1 A.—

When a shallow longitudinal cut is made on the ventral side through the skin along the lateral margin of the right row of appendages, the temperature sense of the carapace lateral to this cut is destroyed. On applying the hand to the right side of the cephalo-thorax, either on its dorsal or ventral surface, no movements are produced, but when the same is done to the left side the legs on both sides are set in motion. The roots of the great tegumen-tary nerves (fig. 48, 2—6, a. m. p.) lie close to the ventral surface along the outer margin of the legs, and as they are the only ones severed by this proceeding, the experiment shows that the temperature impulses travel centripetally along the anterior and posterior hæmal nerves of the thorax, not in all directions through the subdermal plexus. It shows also that the temperature impulses not only pass up and down the crura on the side stimulated, but on to the opposite side as well. This is in marked contrast with the gustatory impulses which give rise to reflexes in those mandibles only that are stimulated.

Exp. B.—

Cutting across the ventral cord just back of the chelaria causes regular raising and lowering of the abdominal appendages about twenty times a minute. They finally come to rest, and are then left in an unnatural position, with the right and left appendages of the same pair crossed over the median line. The mandibles are pressed firmly together in a sort of tetanus, the line of the meeting being irregular and un-symmetrical. Strangely enough, stimulation of the mandibles with food produced, in this instance, no regular movements of mastication. But a warm hand placed on either side of the cephalo-thorax produced immediate and simultaneous movements of the legs of both sides. The temperature reaction of the cephalo-thorax, then, is apparently not influenced in the least by section of the ventral cord, but the respiratory, and perhaps the gustatory reflexes are strangely affected.

Exp. C.—

In this experiment the crab operated upon had already had the mandibles belonging to the second right appendage removed, but was otherwise in perfect condition. A deep median longitudinal cut was made, severing (as shown by post-mortem examination) the post-oral cross-commissures of the crura, and cutting through the junction of the crura in the vagus region (Pl. 4, fig. 48). The animal lived nearly two months in apparently good condition. It was then killed to make sure of the direction of the cut. During this period it ate with the normal movements of the mandibles, except that they worked perhaps a little more slowly than usual, and the cheliceræ were not brought into action. On placing the warm hand on one side of the cephalo-thorax of the quiescent animal, responsive movements of the legs on both sides were at once produced. The temperature reaction was unaffected.

Exp. D.—

In this specimen the crus of the left side was sectioned just back of the second pair of legs. The results were very clearly marked. After about five hours, when the crab had become perfectly quiet, placing the hand anywhere on the left side of the cephalo-thorax produced no movements what ever of the appendages back of the section, and only feeble movements of those on the same side in front of it. On placing the hand on the right side, however, all the legs on that side were immediately set in motion, and continued to move as long as the hand was held there, but they ceased immediately the hand was removed. These results could be produced repeatedly without any perceptible variation. After eighteen hours, during which period the crab had lain on its back on a table without attention, the reaction was less vigorous, but still very prompt and decided. There was no movement of the mandibles after this period in response to stimulation of the gustatory spines. Unfortunately I did not try this at an earlier stage ; neither did I try, as I should have done, heat stimulation of the appendages back of the section in the left crus. These experiments are sufficiently definite, however, as regards the course of the temperature impulses, for they show that, starting in the cephalo-thorax, they travel inward along the hæmal nerves to the corresponding crus, which they ascend to the fore-brain ; from there they must descend along both crura to the pedal nerves.

If, as seems probable from this experiment, the temperature centre is somewhere in the fore-brain, we ought to be able to destroy all temperature reflexes by cutting both crura close to the brain. This is very nearly what takes place, as shown by—

Exp. E.—In this specimen both crura were cut completely across, just back of the second pair of appendages. There was much loss of blood, and all the reactions were feeble. The only spontaneous movements were those of the cheliceræ and second pair of appendages. Five hours after the operation no response could be produced by heat stimulation of the sides of the cephalo-thorax, but feeble movements of the legs could be produced by breathing on them directly. These experiments indicate the existence of subordinate temperature centres in each crus (see also experiment on amputated chelæ, p. 16), and prove that the main temperature centre is located somewhere in the fore-brain.

B.There is some doubt in regard to the position of the Gustatory Centre. I did not have this point so much in mind during these experiments following section of the crura, and I did not always test for gustatory reactions. The centre for the mandibular gustatory organs probably lies in the forebrain. One would naturally suppose, however, from the way a given leg can be made to chew when its gustatory organs are stimulated, that there was in each leg an independent gustatory centre, which I at first supposed might be located in the ganglion of the pedal nerves. However, if this be so, it is hard to understand why sectioning either the ventral cord, or the crura back of the fore-brain, should stop the reflexes, while a median longitudinal section across their union in the vagus region should have no effect (see pp. 37-8). There is also a difficulty in the fact that I have amputated the whole leg, including a portion of the crura with the attached pedal nerve and its ganglion, and on stimulating the gustatory spines with food have failed to produce reflex contraction of the leg, although movements could be easily produced by applying the electrodes directly to the pedal ganglion or nerve.

C. Structure of Temperature Organs.—

It is not so easy to identify the temperature organs as the gustatory ones. However, I have found close beneath the epidermis, in all the parts examined, including various regions on the upper and lower walls of the carapace, the gills, the legs, &c., a loose subdermal plexus of nerve-fibres and ganglion-cells. The cuticula of all these regions is perforated with canals under which are buds, in nerve-supply and structure like those in the mandibles and in the olfactory region. As I can find nothing else there that looks like sense organs, it is very probable that these buds are the organs that are specially susceptible to changes of temperature.

D. Function.—

The temperature sense seems to be remarkably acute in Limulus. I know of no other Invertebrate that approaches it in this respect. As Limulus spends the fall and winter months in deep water, where there is comparatively little variation in temperature, it is hard to imagine what this sense can be used for if not to aid the animal in migrating to warmer shallow water during the breeding season.

There are some wart-like sense organs about 5 mm. in diameter on the endopodites of the abdominal appendages, to the structure and function of which I have not given special attention. They have already been briefly described by Gegenbaur. The underlying cuticula is richly perforated with peculiarly shaped canals, and chitinous tubules extend into them similar to those in the gustatory spines. They are sensitive to ammonia vapour, tactile impressions, and temperature changes, but stimulation of them in this way does not produce any definite reflex action. If a camel’s-hair brush be drawn gently across the terminal joint of the expodites to the abdominal appendages, or over the region about the olfactory organs on the abdominal appendages, prompt movements of the legs and abdomen follow. This is not the case when the basal joints of the legs, or the soft flexible skin about the joints of the legs, or about the anus, are brushed.

If we review the results of our observations described in the preceding pages, we see that there are two distinct kinds of sense organs in Limulus. There are the single-celled sense organs, each with a long chitinous tubule, the best representations of which are found in the mandibular spines and in the chelæ of all the walking appendages; they are pre-eminently gustatory: a slightly modified kind in the chelæ probably serve as temperature organs.

The second kind are rounded, solid clusters of gland-like cells, containing a large multipolar ganglion-cell provided with a chitinous tubular prolongation, the distal end of which terminates in canals near the outer surface of the cuticula. These sensory buds are distributed over the whole body, but they are much more abundant in some places than in others. They are as a rule innervated by delicate branches from an everywhere present subdermal nerve-plexus, which is itself connected with branches of the tegumentary nerves; but special aggregations of buds may be supplied by special nerves, such as the olfactory buds, and the gustatory cells and buds of the mandibles ; both sets of the latter organs being supplied by the two branches of the mandibular nerve (coxal nerve of scorpions, supra-branchial nerve of Vertebrates).

All these sense buds or sense cells in Limulus multiply by division. This process is easily studied in the buds of the inner mandibles, and in the gustatory cells of the mandibular spines. The division begins at the summit of a cuticular canal, and gradually extends down into the organ, the process being the same in both sense cells and sense buds. The chitinous tubule itself does not divide; a new one is formed alongside of the old one ; a longitudinal constriction then appears at the summit of the old cuticular canal, dividing it into two diverging arms, the separation of which gradually progresses toward either the sense buds or cells, as the case may be. I do not know how the latter divide.

There are at least five varieties of these two types that can be recognised by their termination at the surface. (1) The olfactory buds are connected with nearly straight canals, which contract near the top into a very narrow slit (fig. 12, e and f). (2) The gustatory buds of the inner mandibles are connected with straight canals, resembling the ones just described, except that they open out by excessively small pores (fig. 12, a, b, and c). Just before reaching the surface the tubule expands into an irregular, spindle-shaped body; beyond this it becomes extremely small, but still it can be followed with certainty to the outer surface, where it terminates in a very shallow depression (fig. 12, a). A dark coagulum of some kind usually fills the cuticular canal, and completely surrounds the chitinous tubule. (3) The buds scattered over the surface of the man-dibles terminate in canals opening by a rather wide slit (fig. 12, d). (4) The gustatory cells always lie in bent canals that gradually taper to a very fine opening (fig. 1). (5) Finally, there is a set of canals like those of the gustatory buds, except that they are capped by short spines of varying shape and size, into which extends a chitinous tubule (fig-10).

The sense buds of Limulus have a certain resemblance to the organs which in the Gephyreans have been described by recent authors either as glands or sense organs. Organs similar to those in Limulus, but generally described as glands, are widely distributed in Arthropods.

The only thing resembling the gustatory cells in the mandibular spines are certain organs in the extremities of the palps and in the first pair of legs of Galeodes, as recently described by P. Gaubert. They resemble the chitinous tubules seen in Limulus, but histological details concerning them are so meagre that it is impossible to identify them with certainty.

I will also call attention to the “olfactory cones” of Mutilla, as described by Franz Ruland. They consist of spindle-shaped clusters of cells from which a delicate hyaline tubule runs to the perforated summit of the overlying cone. At the base of the tube is a spindle-shaped swelling with longitudinal striations resembling those on the spindle of the gustatory cells of Limulus. The organ also contains a single large nucleus that may be compared with that in the sense buds of Limulus.

The young stages of the sensory buds are much alike, whatever the subsequent modification. They are probably derived from the multiplication of a few ectodermic cells ; but however that may be, the cells soon arrange themselves to form a rounded body with a small central lumen (fig. 23). The buds in this early stage stain more deeply in osmic acid, and have a different appearance from that of the older buds (fig. 22). The large ganglion-cell and the halo around the central cavity are much more conspicuous at this time than afterwards. Moreover the ends of the cells that converge toward the central cavity are provided with refractive rod-like thickenings, which are highly suggestive of the rods on the ommatidial cells of the lateral eye.

Very similar figures are seen in the inner mandibles of immature specimens, as in fig. 8. Here one can see a distinct fibrous process passing outward from the ganglion-cell, which in this instance lies well to one side of the central cavity.

These buds, which in the young are found in all parts of the body, and at first are everywhere alike, finally undergo various modifications. Some may degenerate, some may become olfactory buds, others gustatory, and still others temperature organs. The resemblance of these buds to ommatidia is so striking that we must include them both in the same category. We can therefore reduce the whole system of sense organs either to isolated sense cells or sense buds, or aggregations of the same. This agrees with my conclusion concerning the sense organs of insects (see ‘Zool. Anz./1890, Nos. 13, 14), where I maintained that the ommatidia of the compound eye were nothing more than specialised cell clusters, which in other parts of the body were supplied with various forms of spines or hairs, and served as tactile, auditory, gustatory, or olfactory organs. Moreover, in my earlier observations on the “Eyes of Molluscs and Arthropods,” I showed that the eyes even of Molluscs were composed of circles of cells or ommatidia, which were also widely distributed over the surface of the body. In Vertebrates we have evidence of the same condition. Isolated sensory cells are there widely distributed, and, while essentially alike in structure and appearance, have very diverse functions. The olfactory and gustatory organs are but aggregates of sense buds like those widely distributed over the body. The presence of similar sense buds in the eye is shown by the circles of rod-cells surrounding the cones.

The Brain of Insects and Myriapods

Among the most remarkable features of the brain of Limulus are its various cavities, its cerebral hemispheres, and its infundibulum. Although some of these parts can be identified with a fair degree of certainty in insects, Myriapods, and Arachnids, the whole appearance of the fore-brain region in the adult Limulus is totally unlike that of any other known Arthropod. On the other hand, it bears such a striking resemblance to a Vertebrate brain that I believe no competent person need hesitate a moment in picking out the corresponding parts.

Before one can understand the structure and development of the brain of Limulus one must first have a clear idea of its structure in those Arthropods in which the cephalic lobes are present in their simplest and most primitive condition. I shall therefore first call attention to certain features of the brain of Myriapods, Insects, and Arachnids, that are probably common to the brain of all Arthropods. I have not paid special attention to the Crustacea, and I do not, unless expressly stated, include them in my speculations on the Arthropod brain.

By a comparative study of the cephalic lobes of Acilius, Blatta, Vespa, Hydrophilus, Scorpions, several species of Spiders, and Limulus, specially prepared to bring out surface contours, I am able to demonstrate that the cephalic lobes of a typical Arthropod are composed of three distinct segments, each containing a segment of the brain, optic ganglion, and optic plate ; between the two latter is an invagination by means of which the optic ganglia are more or less impeded. These results have been in part confirmed by Wheeler and Heider. Viallanes, who has made a careful study of the anatomy and physiology of the adult brain, has quite a different conception of its structure. Embryological studies, however, do not support his views ; they show that each of his three segments, protocerebron, deutocerebron, and tritocerebron, comprise very heterogeneous centres, not at all arranged according to their morphological affinities. St. Remy, in his valuable work on the brain of Myriapods and Arachnids, has followed Viallanes. For lack of space I cannot review the works of these two authors as I would like. But in what follows I shall try to show that their basis of classification of the brain-lobes is founded on vital misconceptions. They fail to recognise the difference between the organs derived from the cephalic lobes and those derived from the ventral cord, as well as the segmental nature of the primitive optic ganglia in such forms as Acilius and Scorpions, and their relation to the compound eye. Moreover, when Viallanes subsequently attempted to confirm his views by embryological study, he not only selected in Mantis a poor type, but mistook the well-known trachealike invaginations for the ganglionic ones described by me, and consequently he is quite right in asserting that they do not give rise to any part of the optic ganglion.

Those who have heretofore touched on the development of the brain of Insects have failed to appreciate the far-reaching morphological importance of the ganglionic invaginations. Korschelt and Heider, in their text-book of embryology, did not understand their relations in Insects, Scorpions, and Limulus. As I consider these invaginations the key to the morphology of the Arthropod and Vertebrate fore-brain, I shall try to explain in more detail my interpretation of their significance.

In studying the development of the convex eyes great confusion and difficulty was at first occasioned by the failure to recognise that two distinct invaginations are sometimes present, one for the optic ganglion, the other, less commonly present, for the eye. Reichenbach and Kingsley made this mistake ; both supposed that the purely ganglionic invagination described by them gave rise either to the whole (Kingsley) or a part (Reichenbach) of the ommateum.

I was the first to show in Vespa that the two invaginations are absolutely distinct, one giving rise to the three lobes of the optic ganglion, the other to the compound eye. The latter invagination, although very deep, probably does not close up, and its outer or middle wall produce the corneagen, as I at first supposed. Strangely enough, the invagination soon straightens out, and the corneagen, as I found out subsequeutly, is formed by the union, over the crystalline cones, of two cells derived from the sides of each ommatidium. The same process undoubtedly occurs in Crustacea.

I studied the subject again in Acilius, and fortunately this insect furnished the most primitive and least modified type of cephalic lobes yet described, and apparently the one from which all others have been derived. I do not by any means believe that the Coleoptera are the most primitive Arthropods. But since we find the Acilius type of fore-brain clearly repeated with more or less modification in other insects, in Spiders, Scorpions, Limulus, and probably with still greater modifications in the Crustaceans (although our knowledge of the last group is too imperfect as yet to speak definitely about them), it must be the nearest in structure to the ancestral type.

A. The Cephalic Lobes of Insects.—

Basing our conclusions mainly on Acilius, we find that in insects the true fore-brain is derived from the cephalic lobes, which are composed of three segments. In each segment one can readily distinguish a pair of brain-lobes, a pair of optic ganglia, and two ocelli opposite each ganglion (Pl. 5, fig. 57). Between the ocelli and the ganglia are three pairs of invaginations, which decrease in depth and extent from the first to the third. The ocelli, after the closure of the invagination, still remain on the margin of the cephalic lobes in their original upright position.

The antennary neuromere is usually regarded as a part of the cephalic lobes. I believe this is a mistake, as can be very readily shown by comparison with Scorpions and Limulus. One finds there, as well as in Acilius, no evidence whatever that it forms part of the true cephalic lobes ; but it is not possible to show this without the proper material properly prepared. I regard the antennary neuromere as morphologically post-oral —it is unquestionably derived from the ventral cord of the trunk. It moves gradually forward till it occupies a position beside, or in front of, the mouth, constituting what I shall call the mid-brain.

B.The cephalic, sympathetic, or the stomodæal nerves are very important landmarks in this region. The anterior stomodæal, or “frontal” ganglion and its nerve arise as a solid linear outgrowth from the median anterior wall of the stomodæum. Its outer or proximal end is united by two cross-arms with the lateral stomodæal ganglia situated either on the median border of the antennary neuromere, or on the oesophageal commissures (Pl. 5, fig. 57). This cross-commissure I shall call the anterior pons stomodæi (a. p. st.). In the later stages it is usually shaped like an elongated V, owing to the fact that the frontal ganglion, which is situated at the apex of the V, is carried a long distance inward by the growth of the oesophagus. From the lateral stomodæal ganglia the lateral stomodæal nerves extend inwards, one on either side of the oesophagus (fig. 57,l, n.). Their exact point of union with the brain I have not been able to determine, because in the forms that I have studied these nerves are either absent or imperfectly developed. However, from all I can gather about them from the works of others, they must arise very near this point ; moreover that is their point of origin in Scorpions and Limulus.

The lateral and median stomodæal nerves expand at intervals into ganglia united with one another by commissural strands. These ganglia and strands are specially well developed in lulus, as shown by Newport (fig. 62).

The labrum, which in Acilius I have shown to be unquestionably a paired organ, arising from the very anterior margin of the cephalic lobes, is innervated, strangely enough, from the roots of the anterior pons stomodæi, a condition which seems to prevail throughout the Myriapods, Insects, Arachnids, and probably the Crustacea. Moreover there appears to be present in a sufficient number of cases to make it typical for the whole group of Insects a small unpaired nerve directed outward and backward from the frontal ganglion toward the labrum (fig. 53). It is obvious that the innervation of the labrum is different from that of any other Arthropod appendage, and I believe it has special significance, as I shall presently indicate.

The lateral stomodæal ganglia are usually situated on the posterior median margin of the antennal neuromere, or sometimes on the oesophageal commissures. They are always united with one another by a large transverse commissure that contains, unlike the other post-oral commissures, many ganglioncells. In the young stages of Acilius (see‘Eyes of Acilius,’ fig. 44) I have figured a segment of the median sympathetic nerve which seems to be united with this commissure to form a rudimentary ganglion, and in the fourth larval stage of the lobster I have found a very distinct ganglion in the middle of this commissure. No doubt this commissure is characteristic of the whole group of Arthropods ; it is well known in Myriapods, Insects, and Crustacea, and I have found it in Limulus. Its apparent absence in the Arachnids is probably due to the crowding together of the neuromeres in the mouth region. This commissure, I am convinced, also belongs to the cephalic sympathetic system; but it may perhaps contain fibres representing the cross-commissures of the antennal neuromere. I shall call it the posterior pons stomodæi.

Besides these nerves, most insects, as shown in the diagrammatic fig. 53, are provided with a system of trunk-sympathetics consisting of a chain of lateral and median sympathetic ganglia ; the latter, as I have shown elsewhere, is derived from the “Mittelstrang” of Hatschek, and probably terminates anteriorly in the posterior pons stomodæi.

C. The Convex Eyes and their Ganglia.—

The convex eyes form such an important part of the brain of Insects that it is of great importance to determine exactly where they belong. They are apparently so intimately associated with the forebrain that there seems little reason to doubt their derivation from the cephalic lobes. But there are strong reasons for supposing that they belong, not to the cephalic lobes proper, but to the trunk, probably to the mid-brain neuromere.

That they did not belong to the cephalic lobes originally is indicated by the fact that in Acilius not a trace of them appears till long after the cephalic lobes, as such, have disappeared (i. e. beginning of the pupal stage), but it is then impossible to determine exactly their relation to the cephalic lobes. In the Scorpion there is nothing comparable to the convex eye. In Limulus they certainly are not derived from the cephalic lobes, although as in insects the optic ganglion does have this derivation. My recent observations have shown that in Limulus the convex eye arises farther forward than I at first supposed. In its earliest stages it lies about opposite the cheliceral segment to which it in all probability belongs ; its subsequent union with the optic ganglion of the cephalic lobes is therefore a secondary affair. The same thing apparently takes place in Acilius, for the convex eyes when they appear at the beginning of the pupal stage, instead of being provided with a special ganglion of their own, become united with the ganglia of the degenerative ocelli.

These facts suggest a very interesting comparison with Myriapods. There seems to be present in nearly all Myriapods a remarkable nerve arising from the optic ganglion and supplying a peculiar sense organ situated at the base of the antennae. St. Remy calls them the nerve and sense organ of Tomosvary (fig. 62). Now I strongly suspect that this sense organ is the rudiment of the convex eye of the higher Arthropods, for it agrees in two important particulars with the convex eye of Limulus, and presumably with that of all other Arthropods, namely, in its situation at the base of the antennae, and in the attachment of its nerve to the ganglion of the larval ocelli.

In Acilius, the compound eyes appear at the beginning of the pupal stage as a sickle-shaped band on the dorsal and median margin of the ocelli. They finally break away from the surface, and their degenerated remains become attached to the under side of the optic ganglia (fig. 60). The latter persist, and form the ganglion of the compound eyes. The conversion of the larval optic ganglion into that of the imago is brought about in the pupal stage by rapid growth along three very clearly marked regions (fig. 60, o. g. 1—3). In all three bands, or centres, which probably represent the three segments of the larval ganglion, there is rapid multiplication of cells, but it is the middle lobe which increases most rapidly, and which forms the main part of the adult ganglion; traces of the other two bands may be seen even in the adult (see my paper on the “Eyes of Vespa”). Now it is a remarkable fact that we find these three identical bands of the pupæofAciliusin the embryos of Vespa, but they are there formed by a direct invagination of the ectoderm of the cephalic lobes. Either the same kind of an invagination or a solid ingrowth is found in a variety of other forms, as in the Hymen-optera,Orthoptera, and Hemiptera—that is, in forms that do not pass through a free ocellate larval stage. This proves that the Acilius type of cephalic lobes is the most primitive, and that in such forms as the Hymenoptera and Orthoptera, &c., important embryological processes are omitted; it also shows that the optic ganglion of the convex eye of insects is formed by the fusion of the three larval ganglia.

As to whether the three frontal ocelli found in many adult insects are derived from the larval ocelli, or are new formations, like the compound eye, is a question of great morphological importance, but we have as yet no evidence upon which to found an opinion concerning them. Their position and number suggest their identity with the three-lobed median eye of Limulus and Crustaceans. Careful investigation of some forms, the Sialidæ, for example, which pass through an ocellate larval stage and possess frontal ocelli in the adult, would probably settle this question. It is certain that early in the pupal stage of Acilius and Cecropia all the larval ocelli break away from the ectoderm and take up their position on the under side of the optic ganglion, where they seem to undergo complete degeneration.

A.The cephalic lobes of Arachnids have at first the same shape and appearance as in insects. They soon divide into three segments, which can be identified with even greater ease than in Acilius, especially the three invaginations of the optic ganglion.

In the first segment of Scorpions1 the ocelli, as well as the distinction between brain-lobe and optic ganglion, have disappeared. The invaginations, which in Acilius are separate, here unite to form a great transverse furrow with thick walls and small dark nuclei (fig. 58, s. l.); the whole lobe sinks below the surface, and moving backward, lies underneath the second segment, where it forms the semicircular lobes (Forgan stratifié, l’organ pédunculé), identical in almost every particular with the semicircular lobes of Limulus and Spiders. The second pair of invaginations are at first exactly like those in insects; but the fold on the lateral margin of the invagination soon advances rapidly inwards and backwards, and uniting in front with a similar fold in the first segment, and behind with one in the third, gradually extends backwards, in a broad amnion-like fold, over the whole of the cephalic lobes. During this process the eyes on the second segment are gradually infolded, so that they finally lie inverted on the middle wall of the fold. They then unite with one another over the median line, and growing forwards come to lie in a common sac at the distal end of a short tube (fig. 42). The eyes of the third segment are not involved in the ganglionic invagination, consequently they remain upright on the surface ectoderm, as in Acilius. Thus almost the whole of the cephalic lobes are invaginated, or, to speak more accurately, they are enclosed by amnion-like folds to form a primitive cerebral vesicle. The floor of the vesicle is formed by the fore-brain or the whole of the cephalic lobes, except that part containing the lateral eyes. The vesicle has a thin roof or pallium, from which arises a median tubular outgrowth, with its terminal pineal eye. There is nothing approaching this in any other animals, except in the Vertebrates, where this condition has long been regarded as typical of the group. I am convinced that this fact alone, when looked at without prejudice, is sufficient proof of the genetic relation between the Arachnids and Vertebrates. It certainly is not inferior to evidence based on the presence of a notochord or gill-slits. However, this fact only furnishes the key-note to the argument that is to follow.

In adult Scorpions and Spiders the brain shows more clearly its origin from three segments than is the case in insects. In fig. 61 I give a diagrammatic view of the brain of Arachnids based on my observations on Scorpions and Spiders. None of the tegumentary nerves are represented, and, excepting the position of the lateral ocelli, the diagram would do as well for one as for the other. I was the first one to point out the homology of the eyes of Spiders with those of Scorpions and Insects in my paper on the ‘‘Segmental Sense Organs of Arthropods.” I stated, p. 601, “In Spiders the structure of the cephalic lobes is the same as that of Scorpions. The two anterior median eyes belong to the second segment, and are homologous with the median eyes of Scorpions, the development being the same in both cases. The three remaining pairs belong to the third segment, and are homologous with the lateral eyes of Scorpions. They are invaginated to form optic cups in the same way as those of Acilius.” Korschelt and Heider have overlooked this statement, for they credit Kishinue and Purcell with having shown the similarity in the development of the median eyes of Scorpions and the anterior median eyes of Spiders, although their papers did not appear until two or three years after mine.

If further confirmation of this interesting fact is necessary, it is found in the structure of the adult brain. In Scorpions the optic ganglia of the second and third segments remain distinct through life. They are carried by the movements of the eyes on to the anterior face of the fore-brain, where they remain as two pairs of conical projections, the anterior pair united with the nerves to the median eyes (fig. 61). In the brain of some Spiders, notably Epeira, according to St. Remy, exactly the same condition prevails.1 But the anterior ganglia supply the median eyes, or “Hauptaugen;” while the others, irrespective of their position on the head of the adult, are supplied by the posterior ganglion, or that on the third segment of the cephalic lobes (fig. 61).

The brain-lobes of the second and third segments gradually become indistinguishable. In their place, in the adult, one sees a pair of deeply stained lobes (c. h., fig. 61), which are probably homologous with the cerebral hemispheres of Limulus.

B. The Mid-brain.—

There can be no doubt whatever that the whole of the cephalic lobes of the Scorpion are homologous with the whole of the cephalic lobes of Acilius. The next neuromere, that of the cheliceræ, must be homologous with the antennal neuromere of insects, because both bear identical relations to the stomodæal nerves.

In Scorpions and Limulus there are four nerves connected with the oesophagus. The most important are the lateral stomodæal nerves, extending from about the middle of the median margin of the cheliceral neuromere inwards along the sides of the oesophagus (figs. 59—61, l. st. n.). What I formerly regarded as the pre-oral cross-commissures of the cheliceral neuromere (a. p. st.) I am now convinced is merely the much shortened anterior pons stomodæi of insects. It is composed at first of numerous ganglion-cells surrounding a cortical “punct” substance, and in some Spiders, according to St. Remy, contains in the centre a ganglionic enlargement that I regard as the remnant of a frontal ganglion. The same commissure, called by St. Remy “pons stomatogastique,” occurs in Myriapods (fig. 62, a. p. st.). It there strongly resembles the anterior pons stomodæi of Arachnids, and yet shows, by the presence of a distinct median ganglion, its undoubted homology with the cross-arms of the frontal ganglion of Insects (compare figs. 58—62).

An unpaired rostral nerve extends outward and backward along the anterior ventral wall of the oesophagus into the rostrum (figs. 59—61, r. n.). In Limulus, perhaps in Scorpions, there are also two lateral rostral nerves arising from the lateral stomodæal ganglion, and extending outwards to the rostrum (l. r. n.). They agree in origin and distribution with the lateral labral nerves of Insects, with which they are homologous.

In Limulus the first post-oral cross-commissure is much longer than the rest, and occupies a different position on the posterior under wall of the oesophagus (figs. 46, 47, c3.). It is difficult to account for this commissure if we do not regard it as homologous with the posterior pons stomodæi of the oesophageal ring of Insects. I cannot identify it in Scorpions on account of the excessive crowding of the neuromeres about the mouth, but there is no reason to doubt that it is present.

C. Development of Lateral Stomodæal Nerves.—

The lateral stomodæal nerves in Scorpions and Limulus arise in part from the walls of the oesophagus. Their development is best studied in scorpions. They first appear in surface views of Stage E (‘Origin of Vertebrates from Arachnids,’ fig. 2, st. n.) as a pair of invaginations on the median border of the cheliceral neuromere. The invagination gives rise to a string of cells which at its inner end is continuous with an evagination of the lateral wall of the oesophagus. By the inward growth of the oesophagus the nerves are gradually drawn out to their full extent; their inner ends are for a long time continuous with the proliferating thickening of the stomo-dæum, and their outer ends terminate in the lateral stomodæal ganglia derived from the thick-walled invaginations on the margin of the cheliceral neuromere.

Thus, if we combine our observations on scorpions and Limulus, we are able to identify in the Arachnids every characteristic nerve and ganglion of the stomodæal system of Insects except the unpaired stomodæal nerve. It is not impossible that further observation on other forms will show the existence of this nerve in Arachnids. It is not easy to overestimate the importance of these facts. They prove beyond doubt that there has been no addition or suppression of neuromeres about the mouth either of Insects or of Arachnids.

It is also obvious that in Arachnids the fore-brain, the cheli-ceral neuromere, and the principal stomodæal nerves are respectively homologous with the fore-brain, the antennal neuromere,and stomodæal nerves of Insects and Myriapods. The “pons stomatogastric” of Myriapods is homologous with the frontal ganglion of Insects, and the cross-arms uniting it with the ganglion of the oesophageal commissure, or what I have called in Insects and Arachnids the anterior pons stomo-dæi. In spiders the same organ is called by St. Remy the “rostral ganglion,” or “lobe rostral.” The lateral rostral nerves and the labral nerves throughout Myriapods, Insects, and Arachnids, including Limulus, are the same, for in all these cases they arise either very near to or directly from the lateral stomodæal ganglion. It is hard to understand how St. Remy, after making his careful study of the brain of Myriapods and Arachnids, could overlook, as he seems to have done, these obvious homologies. It was probably due to the false a priori assumption that the antennæ are absent in Arachnids, and that their cheliceræ correspond to the mandibles of insects.

We are thus able to reduce the fore- and mid-brain of Myriapods, Insects, and Arachnids to the same ground plan. Such a comparison has been heretofore impossible, owing to the absence of the necessary embryological data. This, however, has been no serious obstacle to those who find it so easy to account for all discrepancies by assuming that one or more neuromeres have been omitted as the necessities of the comparison demanded. But there is apparently no biological law more constant than that head and anterior trunk segments, once formed, are never entirely omitted, or new ones intercalated between them ; and a careful study shows that the head of Arthropods offers no exception to this law. On the other hand, to assume that there have been such changes raises an insurmountable barrier to any satisfactory comparison between the brain of Arachnids and other Arthropods. There is no reason to doubt that the brain of Crustacea will fall in line with the above comparisons.

D. Comparison with Annelids.—

Still another advantage to be derived from my interpretation is that it enables us to compare the cephalic lobes and stomodæal nerves of Arthropods with those of Annelids. According to Lang, the paired stomodæal nerves of Annelids arise from the oesophageal collar, that is near the point of union of the ventral cord with the brain, just as they do in Arthropods. According to Kleinenberg, they originate in Lopadorhynchus from the lateral margin of the first post-oral neuromere. The origin of this nerve, therefore, in Annelids and in Arthropods marks the point of union of the neuromeres of the head and trunk. Moreover, since the stomodæum nerves arise from the first post-oral neuromere of Annelids and from the first neuromere of the ventral cord of Arthropods, i. e. from the antennal neuromere of Insects and Myriapods, the cheliceral of Arachnids, and the first antennal of Crustacea, these neuromeres must be homologous.

E. Nature of Stomodæal Nerves.—

The remarkable mode of development of the stomodæal nerves from the walls of the stomodæum, their constant presence, and their voluminous size in the early embryonic stages emphasises their importance, and indicates that they belong to a system quite apart from that of the fore-brain and ventral cord. If so, what is their significance ? The stomodæum undoubtedly represents the invaginated ectoderm formerly surrounding a primitive mouth leading directly into the mesenteron. That primitive mouth now lies at the junction of the mesenteron with the stomodæum. We have only to turn the stomodæum back to its primitive condition as ectoderm surrounding the mouth, in order to obtain a clearer idea of the original position of the stomodæal nerves. I have made this change in the typical insect nervous system shown in figs. 54, 55, and 56, and the transformation is very suggestive. With only slight modifications of the nerve-rings back of the oesophagus, the pontes stomodæi, with the frontal and the lateral stomodæal ganglia, are seen to form a circumoral nerve-ring ; and by continuing the nerves, uniting the lateral sympathetics, p. p. n., till they meet on the dorsal side between the body and the head our ideal insect embryo appears like the larva of Lopadorhyn-chus, or of an advanced trochosphere ; the stomodæal nerves forming on the sub-umbrella a system of circumoral nerverings, such as we might expect to find in a Coelenterate. The labrum is carried forward by the evagination of the oesophagus to the umbrella, and its nerves now appear as umbrella nerves originating from a circumoral nerve-ring.

This transformation renders the homology of the labrum with the pre-oral antennæ of Annelids, as suggested by Korschelt and Heider, very plausible, and at the same time it explains their remarkable innervation from the stomodæal ganglia. The connection of the anterior ends of the sympathetic nerves of the trunk is not known. If they are connected with the system of stomodæal nerves, as indicated in the figures, they might be regarded as sub-umbrella nerves, drawn out to their present extent by the growth of the trunk. The median sympathetic nerve would then be antemeric with the unpaired stomodæal nerve, as in fig. 54. It is obvious, on inspection of the figures, that the invagination of the subumbrella must have been greater in front than elsewhere, for the lateral stomodæal ganglia are carried only to the edge of the permanent mouth, while the frontal ganglion is carried far into the oesophagus, bringing the labrum up to the anterior border of the mouth.

The advantages of this view are obvious. It affords a means of identifying, approximately, the” trochosphere” in the cephalic lobes of Arthropods; explains, among other things, the anomalous innervation of the labrum, and the limitation of the stomodæal nerves to the ectodermal portion of the alimentary canal. I maintained, in my paper on “Acilius,” that the segments of the cephalic lobes of Insects were originally postoral, and comparable with those in the trunk. While I still recognise the great similarity of trunk and cephalic segments, I do not now believe that they are morphologically identical. The constant presence of the larval ocelli, and the absence of typical appendages, besides other considerations, indicate a sharp demarcation between the segments of the cephalic lobes and those of the body.

A.The cephalic lobes of Limulus, before they divide into segments, resemble more closely in shape and general appearance those of Insects and Scorpions than those of Crustaceans. We find there the same three pairs of invaginations seen in Acilius and Scorpions, but so strangely modified as to be at first sight hardly recognisable.

The first change that takes place is the formation of a great furrow along the whole of their anterior margin. As the furrow deepens, the lobes become a little shorter, and assume the shape shown in fig. 61 ; at the same time they come to be differentiated into four distinct swellings. The lateral ones constitute the optic ganglia (op. g.), or, as I shall sometimes call them, the thalamencephalon, owing to their double relation to the nerves of the lateral eyes and olfactory organ, and to the way they finally become incorporated into the adult brain. In front of the optic ganglia the furrow becomes deeper and broader, and constitutes the invaginations of the optic ganglia. Just in front of the latter, and united with them by thick nerves, is a pair of oval thickenings, the primitive olfactory organs (p. ol. o.). The compound eyes, with which this optic ganglion is subsequently united, have not yet appeared. They are first seen near the outer end, and a little back of, the ganglion, as though they belonged to the cheliceral segment.

The optic ganglia of Limulus evidently correspond to the optic ganglia of the third segment of Scorpions and Insects, but the opening to the invagination is at right angles to the longitudinal axis of the body instead of parallel with it, as in the early stages of the Scorpion (fig. 59). But even in Scorpions it is at right angles with the long axis of the body at a later period. The sense organ on the anterior margin of this invagination in Limulus, the primitive olfactory organ, must be homologous with the lateral eyes of Scorpions. In the anterior median part of the cephalic lobes another enlargement of the furrow appears (s. l.) ; its walls are thicker and stain deeper than elsewhere, forming two oblong, slightly thickened lobes, very conspicuous in surface views, and having the same appearance as the semicircular lobes of Scorpions and Spiders, with which they are unquestionably homologous. This is shown not only by their position and appearance in the early stages, but by the fact that they develop into the same kind of organs in the adult. Back of the semicircular lobes are two poorly defined swellings (br.2 and br.3), that I regard as the second and third segments of the fore-brain. From the anterior lobes arise the cerebral hemispheres; the posterior ones do not undergo any marked specialisation, they are concealed by the subsequent backward growth of the cerebral hemispheres, and seem to bear the same relation to the rest of the fore-brain that the “tween-brain” does to the cerebral hemisphere in Vertebrates.

About midway between the invagination of the optic ganglia and that of the semicircular lobes appears a small pore, which rapidly moves forward and inward till it lies in front of the invagination of the cephalic lobes. The position of this pore at first made me regard it as belonging to a segment in front of the semicircular lobes, as I stated in my paper on the “Origin of Vertebrates from Arachnids.” Further study, however, convinced me that it originates between the invagination of the semicircular lobes and that of the optic ganglion, and consequently it belongs to the second segment of the cephalic lobes. The pore leads into a short tube formed by an invagination of the ectoderm. The tube, although easily seen in sections, is not very clear-cut, and the lumen extends a short distance only into its interior. The distal end of the tube is not specialised, and shows no trace of the eye that is subsequently developed from it. These two tubes soon unite to form a common tube opening by a large pore, situated some distance in front of the brain (Pl. 3, fig. 24). Two bands of ectoderm, in which there is a shallow groove, lead on either side to the point where the original invaginations of the eye-tubes were situated (fig. 24, c.m. e. t.). The distal end of the tube is now slightly swollen, and constitutes the “anlage” of the median eye. The paired invaginations of the median eyetubes in Limulus, therefore, are homologous with the invaginations on the second segment of Scorpions, with which they agree in their relation to the other invaginations. They are, however, much smaller than those of Scorpions, and instead of advancing backward and inward over the cephalic lobes, they advance forward and inward to a point in front of the brain, where they unite to form a single invagination extending right into the yolk, and apparently having no connection with the cephalic lobes. A comparison of figs. 24 and 25 and the sections shown in figs. 41 and 42 will show that the opening into the median eye-tube really has much the same position in Limulus as in the Scorpion, so that there can be no doubt that the median eyes in these two forms are homologous. The strange position of the median eye-tube in Limulus is caused partly by the forward migration of the invaginations, and partly by the shortening of the cephalic lobes, which brings all the invaginations into a nearly straight transverse line, instead of being distributed along a broad semicircular curve, as in Scorpions. Moreover the rapid invagination of the semicircular lobes and their backward growth under the rest of the brain have something to do with the apparent separation of the median eye-tube from the brain (figs. 41 and 42).

A narrow commissure, similar to that seen in Myriapods and in the early stages of Acilius, unites the right and left halves of the cephalic lobes (compare figs. 57, 59, 61, 62, c′.).

B. The Mid-brain.—

The first post-oral neuromere, that of the cheliceræ, becomes intimately united with the cephalic lobes, as in Insects and Scorpions. Its stomodæal nerves, with one exception, are identical with those of Scorpions. There is a large “lateral stomodæal ganglion” on the median margin of the cheliceral neuromere, from which two large lateral stomodæal nerves extend inward the whole length of the stomodæum (figs. 43, 46, 49,61, st. n.). There are also a small median and two lateral nerves extending outward and backward along the oesophagus to the rostrum, comparable with the paired and unpaired labral nerves of Insects and Myriapods (figs. 47 and 48). The first post-oral commissure differs from the others in that it is isolated from the rest, and forms a long loop around the posterior under side of the oesophagus. The other commissures extend straight across, and are bound together by firm connective tissue. I have not traced the ends of this commissure up to the lateral stomodæal ganglion, but nevertheless it seems to me very probable that it represents the” posterior pons stomodæi” of insects and Myriapods. The commissure just in front of the mouth unquestionably corresponds to the anterior pons stomodæi of insects and Myriapods.

I can find nothing in Limulus corresponding to the optic ganglion of the second segment in Scorpions. This seems to be due to the fact that the median eye-nerves in Limulus have shifted their points of attachment from the neural surface of the brain to the semicircular lobes on the opposite side.

C. Later Modifications of the Brain.—

After the stage shown in fig. 59 the cephalic lobes rapidly change in aspect, and one finds less and less resemblance between them and the brain of other Arthropods; while, on the other hand, their resemblance to the fore-brain of Vertebrates becomes more apparent.

Among the important changes that take place in the disposition of the parts is the shortening of the optic ganglia, and the forward movement of their distal ends ; at the same time they are drawn inward and backward till they lie in about the middle of the hæmal surface of the brain, enclosed by a common envelope for the brain and optic ganglia (fig. 49). I know of no other Arthropod in which the optic ganglia have this position. They usually project from the sides, or, as in most Arachnids, from the neural surface of the brain. In their position on the hæmal surface the optic ganglia of Limulus resemble the thalamencephalon of Vertebrates, with which I regard them homologous. The other important changes that take place are in the cerebral hemispheres. They arise from disc-like thickenings of the cephalic lobes, and correspond in position with the brain-lobes of the second segment of Scorpions (compare figs. 24, 59, and 61, c. A.). They grow upward at first, and then their summits spread out mushroom-like, till they completely conceal the remainder of the cephalic lobes, including even the greater part of the segment of the midbrain (compare figs. 24, 25, 43, 47—52).

By this method of growth a series of chambers are formed, some disappearing early, others persisting in the adult, that correspond to certain cavities of the Vertebrate brain, such as the fifth ventricle, the lateral ventricle, the primary cerebral vesicle, and the cavities leading from the third ventricle into the lateral eye-tubes.

We shall now describe these changes in more detail. I have carefully studied these stages, both in surface views and in sections, and have constructed a partial wax-plate model of an embryo in about this stage. In fig. 24 I have represented the cephalic lobes as they would appear in surface views. Some details have been omitted, and the clearness of some parts has been exaggerated, and in order to save repetition of figures strict attention was not paid to synchronism. It is therefore not so much an accurate picture of one stage, as a diagram of two or three closely joined stages. A series of sections of this period, drawn as accurately as possible, will serve to make the meaning of the drawing clearer, and will also show to what extent it is diagrammatic. Such a series of longitudinal sections is shown in figs. 35—40. They are taken from a little earlier stage than that in fig. 24, and illustrate the continuity of the invagination cavities. There were fourteen sections in the series, of which the fourteenth is shown in fig. 35 ; the eleventh, ninth, fifth, and second, in the four succeeding figures. The other sections of the series showed nothing of interest, and so were not represented. The position of the sections is shown by the dotted lines in fig. 24. The invaginations do not persist very long, and the method of disappearing varies somewhat in the different parts. In fig. 39 the ectoderm has already broken away from the anterior wall of the invagination, and is now pushing its way backward over the optic ganglion, in order to unite with the thin ectoderm behind it. The anterior wall (a. w.), which assumes a little darker colour, finally fuses with and forms a part of the definitive optic ganglion. A section through this region in a little later stage (fig. 33) still shows traces of the anterior wall of the fold (a. w.). The figure also shows the characteristic way in which the thick layer of ectoderm, constituting the ganglion, becomes tilted over, so that its inner surface, where the medullary substance is just appearing, is turned outward. Another section of the same stage, across the distal end of the optic ganglion, and showing the first traces of the convex eye, is shown in fig. 44.

On the median side of the optic ganglion, to go back to the stage we started with, the invagination is already quite faint (figs. 37 and 38, i. v.), and in the next stage disappears, both walls forming brain tissue. The same is true of the semicircular lobes (fig. 36, i. v. sl.) although the invagination cavity and its walls can be distinguished as such for a considerably longer period than in the optic ganglion. In the median section (fig. 35) one sees the median eye-tube cut lengthwise; the relatively small commissure uniting the halves of the brain (m. c.), and, at the tip of the rostrum, a great mass of transitory tissue (r. m. s.), the nature of which could not be determined. In fig. 26 a cross-section of the anterior margin of the cephalic lobes of the same stage shows the extent of the two invaginations for the semicircular lobes.

Shortly after this stage the cerebral hemispheres become more conspicuous. An outline of a section showing their appearance at this time is seen in fig. 27. The whole brain here appears to be a very thick, proliferating layer of ectoderm, with no differentiation into an overlying hypodermis. The underlying semicircular lobes are seen with their cavities nearly obliterated (c. sl.).

Soon after this the ectoderm begins to advance in an obscure fold over the cerebral hemispheres, as in fig. 28, which represents a section just back of the posterior margin of the semicircular lobes. In some cases one can see a doublewalled fold like that on the lower left side of the figure, but usually there is only a single layer, the edge of which creeps over the hemispheres, hugging closely to their outer surfaces. Where this layer has passed, one sees a layer of thin cells (b. s.), that probably develop into a part at least of the brain envelopes. Whether this layer comes from the brain itself, the mesoderm, or the middle wall of the ectodermic fold, could not be determined.

Similar but more distinct medullary folds are seen advancing over the margin of the mid-brain and ventral cords. Figs. 29—32 are selected from a series of cross-sections to illustrate these folds. In fig. 29 the section passes just back of the cerebral hemispheres in fig. 30, where the folds show best, through the mid-brain, just in front of the cheliceræ ; in fig. 31 back of the second post-oral appendage ; and in fig. 32 through about the middle of the third post-oral neuromere. These sections show that the nervous system is not separated from the surface by a process of delamination like that which occurs in nearly all other Arthropods, but by an infolding from the lateral margins, very similar to that which takes place in Vertebrates, especially of such forms as the sturgeon, as described by Salensky. The folds in the post-oral region never advance far enough to meet over the median line. What becomes of the middle layer of the fold when it does occur could not be determined. Over the middle of the ventral cords, mainly over the “Mittelstrang,” there is formed by delamination a thin layer of superficial ectoderm, which probably unites with the medullary folds to form the continuous layer that finally covers the ventral cords.

The margin of the ectodermic fold advances forward and medianly over the cerebral hemispheres, as shown by the line m. c. f., fig. 24. At the same time the narrow bridge of ectoderm leading to the pineal eye-tube is drawn backward, diminishing the clear triangular area behind it. Its anterior lip then unites with the advancing fold on the cerebral hemispheres, so that the surface of the hemisphere, not yet covered by the ectoderm, has a contour something like that shown in fig. 25, c. h. The cross-bar of the ectoderm is finally converted, by the union of its anterior and posterior lips, into a tube opening by a small round pore immediately over the proximal end of the median eye-tube. This pore, which I shall call the anterior neuropore (n. p.), leads directly into the median eye-tube and through the cross-tube into all the cavities of the fore-brain. The lips of this pore also represent the last point of attachment of the fore-brain with the surface ectoderm.

Meantime the cerebral hemispheres have increased rapidly in size. From the very start each hemisphere shows a distinct separation into two lobes—a posterior one, drawn out into long sharp points, and an anterior lateral lobe. Both lobes are separated from each other by a deep fissure, in the angle of which lies the slender fibrous peduncle on which the hemisphere is supported (fig. 25, p. ch.). The shape of the hemispheres in the second larval stage is accurately shown in fig. 47. The fore-brain region is drawn from a wax-plate model, the rest from dissected specimens. The posterior lobes are not so slender and pointed as at an earlier stage, and both lobes show the first traces of the convolutions so characteristic of the brain at a later period. The hemispheres have united with each other along the median line, and the anterior lateral lobe is beginning to grow around or. to the under side of the brain. Two cross-sections of the brain in this stage are shown, one through about the middle of the hemi spheres (fig. 51), another, farther back (fig. 50), showing the growth of the lobes over the rest of the brain. A third cerebral lobe, the median internal lobe, or “corpus striatum,” is now visible on the anterior inner face of each hemisphere (fig. 51, c. s.). It is an oblong lobe, extending about half the length of the median face of the hemispheres. Even at a much later period its thick cortical layer of cells is never convoluted like the rest of the hemispheres, and it contains a great medullary core, terminating blindly in front, and behind in a great bundle of medullary substance that forms a part of the cerebral peduncles (fig. 43, p.c.s.). comparing figs. 47—49 a fair idea of the enormous increase in size of the cerebral hemispheres may be obtained. These are all young specimens, however; the hemispheres are still larger in the adult, as may be seen in fig. 52, a camera drawing of a section through about the middle of the brain. The extraordinary convolution of the cortical substance, formed of very small, deeply stained nuclei, stands out in sharp contrast with the unstained medullary portions. One should observe the slender peduncles of the hemisphere, the large space or vesicle (v.5), sometimes filled with a mass of loose connective tissue, and the now much-reduced semicircular lobes (s. l.).

The semicircular lobes undergo strange modifications. The two curved lobes seen in fig. 59 grow inward and backward beneath the braiu till they cover its whole median haemal surface. They are seen in cross-section in fig. 27, and their extent is dimly shown through the rest of the brain in figs. 24 and 25. They reach their greatest relative extent in the second larval stage, as shown in fig. 46 s. I. Some time before this there are split off from the median eye-tube two clear nerve-strands, which now terminate in conical swellings on the anterior ends of the semicircular lobes (fig. 46). The latter are now joined with each other at their posterior ends, and no longer show any trace of their dual origin. The lateral and posterior margins of the semicircular lobes contain small, densely crowded, and deeply stained nuclei, which are apparently multiplying rapidly, and when the brain is stained and seen whole as a transparent object these cells appear as a very conspicuous narrow band, showing clearly the peripheral boundaries of the semicircular lobes. This narrow dark band is best seen in larvæ about four or five inches long, but may be distinguished even in the adults. After the stage shown in fig. 46 the semicircular lobes become narrower and relatively m aller (fig. 49). Its cortical layer is always smooth, and contains many large ganglion-cells. Within the lobes is a narrow stratified band of fine dense medullary substance, something like a horseshoe in shape. Its arms are directed forward and outward, and terminate blindly in the apex of the lobes : the curved posterior part constitutes the anterior commissure (fig. 43, a. com.).

The optic ganglia, soon after their first appearance, divide into four medullary cores or centres, each covered with a cortical layer of ganglion-cells. Three of these lobes are seen in fig. 49; the fourth, which differs in some respects from the rest, is situated at the base of the ganglion, concealed by the semicircular lobes.

D. Comparison with Vertebrata.—

It cannot be justly denied that the brain of Limulus has a strong superficial resemblance to a Vertebrate brain, and it is equally certain that there is nothing else like its adult condition among other Arthropods. Is this resemblance real—that is, is it profound and varied, indicating genetic relationship, or merely a superficial resemblance, which on closer inspection is seen to be due to some deceptive quirk, but being in other respects totally and irreconcilably different? It must be either one or the other. Either Limulus is closely related to the Vertebrates, and will show this by a fundamental similarity of structure, or else it is a world apart, and will show this also by an equally profound difference iu structure. The resemblances already pointed out can hardly be called superficial, involving as they do the similarity in the number of neuromeres, nerves, and sense organs. I shall not discuss these points now, but shall consider the various vesicles and invagination cavities, which are so strikingly like the cavities of a Vertebrate brain that the resemblance cannot be due to a mere coincidence, or to similarity of function. Before studying the development carefully, it was a great puzzle to explain how the adult brain of Limulus could resemble so closely the brain of Vertebrates, and yet one be solid and the other hollow. How would it be possible to convert the solid brain of Limulus into the hollow one of Vertebrates, without at the same time destroying its resemblance to the Vertebrate type? A careful consideration indicates that we shall not have to deal with this difficulty, for the brain of Limulus already contains, either potentially or actually, all the important cavities of the Vertebrate brain. In order to show this, let us go back and consider the manner in which the brain and nerve-cord are folded off from the surface. The imperfect folds of ectoderm on the lateral margins of the ventral cord, and the single layer of cells advancing over the cerebral hemispheres, are, I believe, morphologically the same as typical double-layered folds, such as the medullary folds of Vertebrates. No one doubts, for instance, that the formation of the brain and spinal cord of Teleosts by solid ingrowth is anything more than a modification of the same process which in other Vertebrates is expressed in continuous folds. Either may be derived from the other. In order to prove the identity of these folds in Limulus with those in Vertebrates, it is necessary to show that they advance over the brain, not as they do now in a typical Vertebrate embryo, but as they did in the ancestral Vertebrate ; or it will be sufficient if we can show that these imperfect folds in Limulus advance in such a manner that, if they were perfect duplicatures, the cavities they enclosed would have the same relation to one another and to the brain-lobes as those in Vertebrates. These requirements, that appear so formidable on any other theory, cau be fairly met in Limulus. It is necessary to remember, however, that in Limulus the advancing layers of cells, representing folds of ectoderm, do not enclose spacious cavities between themselves and the brain ; and that neither these cavities nor the invagination cavities persist till the adult stage is reached.

Lateral Ventricles.—

Referring now to figs. 24 and 25, it is evident the fold (m. c. f.), growing over the cerebral hemispheres, encloses a very broad but flat cavity, corresponding to the lateral ventricles of Vertebrates. The cavities are as extensive as the whole upper surface of the cerebral hemispheres, and they increase in extent as the latter increase in size. They communicate with each other in front (fig. 25), as in many fishes, and below with the cavity of the semicircular lobes or infundibulum. The roof consists of a thin, non-nervous membrane or pallium, and the floor of the thick posterior, anterior lateral, and the median internal, lobes of the cerebral hemispheres.

The Infundibulum.—

On comparing figs. 24, 25, 41, and 43, it is seen that the anterior wall of the cerebral hemispheres passes downward and backward into the cavities of the semicircular lobes (s. l.). These two cavities, which are at first separate (fig. 59, s. l.), disappear about the time the lobes in which they lie unite. Had they persisted a little longer they would communicate with each other (as they do in Scorpions), and this broad, backwardly directed cavity thus formed would be similar to that in the infundibulum of Vertebrates. In my first paper on the “Origin of Vertebrates” I advocated the old view that the infundibulum represented a primitive oesophagus. I have now abandoned that position for what I consider a much stronger one, and we are thus left to account for the ancestral oesophagus in some other way. It is possible that the primitive Arthropod oesophagus broke through the narrow band of nerve-tissue in front of it, and moving forward, was converted into the one we now see in Vertebrates. However that may be, there are excellent reasons for regarding the semicircular lobes of Arachnids as homologous with the infundibulum of Vertebrates : (1) they agree in a general way in shape, and in their position on the hæmal surface of the brain; (2) they develop from invaginations in such a way as to bend the anterior end of the brain-tube downward and back wards on to the hæmal surface, and form cavities communicating directly with the third and the lateral ventricles ; (3) their relations to the median or parietal eye are apparently the same in both Limulus and Vertebrates.

Concerning the last fact, it seemed at first impossible to satisfactorily account for what is apparently a great difference in the position and attachment to the brain of the parietal eye in Scorpions, Limulus, and Vertebrates. But these differences are very easily explained, and when thoroughly understood, help to strengthen the comparisons already made. For instance, in Vertebrates the parietal eye arises from the middle of the roof to the thalamencephalon,back of the cerebral hemispheres. In Limulus it seems to be about as far as possible from its apparent position in Vertebrates, for it lies in front of the cerebral hemispheres, and has its roots inserted into what, in Vertebrates, would be the floor of the infundibulum (figs. 43—49). But on looking at the spider’s brain (fig. 61), which for the point under consideration will serve as well as that of the scorpion, we see that the parietal eye lies back of what corresponds to the cerebral hemispheres in Limulus (c. k.); and this position, as shown by the whole course of development, seems to be the primitive one for Arachnids. But by elongating its divergent nerve-roots the median eye can be moved forward in front of the cerebral hemispheres, so that its roots, instead of describing a half-circle around the posterior neural surface of the fore-brain, lie like an inverted Y on its anterior hæmal surface; the points of attachment, however, will be the same in either case. In Limulus this very change has taken place, owing to the movement of the eye farther and farther forward on to the hæmal surface. It is a very significant fact that in mammals there are two bands, the peduncles of the parietal eye, that extend around the “tween brain” and terminate in the posterior wall of the infundibulum, in the “corpora albicans.” I have seen something similar to these bands in the brain of Petromyzon, but have not yet had time to work them out carefully. I am not aware that their existence has been described in fishes, but it is hoped they will soon receive the careful attention they deserve. However, if we can regard the course and termination of these peduncles in mammals as typical for the Vertebrates, it is obvious that they correspond to the divergent roots of the parietal eye nerve of Limulus. On turning to fig. 43, it is evident that the parietal eye could be moved back to the position it occupies in mammals under the backwardly projecting lobes of the hemispheres, for the diverging roots (I. n. m. e.) could be slipped over the hemispheres, and when shortened would form nerve-bands or peduncles encircling the “tween brain” and terminating in the infundibulum. By turning to figs. 24, 25, and 59, it is seen that the olfactory organ would not stand in the way of this change provided it were made early enough, since the olfactory organs do not unite in the median line, and the median olfactory nerve is not formed, till long after the parietal eye has assumed its final position. The difference, then, between the position of the parietal eye in Limulus and Vertebrates is more apparent than real, and is probably due to the fact that Limulus moves about on its neural surface, and the parietal eye, to be of any use, must be on the opposite side ; the extent to which it has wandered from its original position on the cephalic lobes is shown approximately by the great length of its nerve in the adult. In Vertebrates the neural surface is turned upward, hence the parietal eye can remain nearer its original position than in Limulus.

The, at first sight, apparently inexplicable position of the parietal eye in Limulus, and its connection with the infundibulum, are seen, therefore, to be in harmony with Vertebrate anatomy as soon as we recognise that the true roots of the nerve to the parietal eye in Vertebrates do not terminate in the roof of the “tween brain,” but in the infundibulum.

The Third Ventricle.—

It is evident that in Limulus the space between the lateral ventricles and the cavity of the infundibulum corresponds to the third ventricle of Vertebrates (compare figs. 25 and 43). The resemblance appears greater when we recollect that the invagination cavity of the optic ganglion leads from the sides into this space at a level midway between the infundibulum and the cerebral hemispheres, just as the tubular lateral eye nerve of Vertebrates leads into the sides of the third ventricle. The principal difference between the optic cavities in the two cases is that in Vertebrates the canal extends the whole length of the nerve to the lateral eye, while in Limulus it only reaches to the root of the nerve. It must have extended further along the lateral eye nerve in forms more closely related to Vertebrates, otherwise the lateral eye could not have been inverted, as it is now in Vertebrates.

If this view is correct, then extending the ganglionic invagination to the lateral eyes in Limulus ought to give rise to conditions similar to those in the lateral eyes of Vertebrates. This is indeed the case. It is a well-known fact that the lateral eyes of Limulus, Trilobites, and the Merostomata are kidney-shaped, a configuration they must necessarily have in order to distribute the ommatidia economically over the convex surface of the cephalo-thorax. The concave edge of the eye is always directed hæmally, as I have shown, in a purely diagrammatic way, in fig. 24. Now if the invagination of the optic ganglion had progressed a little farther along the nerve the whole eye would have been invaginated, and the ommatæum would then form at the end of a long tube a kidney-shaped retina, but with its concave edge turned in the opposite direction from before. As a kidney-shaped retina would be no longer of any advantage it would tend to assume a circular outline ; but owing to the peculiar distribution of the nerve-fibres and the predetermined method of growth, this could be most economically and advantageously accomplished by bringing the halves of the concave edge together, thus producing a choroid fissure, the position and direction of which would be like that in Vertebrates. In other words, the kidneyshaped retina of Vertebrates is due to the fact that the retinal cells multiply faster on the convex margin than elsewhere.

This method of growth forces the concave margins together and produces a choroid fissure. The shape and method of growth of the vertebrate retina are inexplicable, unless we assume it to be inherited from ancestral forms whose eyes must of necessity have had the shape and position that the eyes of Vertebrates would have if carried back to the exterior where they originally belonged. The only Invertebrates having such eyes are the marine Arachnids, Trilobites, Limulus, and Mirostomata, &c.

If we turn again to fig. 24 we see that the space over the mid-brain uncovered by the marginal folds (m. f. cr.) represents the imperfectly roofed-over cavity of the Sylvian aqueduct, and that it extends forward around the posterior median margins of the hemispheres into the uncovered lateral ventricles as a shallow groove (F. M.) which corresponds in position to the foramina of Monroe. Before a complete roof can be formed to the mid-brain (Ne.1), and to the third forebrain segment or the “tween brain” (Ne.3), by the union of their medullary folds, the cerebral hemispheres grow backward, as they do in Vertebrates, and, uniting in the median line, completely cover them (fig. 25). Therefore the cavity in Limulus corresponding to the “iter” must communicate freely above with the fifth ventricle, or the space between the inner median faces of the hemispheres (fig. 52, v.). But if the marginal folds of the embryo persisted they would occupy about the position indicated by the dotted line in fig. 52, and if they united completely two distinct chambers (v.5 and i.) would be found corresponding exactly to the “iter” and to the fifth ventricle of Vertebrates.

If the marginal folds of the ventral cord united (fig. 32) they would produce a medullary canal, bounded at the bottom by the Mittelstrang, on the side by the ventral cords, and on the roof by the united medullary folds.

Owing to the divergence of the cords in the hind-brain region (fig. 48) a great rhomboidal cavity with a thin roof would be formed that would correspond to the fourth ventricle.

To sum up what has preceded, we find that in the brain of Limulus there is an actual or potential agreement throughout with the Vertebrate brain. There is (fig. 43) the folding of the anterior end of the cerebral vesicle downwards and backwards to form the infundibulum ; the upward growth of the co-ordinating centres, or the cerebral hemispheres, and their expansion in all directions at the summit to cover the rest of the brain. There are the lateral ventricles communicating in front with each other, with the cavity in the epiphysis, or median eye tube, and with that of the olfactory lobes, below with the infundibulum, and backwards and downwards with the “iter” and the third ventricle. Its roof is a thin membrane or pallium ; its floor the three great lobes of the cerebral hemisphere—the posterior, the anterior lateral, and the median internal or corpus striatum.

There is the great cavity in the centre of the brain corresponding to the imperfectly separated third and fifth ventricles and the “iter.” It is bounded in front by a thin membrane, “lamina terminalis” (fig. 43, l. t.), above by the corpora striata (c. s.), and on the sides by the cerebral peduncles and optic thalami, and on the floor by the middle and inferior commissures. It communicates anteriorly and above with the lateral ventricles, behind with the unroofed “iter,” and on the sides with a cavity leading to the roots of the lateral eye nerves.

There are three commissures to the brain—

one the inferior commissure in the roof of the infundibulum (a. com.), the very large middle commissure in the floor of the third ventricle (m. com.), and the posterior commissure in what would be the roof of the mid-brain, just behind the posterior end of the cerebral hemispheres (p. com.).

There is the fourth ventricle, a large rhomboidal space imperfectly covered by the medullary folds (fig. 48). Its floor is composed of the united cross-commissures of the thoracic neuromeres in the position of the future “pons,” and its sides are formed by the great diverging masses of nerve-substance, or crura, leading up to the fore-brain.

It is evident that the principal differences between the forebrain of Limulus and that of Vertebrates is a difference in degree and not in kind. In Limulus the infoldings are obscure, in not always showing the duplication of layers ; individualized, in that in each important brain-lobe they progress independently of the others ; and they are incomplete, in that they do not always unite so as to entirely enclose the infolded parts. Moreover some important cavities, such as the lateral ventricles and the cavities in the optic ganglia and infundibulum, disappear at an early stage, so that their relations to the permanent cavities are not very obvious. In Vertebrates, on the contrary, the infoldings are very simple because they have lost their individuality by fusing into one continuous fold, which appears very early, and is usually completed before the brain-lobes are specialized. In Limulus the phylogenetic processes from the very first stages dominate over the pure mechanics of ontogeny. In Vertebrates it is almost the reverse; purely embryological processes prevail at first, afterward phylogenetic ones are manifest.

There is no greater difficulty in identifying all the characteristic features of the Vertebrate brain in Limulus than there is in comparing a fish brain with that of mammals. No other Invertebrate will permit any approach to such comparison. On the other hand, it is impossible that the comparisons, such as I have instituted, could be carried so far without breaking down, unless they rested on a sure foundation of actual facts and correct premises.

It remains now to say a few words concerning the parietal eye and the olfactory organ. If the relation of the parietal eye to the primary cerebral vesicle and to the infundibulum are mere coincidences, having nothing to do with the genetic relationship of Limulus with Vertebrates, then in all probability the resemblance will cease there ; it certainly cannot extend any farther, say to the structural details of its nerves and its terminal portions. But the resemblance does go a great deal farther, as we shall now see.

After the median eye-tubes in fig. 59, p. e., unite, a single tube is formed like that in figs. 24 and 25. The solid distal end (fig. 35, m. e.) soon divides into three lobes; the two outer ones just before hatching become filled with black pigment, and form the retinas to the ectoparietal eyes (m. e.); the inner, unpaired, one forms the endoparietal eye (m. e′.). There is no distinct cavity in these lobes, but as they are formed from the walls of an invaginated tube, and as the two outer ones are homologous with the median eyes of scorpions, where such cavities are present, we must regard them morphologically as sac-like diverticula from the end of a tube. Their similarity to the common vesicle of the median eyes of scorpions and their relation to the cerebral vesicle is easily seen in figs. 41 and 42. During the first larval stage the endoparietal eye is a solid cylindrical mass of cells with rod-like thickenings in their walls, like those in true retinal cells, and filled with dense white pigment (fig. 63). In sections the crumpled refractive plates on these degenerate retinal cells look like coiled or zigzag fibres something like those in fig. 20. As the animal grows older this mass of aborted retinal cells buries itself deeply in the underlying tissue, away from all connection with the exterior (fig. 74). In the adult it usually lies below a conical tubercle, situated a little behind the two lenses to the median eyes. In many old specimens this tubercle is replaced by a clear, transparent spot, which is no doubt the remnant of a lens. As the endoparietal eye must itself have been formed by the fusion of paired retinas, it is evident that the distal end of the parietal eyetube contains the retinas of four originally distinct eyes. These facts are all the more interesting since Claus has shown that the median eye of Crustacea, which in some cases is composed of three distinct eyes, is formed by invagination just as in scorpions and Limulus.

The distal portion of the primitive eye-tube is converted bodily into the median eye-nerve. Just before hatching its distal end splits up into four branches, two of which plunge directly into the median diverticulum or endoparietal eye (fig. 63, n. en. p. e.), and the other two pass to the paired retinas of the ectoparietal eyes (figs. 63—65, ec. p. e.). Two delicate nerve-strands split off at a very early period from the proximal end of the primitive eye-tube and its diverging arms (figs. 35, l. n. m. e. and fig. 36, r. m. e. n.). Compare also surface view (figs. 46 and 47, r. m. e. n.). These two nerve-roots subsequently divide (fig. 49), so that there are at least four main roots to the nerve, not including the epiphysis. Two of these roots terminate in conspicuous medullary nuclei attached to the horseshoe-shaped medullary core in the interior of the semicircular lobes (p. e. c.). From each nucleus a delicate strand passes laterally to the peduncle of the lateral optic ganglia (s. t.p.).

The epiphysis, or what is left of the median eye-tube after splitting off the roots to the parietal eye-nerve, remains for some time unchanged, as in fig. 43, m. e. t., also figs. 46—49 and 68—70. Finally its lumen disappears, and several swellings appear in it, composed of small nuclei like those in the cerebral hemispheres. In specimens four or five inches long these swellings may be seen em-beddedinthe brain evelope, and extending downward from the root of the median olfactory nerve toward the parietal eye-nerve (fig. 49, m. e. t.). After this stage the epiphysis loses altogether its connection with the root of the parietal eye-nerve, and then disappears completely.

We have in the parietal eye seven distinct structures, which should be constantly borne in mind in making any comparison with the parietal eye of Vertebrates. (1) The primary brain diverticulum or primitive parietal eye-tube; (2) the paired, and (3) the unpaired diverticula at its distal end; (4) the solid nerve-stalk to these diverticula; (5) the epiphysis, or remnant of the primitive parietal eye-tube after splitting off the (6) roots of the median eye-nerve, and finally (7) the large blood-vessel accompanying the nerve. The primary parietal eye-tube of Limulus corresponds to the epiphysial outgrowth from the brain-roof in Vertebrates. The ectoparietal eyes containing the two separate retinas, and the ventral diverticulum or endoparietal eye filled with white pigment, correspond to the two distinct terminal organs found in some Vertebrates. I venture to suggest that in Hatteria the “capsular-like structure,” described by Spencer, at the base of the eye where the nerve enters, corresponds to the endoparietal eye of Limulus. Again, the extraordinary presence of dense pigment in the middle of the outer wall of the eye in Varanus giganteus, as described by the same author, may be regarded as the remnant of the pigmented epithelium originally separating the paired retinas of the ectoparietal eyes of Limulus and scorpions ; and finally it is possible that the two separate pineal eyes that occur in certain reptiles, such as Anguis and Lacerta, may be due to shortening of the primary évagination, so that the ecto- and endo-parietal eyes arise as two independent outgrowths from the roof of the thalamen-cephalon.

The distal end of the primitive eye-tube in both Limulus and Vertebrates is converted bodily into the pineal eye-stalk. The proximal end in Limulus, after giving rise to the nerveroots, persists as an inverted T-tube ; the upright arm unquestionably corresponds to the epiphysis in Vertebrates, and the cross-bar, perhaps, to the ganglion habenulæ. The nerveroots correspond to the “peduncles” of the pineal eye as seen in mammals. In both Limulus and Vertebrates a large bloodvessel accompanies the nerve to the pineal eye. I formerly regarded the epiphysis as one of the nerves to the parietal eyes, but it now seems to me to be nothing but the ectoderm along which the parietal nerves formerly extended. The nerves separate from it just as the peripheral nerves separate from the overlying surface ectoderm. It may be compared to the epithelium lining the interior of the hollow lateral eyenerves.

A remarkable feature of the parietal eyes of Limulus and of Vertebrates is the presence in them of great quantities of dense white pigment. In Petromyzon the solubility of this pigment in weak acids and its intensely white glistening appearance in reflected, and deep black appearance in transmitted light has been the cause of contradictory statements concerning it, some claiming it is black, others white, and others that it is absent, according to the method of preparation or the light (transmitted or reflected) by which it has been examined. I have examined the white pigment in the parietal eye of Limulus side by side with that in the parietal eye of Petromyzon, and have found them to be identical in appearance. In Limulus, as in Petromyzon, the white pigment is much more soluble in dilute nitric acid than the black. White pigment is comparatively rare in Invertebrates, and, so far as I know, is confined to the Arthropods. It is present about the base of the ommatidia in the compound eyes of the crustaceans Pinæus and Galatea (Patten), and in Limulus is found in the infolded margin of the young lateral eyes, and in the choroid plexus lying in front of the brain. But nowhere in Arthropods is it more abundant than in the endoparietal eye of Limulus. This eye is apparently nothing but a solid mass of the pigment, and when ruptured with needles it flows out in a dense white chalky stream. Ahlborn claims that the white granules in Petromyzon are composed of calcium phosphate, like the “brain-sand” found in the parietal eye of the higher Vertebrates. It would be very interesting to learn its composition in Limulus.

Gaskell has compared the parietal eye of Vertebrates and Crustaceans, but he failed to see the real points of resemblance between them, for he knew nothing about their development in Arthropods. There is no more resemblance between the parietal eye of Vertebrates and the ocellus of Acilius, as he puts it, than there is between the lateral eyes of Vertebrates and those of a Cephalopod. If we fail to take into account the development of the parietal eye in Arthropods there can be no trustworthy grounds of comparison between it and the parietal eye of Vertebrates.

While there remains a good deal of doubt concerning the significance of certain parts of the parietal eye in Limulus and Vertebrates, that they are homologous with each other as a whole is, I believe, beyond question.

We have in the olfactory organ of Limulus a structure presenting the most striking and unusual features. It is as different from the other cerebral sense organs as the olfactory organ of Vertebrates is from the pineal eye. The features that in Limulus distinguish it from all other sense organs are the very ones characteristic of the olfactory organ in Vertebrates. The organ itself consists of an upright epithelium containing a large number of bud-like sense organs comparable with the sense organs described by Blaue in the olfactory organ of fishes. The whole organ, as it exists in the adult Limulus, may be compared to the unpaired olfactory organ found in the Cyclostomata. It is composed of three distinct parts—a pair of primitive sense organs growing forward from the brain, and a subsequently formed part lying between them.

It is probable that the dormant individuality of these parts has reasserted itself in the higher Vertebrates, and caused the separation of the olfactory organ into its constituent parts, or the olfactory organ proper, and the organ of Jacobson. The distribution of the nerves supports this view. In Limulus there are three distinct nerves : the paired lateral and hæmal ones, having their roots in one of the centres of the optic ganglia, or in the optic thalami ; and an unpaired, median, neural nerve terminating in the cerebral hemispheres. The median nerve, which in its early stages shows evidence of its paired origin, arises at a late period as an outgrowth from the cerebral hemisphere. It is a new formation, in histological structure and origin utterly unlike any other Arthropod nerve. All four of these nerves are represented in Vertebrates, for it is now known that each olfactory nerve of the higher Vertebrates is represented in Amphibia by two distinct nerves, which have been likened to the dorsal and ventral roots of a spinal nerve.

But if this were so they would differ from all other spinal nerves in that both dorsal and ventral branches supply sense organs. Moreover, on any supposition they are entirely different from those belonging to the other sense organs of the fore-brain, such as the lateral and parietal eye, and the auditory organ. This condition is quite inexplicable on any theory founded On Vertebrate anatomy. But this very thing occurs in the olfactory organ of Limulus, although the meaning of it cannot be explained any more there than in Vertebrates.

Now in Pipa dorsigera and Epicrium glutinosum, according to Wiedersheim, there are four distinct olfactory nerves—a neural and hæmal pair. The Saracins in their Ceylon work, and more recently Burckhardt (‘Z. f. w. Zool.’ 1891), have further shown that in Ichthyophis and Triton the hæmal pair innervate the organ of Jacobson, and the neural pair the rest of the olfactory organ. Exactly the same relations prevail in Limulus ; the hæmal pair are also lateral, and they supply a distinct lateral depression in the olfactory organ. The neural pair in Limulus innervates the greater part, if not the whole, of the definitive olfactory organ ; they are formed by an outgrowth from the cerebral hemispheres ; they contain the olfactory bulbs or lobes, and they are composed of pale refractive nerve-fibres with a structureless nucleated sheath, differing strikingly from the thick-walled apparently empty nerve-tubes seen in the lateral olfactory and other nerves moreover, as they supply by far the greater part of the olfactory buds with nerves, they may be regarded as the olfactory nerves proper. They therefore agree in these important morphological and histological features with the dorsal pair in Amphibians. In the higher Vertebrates all four nerves seem to have united, and the features of the median pair impressed on both.

I am not aware that the course of the roots to the olfactory nerves is accurately known in the lower Vertebrates; but in the Mammalia there are four large medullary nuclei in the thala-mencephalon, in one of which terminate the lateral roots to the olfactory nerve. It certainly is not without significance that the optic ganglion of Limulus, which for other reasons may be regarded as homologous with the thalamencephalon, also contains four nuclei, and from one of them (fig. 49) arises the lateral olfactory nerve.

But we have not yet reached the end of the similarity between the olfactory organs of Limulus and Vertebrates. In Vertebrates the olfactory organs are supposed to belong to a system of supra-branchial sense organs, from which arise the scattered sense organs of the head and lateral line. This supposition is based on the similarity between the method of development of the organs in both cases, and by the fact that the olfactory organ in many fishes is at first composed of a collection of sense buds similar to those in, or originating from the supra-branchial sense organs. Now, although I do not attribute much weight to these arguments, it is important to observe that we have in Limulus the same relation between the olfactory and other sense organs that we have in Vertebrates. We have, for instance, in the thorax of Limulus a series of sense organs and ganglia supplied by a purely sensory nerve, which in position and connection is much like the supra-branchial nerve of Vertebrates. Before I had paid much attention to these organs in Limulus I was led, from a knowledge of these relations in scorpions, to regard the epidermic thickening that gave rise to the “coxal ganglion” of Scorpions as homologous with a supra-branchial sense organ of Vertebrates. My recent observations on Limulus confirm this comparison, and broaden its significance in a most unexpected manner. In scorpions the great ganglionic thickening on the median edge of the coxal joint is a transitory structure, and there is no permanent or definite sense organ found there.1 In Limulus, how-ever, we find exactly the same nerves and the same ganglia, but they are connected with permanent organs of taste. Moreover some of these sense organs are single cells, strangely modified, and recalling the isolated gustatory, olfactory, and so-called “tactile” cells so widely distributed in Vertebrates ; others are sensory buds, practically identical with those scattered so Uniformly over the body, as well as with those in the olfactory organ.

There certainly can be no doubt that the great masses of sense organs in the jaws of Limulus have disappeared in Scorpions, leaving nothing but an enormous ganglion and nerve to indicate their former existence. This fact is of special interest in this connection, for it is exactly what Beard supposed had taken place in the supra-branchial sense organs of Vertebrates.

In the free swimming merostomata-like ancestors of the Vertebrates, we may assume that a change like that in the Scorpion has taken place. With the transformation of the appendages into gill arches the endopodites or the gustatory spurs of the walking appendages were probably reduced to mere sensory patches, appearing in the ontogeny of Vertebrates as transitory gangliogenic thickenings of the ectoderm, or supra-branchial sense organs. I have already pointed out that in Scorpions (“Origin of Vertebrates from Arachnids”) the coxal sense organs, ganglia, and nerve, were like the supra-branchial sense organs of Vertebrates in their development and distribution and relations to the rest of the nervous system. Now we can show, in addition to this, that in Limulus the mandibles may be regarded as segmental gustatory organs. They are centres composed of aggregations of gustatory buds and cells, and from them, or near them, may arise diffusely distributed sense buds. The segmental aggregates and the diffuse organs in Limulus are largely gustatory, and the same is in all probability true of these organs in Vertebrates, various theories to the contrary notwithstanding.

Moreover the gustatory cells in Limulus, and the various sense buds arising from the supra-branchial sense organs in Vertebrates, are arranged in straight lines, a condition, so far as I know, found in Arachnids and Vertebrates only. Now whether this arrangement in lines is due to the method of growth or to a physiological necessity is doubtful. Nevertheless this condition may be taken as evidence of genetic relationship between the forms in which it occurs. And finally, this being what I regard as of most importance, we see that in Limulus the olfactory organ, besides its other resemblances to the Vertebrate organ, contains the same kind of buds as those in the segmental gustatary organs. As is well known, the same thing occurs in Vertebrates. Now we might suppose, as has been done in Vertebrates, that the olfactory organ is serially homologous with the mandibular gustatory organs. But the whole development of the organ and its nerves shows that in Limulus this view cannot be entertained for a moment. The olfactory organ in Limulus is obviously a new growth on an old foundation. The latter, it is true, is a segmental sense organ, but it belongs to the series including the ocelli and compound eyes, not to that of the thoracic appendages.

The same interpretation applies, I believe, to the olfactory organ of Vertebrates, as Limulus appears to be less intelligent than animals like Lobsters, Scorpions, and Spiders, in which there are no cerebral hemispheres or any organ that may be regarded as a psychic centre. It is very surprising that it should possess gigantic cerebral hemispheres, and in shape, structure, and development like those of Vertebrates. This may be a coincidence. But can it be a coincidence that in both Limulus and Vertebrates these cerebral hemispheres are connected with only one pair of nerves ? Is it a coincidence that these nerves arise from the same brain region, have the same lobes, and supply sense organs that show striking similarities in position, structure, development, and function ? It cannot be. Not one of these characters obtains elsewhere, and, in my judgment, it is impossible that all of them should occur in Limulus and Vertebrates unless they are genetically related.

In any near relative of Limulus there might be from five to ten pairs of supra-branchial nerves, according as the cheliceræ and the vagus segments retained their sense organs. In Limulus there are five pairs, those belonging to the post-oral thoracic appendages (figs. 47, 48, md. n. 2—6). This number corresponds approximately to the number of supra-branchial sense organs in Vertebrates, and affords us a satisfactory explanation of their relation to the gill-arches and their absence from the trunk in the early stages of development.

I have already pointed out in my paper on the “Origin of Vertebrates” how the cephalo-thoracic neuromeres of Scorpions and Limulus are comparable with the entire brain of Vertebrates. This is clearly shown by the similarity in the grouping of the neuromeres, and by the modifications these groups have undergone.

The modifications and homologies of the three segments constituting the fore-brain of Limulus and Vertebrates have already been considered. The mid-brain of Vertebrates is generally conceded to consist of a single neuromere, distinct from those in front and behind, and characterised by the fact that it is provided with the only pair of cranial nerves arising from the neural surface of the brain. We find the same isolated neuromere in Arthropods, namely, the one belonging to the antennal segment of Insects and Myriapods, and the cheliceral segment of Arachnids. In the Arachnids it is easily distinguished from the true fore-brain on the one side and from the post-oral neuromeres on the other. Owing to the position of the cheliceræ, close together in front of the mouth, their nerves invariably arise from the neural surface of the neuromere, while all the others, except, perhaps, in the vagus region, arise from the sides.

The hind brain of Limulus (fig. 48, H. B.) is composed of five typical thoracic neuromeres, each having the following four pairs of nerves :—(1) The mandibular nerves (md. n., 2—6) are purely sensory, and consist of two branches which terminate in the gustatory organs in the mandibles. They arise from the neural surface of the ganglion at the root of the pedal nerve, and usually possess well-defined swellings that contain a few small ganglion cells scattered through a mass of interwoven fibrous substance. Sections through the appendages of halfdeveloped embryos (fig. 72) show two sensory thickenings, one corresponding to the inner mandible (i. md.) and the other to the outer (o. md.). In figs. 71 and 73 are seen ganglion cells separating from the sense organs to form the ganglia of the mandibular nerves in the adult (fig. 3, md. n.g.). The mandibular nerves correspond to the supra-branchial nerves and ganglia of Vertebrates. (2) The pedal nerve is a mixed nerve arising from a large neural ganglion common to it and the coxal nerve ; it supplies the sense organs and muscles of the walking appendages. In Scorpions it is at an early stage probably connected with the lateral segmental sense organs, but the existence of such a connection could not be demonstrated in Limulus,. owing probably to the transitory nature and imperfection of the segmental sense organs. This nerve corresponds to the ventral or branchial nerve of Vertebrates, with which it agrees in innervating the only important muscles of the head, namely, those of the visceral arches. All the other head muscles of Arachnids and Vertebrates, owing to the complete anchylosis of segments, have disappeared.

The neural and lateral ganglia of Beard probably correspond respectively to the pedal ganglion and the ganglia to the segmental sense organs of Limulus and Scorpions. The anterior and posterior hæmal nerves correspond to the pre- and post-trematic of Vertebrates.

All the nerves of the hind brain of neuromeres of Limulus agree with those of Vertebrates in being separate, while the same nerves in the trunk of both Vertebrates and Arachnids are more or less united. This is especially true of the Scorpion, where I have shown that the abdominal nerves are built on the type of true spinal nerves, for their proximal ends remain separate to form two roots, the dorsal or neural one being ganglionated, the hæmal one being non-ganglionated.

The accessory brain forms the fourth and last brain region of Vertebrates and Arachnids. It is formed of from two to four neuromeres derived from the trunk and added at a comparatively late period to the head. In spite of their late origin, they form the most specialized and completely fused neuromeres of the head. In both cases almost every trace of the metameres to which they belong has disappeared, consequently their nerves have wandered backward to new territory. The nerves to these four neuromeres in Scorpions are very unlike all others in front or back of them, for the four ganglionated neural nerves are fused into one group, and the eight hæmal ones into another. They are, on the other hand, strikingly like the vagus nerves of Vertebrates in their direction, distribution, and general appearance (see “Origin of Vertebrates from Arachnids,” fig. 1). In Limulus there are only one or at most two neuromeres in this region, and therefore it has not such striking vagus characters as in Scorpions.

The presence of these vagus segments is not confined to Scorpions and Limulus, but is of very wide occurrence in Arthropods. The last thoracic and first two or three abdominal segments in Insects, and the four abdominal appendages bearing segments in Spiders, probably belong here. Their presence in trilobites is shown by a well-marked “cervical suture” like that which in the adult Limulus marks the presence of these segments.

After we have torn off the deceptive Arthropod mask that disguises Limulus we discover that the nervous system, with all its complex and intricate modifications, shows as a whole a profound structural similarity to that of Vertebrates. This similarity extends to important aggregations of parts, as well as to many minute details, all of which could not on any reasonable assumption have occurred accidentally or be due to similarity of function or condition. The mere enumeration of the resemblances which my as yet superficial study enables me to point out ought to convince the most sceptical that we are working in the right direction. Is there any group of Invertebrates besides the Arachnids which by any reasonable assumption can be made to resemble Vertebrates in the way that Limulus does ? How far do the comparisons of either Ascidians, Balanoglossus, Nemertians, and Annelids with Vertebrates carry us? They explain nothing either in Vertebrate anatomy or in phylogeny, and only serve to render the whole problem of the ancestry of the Vertebrates hopelessly perplexing and obscure. If we accept the Arachnid theory all this is changed, for we can see, dimly perhaps, some way out of the intricate problems bound up in the morphology of the Vertebrate head. It is true many great problems will be left unsolved, but their discussion will be shifted to the vast field of Arthropod morphology, where there is hope of their ultimate solution. And the problems involved are great ones, notwithstanding a dawning tendency to subordinate phylogenetic questions to profound studies on the mechanics of cell division and kindred topics. If the Arachnid theory is true, the Ascidians, Amphioxus, and perhaps Balanoglossus and the Echinoderms become degenerate phyla, bearing somewhat the same relation to the Vertebrates that the parasitic Copepods do to the Arthropods. It will furnish us with a new basis for embryological interpretation, especially regarding the problems connected with the formation of germ layers. If the Arachnid theory is true, many current views on phylogeny, ontogeny, and important problems in comparative anatomy are based on false conceptions, and must be revised. That this is no exaggeration is obvious from the objections most frequently urged against the Arachnid theory, ones I had not anticipated would have any weight. Instead of considering the question on its own merits, it has been objected that “it was contrary to all our preconceived ideas,” and “it is quite impossible to conceive highly specialized animals like Arachnids giving rise to such highly specialized animals as Vertebrates.” I do not repeat these objections to give them serious consideration, but only to emphasize the fact that the solution of the Vertebrate problem is not merely the filling in of a gap in our system of classification, but this solution will necessitate the reconstruction of a vast deal of that preconceived morphological philosophy which at present forms the most serious obstacle to the perception of the genetic relation between Vertebrates and Arachnids.

GRAND FORKS, N. D. ; Nov. 29th, 1892.

Illustrating Mr. William Patten’s paper “On the Morphology and Physiology of the Brain and Sense Organs of Limulus.”

ExplanatioN OF Plates 1 and 2.

Reference Letters to Plates.

ax. n. Axial nerve, bl. v. Blood-vessels, c. c. Cuticular canal, ch. t. Chitinous tubule, ch. t′. Proximal end of chitinous tubule, g. c. Ganglioncells. g. o. Gustatory organs, g. p. n. Ganglion to the pedal nerve, gs. c. Gustatory cells, g. and t. or. Gustatory and temperature organs. i. md. Inner mandible. l. ol. n. Lateral olfactory nerve, m. Muscle, md. n. Mandibular nerve, m. i. m. Flexor muscles to inner mandibles, m. ol. n. Median olfactory nerve, n. Nerve, n. c. Slender nerve-like cells surrounding the base of the chitinous tubule, n. co. Nerve collar. n. and n′ Primary and secondary nuclei of the gustatory cells, n.fbl. Cone of nerve-fibrillæ inside the spindle, o. md. Outer mandible, p. g. Yellowish-brown pigment granules, p. n. Pedal nerve, s. e. c. Swelling in cuticular canal, s. ch. t. Swelling on chitinous tubule, sp. Spindle on gustatory cells, sp.f Spiral fibre, s. sp. Canal containing pigment and fibrous cells leading to spines. t. o. Temperature organs, v. Deeply stained varicosities on the nerve-fibrillæ Within the spindle, w. p. White pigment-cells.

PLATE 1.

FIG. 1.—Longitudinal section through the anterior wall of one of the stout gustatory spines of the mandibles, showing the cuticular canals, each containing a chitinous tubule.

FIG. 2.—A gustatory spine from the mandibles, seen from its anterior surface, and showing the lines of pores into each of which runs a chitinous tubule, × 30.

FIG. 3.—Third appendage on right side of an adult female, seen from in front, with the anterior wall removed, and showing the nerve to the appendage with its mandibular branches. Natural size.

FIG. 4.—Nucleated end of one of the gustatory cells of the mandibular spines, showing the peculiar yellowish-brown granules that are sometimes found in these cells. Isolated by Bela Haller’s fluid.

FIG. 5.—Distal end of a gustatory cell from the mandibular spines, showing fibrillar structure of the cell body and the spindle with its internal cone of fibrillæ, each fibrilla having an enlargement at the base of the spindle, and all converging toward the apex, where they unite to form the axial nervebundle that runs outwards through the chitinous tubule. × about 2000. Macerated in Haller’s fluid and stained in acetic acid carmine.

FIGS. 6 and 7.—Spindles from gustatory cells of mandibular spine, showing the partly detached nerve-cells, and the projecting axial nerve-bundle, which in Fig. 7 is broken up into its constituent fibrillæ. Haller’s fluid and methyl green.

FIG. 8.—Very young gustatory bud from the inner mandible, showing the circle of refractive rod-like plates and the single ganglion-cell with its fibrous prolongation.

FIG. 9.—Two sense buds from the olfactory organ, showing the chitinous tubule, central ganglion-cell, and the nerve-plexus. Slightly diagrammatic.

FIG. 10.—Three cuticular canals from the base of a mandibular spine. A. Larger canals with short spine in a saucer-shaped depression at the summit. May be one of the temperature organs. B. Ordinary gustatory canal, c. Same, with a few bead-like globules in the chitinous tubule.

FIG. 11.—Surface view of the cuticle from the chelæ of the walking appendages of a young Limulus about 8 inches long, showing the two kinds of sensory pores, the gustatory pores at g. o., and the temperature organs at t. o.

FIG. 12.—The nucleated portions of two gustatory cells from the mandibular spines, showing that they are formed of two imperfectly fused cells similar to the double ones described by the author in ‘The Eyes of Molluscs and Arthropods.’

FIG. 12 A.—Outer end of a cuticular canal of the inner mandibular gustatory buds. a. In longitudinal section, b. Seen from above, c. Gustatory canal (outer end) from the chelæ.

FIG. 12 D.—Surface view of the outer end of a sensory bud of canal, from the thorax.

FIG. 12 E.—Same view of olfactory bud of canal from olfactory organ of female.

FIG. 12 r.—Same of male. All of Figs. 12 A—D drawn to same scale.

FIG. 13.—One of the node-like swellings in the cuticular canals seen in Fig. 1 much enlarged, and showing the chitinous tubule within and the spiral fibre.

PLATE 2.

FIG. 14.—Isolated chitinous tubule from the gustatory buds of the mandibles, showing the thin, granular, protoplasmic investment, with the spiral lines. Treated with caustic potash.

FIG. 15.—Isolated ganglion-cell from the centre of a gustatory bud. Hert-wig’s mac. fluid, 5 days.

FIG. 16.—Chitinous tubules from the mandibular gustatory buds. In A the tubule is stained dark by the acetic acid carmine, and is surrounded by two sinuous, refractive and colourless fibres, B shows the coiled tubules that are frequently found in the mandibles.

FIG. 17.—Longitudinal horizontal section of the median olfactory nerves and olfactory lobes of a young Limulus about 4 inches long. The lateral olfactory nerves, I. ol. n., which bend outwards beneath the olfactory organ, are cut transversely. × 90.

FIG. 18.—Outline of the fore-brain, olfactory organ, and nerves, of an adult female Limulus. x 4.

FIG. 19.—Surface view of the olfactory organ of an adult female, showing the distribution of the cuticular canals leading to the olfactory buds. × 15.

FIG. 20.—Longitudinal section through the base of the lateral olfactory nerve, showing the ommatidia-like clusters of cells with their refractive, rodlike thickenings. × 200.

FIG. 21.—Cross-section of the adult olfactory organ. The outer layers of the cuticula have been removed. × 46.

FIG. 22.—Cross-section of the olfactory organ in a young Limulus about 7 inches long. Flemming′s fluid (strong). × 90.

FIG. 23.—Portion of the preceding figure still further enlarged. The thickness of the cuticula is not increased in the same proportion, ×550.

EXPLANATION OF PLATES 3 and 4.

Reference Leiters to Plates 3 and 4.

a. 2—6. Anterior hæmal nerves of the second to the sixth neuromere. A. B. Accessory brain, a. e. Anterior edge of dactylopodite. a. w. Anterior wall of the optic ganglion invagination, a. w. c. Anterior wall of cerebral vesicle, b. e. Last point of union of cerebral hemispheres with surface ectoderm. c. Posterior fore-brain commissure, c2. First post-oral commissure. c. A. Cerebral hemispheres, c. A′. Uncovered part of cerebral hemispheres. c. m. Canal between the cerebral hemispheres and the thin median ectoderm. c. m. e. t. Canal leading to the median eye-tube. com3. Posterior cerebral commissure, c. r. crura cerebri, c. s. Internal cerebral lobe = corpus striatum, c. st. Invagination cavity of the semicircular lobes, cu. Cuticula. c. v. Cerebral vesicle. D. S. Dorsal surface. F. B. Fore-brain, g. l. ol. n. Ganglion to the lateral olfactory nerve, g. n. 1—5. Nerves to gills, g. p. n. Ganglion to pedal nerve = “neural ganglion” of a Vertebrate cranial nerve. g. st. n. Ganglion to stomodæal nerve. H. B. Hind brain. A. n. 2—9. Hæmal nerves or peripheral tegumentary nerves. i. Unroofed space corresponding to “iter.” i. v. Continuation of the invagination of the optic ganglion with that of semicircular lobes. i. v.op. g. Invagination cavity in optic ganglion. i. v. sl. Cavity of semicircular lobes. l. e. Lateral eye. l. e. n. Lateral eye-nerve. l. ol. n. Lateral olfactory nerve. l.t. Lamina terminalis. m. 2—6. Median hæmal nerve of the 2—6 neuromeres. M. B. Mid-brain, m. c. Middle commissure of the brain, m. c. 1—2. Two cortical masses of ganglion-cells in front of cheliceral nerves. m. cbr. I, Median cerebral lobe. m. c.f. Margin of fold growing over cerebral hemispheres, md. n. 1—5. Mandibular nerve, or nerve to the endopodite. m. e. Median eye. tn. d. Diverticulum from median eye. m. e. t. Median eye-tube. m.f. Furrow leading from uncovered portion of cerebral hemispheres to the uncovered portion of the crura, m.f. c. r. Margin of the ectodermic fold growing over the crura, m. ol. n. Median olfactory nerve-tn. p. com. Medulla of posterior commissure, m. si. Mittelstrang. mi. n. nerve to chelaria. ne. 1—4. Neuromeres. n. p. Anterior neuropore ; leads into brain cavity and into median eye-tube. ol.b. Olfactory buds. ol. l. Olfactory lobes, ol. o. Olfactory organ, op. g. Optic ganglion, o. g. 1—3. Lobes to optic ganglion, op. n′. Nerve to operculum, p. 2—6. Posterior hæmal nerve of 2—6 neuromeres. p. c. Posterior commissure, p. c. s. Peduncle to internal cerebral lobe (corpus striatum), p. e. Posterior edge. p. e. c. Medullary nucleus at root of parietal eye-nerve, p. g. Pigment granules, p. n. 1—6. Pedal nerves, p. op. g. Peduncle to optic ganglion. p. ol. o. Primary olfactory organ, p. st. Stalk to the pigmented plexus arising from the primary olfactory organ, r. Rostrum, r. m. e. n. Roots to median eye-nerve. r. m. e. t. Remnants to median eye-tube. r. ms. Rostral mesoderm. s.l. Semicircular lobes = infundibulum = Porgan stratifié. st. g. Stomodæal ganglion, st. n. Stomodæal nerves, s. t. p. Strand connecting the nucleus to parietal eye-roots with the peduncle of optic ganglion. t. e. e. Thin ectoderm between cerebral hemispheres, th. r. Thickened ring surrounding the cephalo-thorax. v. 5th ventricle, va. n. Vagus neuromeres and nerves, v. s. Ventral surface.

PLATE 3.

FIG. 24.—Slightly diagrammatic view of the brain of an embryo in a stage when the legs and from two to three pairs of gills are developed. The drawing is constructed from surface views and sections, and is intended to show the relations of the various invaginations and the lines along which the imperfect ectodermic folds (m. c.f, m.f. c. r.) advance to enclose the brain.

FIG. 25.—Similar view of an older embryo, showing the greatly increased cerebral hemispheres and the areas still uncovered by the advancing folds, m. c.f. and m.f. er.

FIG. 26.—Camera drawing of a cross-section through the brain in a stage like that shown in Fig-19. The section passes through the end of the line, p. inf.

FIG. 27.—Cross-section of a brain in a later stage than that shown in Fig. 24, and in a plane that would pass through about the end of the line c. m. of Fig. 24.

FIGS. 28—32.—Cross-section of the brain and nerve-cord in about the same stage as that shown in Fig 24.

In Fig. 28 the section plane passes through about the middle of the cerebral hemispheres (compare Fig. 24).

In Fig. 29, just back of the cerebral hemispheres.

In Fig. 30, just in front of the first pair of appendages.

In Fig. 31, through one of the lateral nerve-cords, just posterior to the second pair of appendages.

In Fig. 32, through the ventral cords, a little back of the middle of the third post-oral neuromere. × 200.

FIG. 33.—Longitudinal section of a later stage than that of the preceding series, passing through the base of the optic ganglion. It shows the anterior wail of the invagination, and first differentiation of the primary olfactory organ, ×200.

FIG. 34.—Part of a longitudinal section in a little older stage than that of the preceding series. The section shows the early stages of the lateral eye, while it is in its primitive position close to the optic ganglion.

FIGS. 35—40.—A series of longitudinal sections through the brain in about the stage shown in Fig. 24. There were 28 sections in the series, section 14, Fig. 25, passing through the sagittal plane. The position of the sections is shown on Fig. 24 by the numbers 35—40. Only those sections are represented that show some variation in the invaginations.

FIG. 41.—Diagrammatic longitudinal section through an early stage of the brain of Limulus to show the relation of the median eye-tube to the anterior wall of the brain.

FIG. 42.—Same of the scorpion.

FIG. 43.—Right half of the brain of a young Limulus about 3 inches long, viewed from its cut surface. The outlines and proportions of all the parts are drawn from a wax plate model, same as that in Fig. 49, and is diagrammatic only in so far as the invagination cavities of the infundibulum, optic ganglion, median eye, and olfactory organ and neuropore are represented as persisting up to this period instead of disappearing soon after their formation.

FIG. 44.—Cross-section through the dactylopodite of a Limulus about 8 inches long, showing general distribution of the cuticular canals for the gustatory and temperature organs. The gustatory canals are most abundant along the anterior cutting edge of the joint. The cuticula about the canals is in some places stained black by the osmic acid used for that purpose.

FIG. 45.—Under surface of a young Limulus 61 mm. long, showing the position of the olfactory organs in reference to the mouth, also its radiating choroid plexus of white pigment-cells. × 2.

EXPLANATION OF PLATE 4.

FIG. 46.—Nervous system of Limulus in its second larval stage seen from the dorsal or hæmal surface. × 66.

FIG. 47.—Cephalo-thoracic nervous system of the same seen from the ventral or neural surface. × 66.

FIG. 48.—Nervous system of Limulus about inches long, seen from the neural surface. Constructed from sections and dissections. The sympathetic system is not represented in Figs 36—38. × 15.

FIG. 49.—Fore-brain region of a young Limulus about 3 inches long, seen from the hæmal surface. The drawing is made from a wax plate model, enlarged about 100 diameters. The drawing is reduced to about 30 diameters.

FIGS. 50 and 51.—Cross-sections of the brain of Limulus in the second larval stage (see Figs. 46 and 47).

In Fig. 50 the section plane lies some distance back of the peduncles, and shows the overlapping of the cerebral hemispheres.

In Fig..51 it passes through the middle commissure, the peduncles of the cerebral hemispheres, and the last connection of the hemispheres with the ectoderm. × 275.

FIG. 52.—Cross-sections of the brain of an adult Limulus, passing through the slender peduncles on which the enormous cerebral hemispheres are supported. The plane of section cuts the anterior border of the middle commissure. Its position is shown on a younger brain by the line 21, Fig. 43. × 15.

EXPLANATION OF PLATE 5.

Reference Letters to Plate 5.

a. p. st. Anterior pons stomodæi. at. n. Nerve to antennæ. at. neu. Antennary neuromere. a. st. g. Anterior stomodæal ganglion, a. st. n. Anterior stomodæal nerve, br. 1—3. Brain lobes, c. Brain commissure. c. e. Convex eye. ch.g. Ganglion to cheliceral nerves, c. I. Cerebral lobes. ch. n. Cheliceral nerve, ec. p. e. Ectoparietal eye. en. p. e. Endoparietal eye. ep. Epiphysis. F. B. Fore-brain, g. hab. Transverse tube of nervous substance, g. v. 1—3. Invagination of optic ganglia, i. md. Inner mandible. i. md. n. Inner mandibular nerve. I. Labrum.-I. e. n. Lateral eye-nerve. I. st. g. Lateral stomodæal ganglion. I. sy. Lateral sympathetic nerve. M. B. Mid-brain = antennal or cheliceral neuromere. m. sy. Median sympathetic nerve, n. ec.p. e. Nerve to ectoparietal eye. n.en.p.e. Nerve to endoparietal eye. n. I. Labral nerve, n. oc. Nerve to ocelli, n. r. p. e. Nerve-roots to parietal eye. o. To. Nerve of Tömösvary. oc. Ocelli, o. e. (Esophagus, o.g. 1—3. Optic ganglia, o.md.n. Outer mandibular nerve. o. md. Outer mandible, p. e. Parietal eye. p. g. Pedal ganglion, p. m. Primitive mouth, p. n. Pedal nerve, p.p. st. Posterior pons stomodæi. s. I. Semicircular lobes, v. c. Ventral cords.

FIG. 53.—Diagram of nervous system of an insect, showing relations of stomodæal and trunk sympathetics to the cephalic lobes and ventral cords.

FIG. 54.—The same, with the stomodæum evaginated, to illustrate what was probably the original distribution of the stomodæal nerves.

FIG. 55.—Hypothetical embryo seen from the side, showing distribution of stomodæal nerves.

FIG. 56.—The same, with the stomodæum evaginated.

FIG. 57.—Diagram of the cephalic lobes and stomodæal nerves of an insect embryo (Holometabolic).

FIG. 58.—Same of a scorpion.

FIG. 59.—Same of Limulus.

FIG. 60.—Brain of Acilius at beginning of pupal stage, slightly diagram, matic.

FIG. 61.—Diagram of a spider’s brain, showing relation of the ocelli to the brain-segments.

FIG. 62.—Brain of a Myriapod (lulus).

FIG. 63.—Cross-section of parietal eye, just after hatching (early trilobite stage); depigmented, × 265.

FIGS. 64—70.—Series of cross-sections of the perietal eye, after first larval moult (trilobite stage) ; not depigmented.

Fig. 64. Section through about the middle of the endo- and ecto-parietal eyes. × 265.

Fig. 65. Section through the root of the eye, showing the four nerves, × 570.

Fig. 66. Section of parietal eye-stalk just below the eye. × 570.

Fig. 67. Section shows the primitive roots to the parietal eye-nerve separating from the epiphysis, × 570.

Fig. 68. Section of the epiphysis just below the preceding section. × 570.

Fig. 69. Section through the base of same, ×570.

Fig. 70. Section through the base, of the epiphysis, showing its union with the transverse tube, and the communication of the cavities in the same. ×570.

FIG. 71.—Cross-section through the base of one of the legs, showing the developing sense organs, nerves, and ganglia in the inner and outer mandibles. Trilobite stage. ×265.

FIG. 72.—Section of same in an embryo just after the shedding of the chorion, ×265.

FIG. 73.—Same just before hatching, ×87.

FIG. 74.—Parietal eyes of a Limulus about 3 inches long. The eyes have been dissected away from the cuticle and surrounding connective tissue, mounted in glycerine and viewed from above. ×38.

FIG. 75.—Three sensory hairs from the cephalo-thoracic shield of Limulus just after the trilobite stage. Their position and relation to the adult sensory hairs were not determined.

FIG. 76.—Sensory bud from thorax of same stage.

1

See pp. 29-30.

1

I subsequently observed similar movements of the second pair of appendages when the olfactory organ of a male was excised.

1

In my later experiments a ligature was drawn under the skin and tied tightly around the nerve, thus obviating the disadvantage of excessive bleeding.

1

See also my figures of the cephalic lobes of the Scorpion “On the Origin of Vertebrates from Arachnids,” vol. xxxi, part iii, of this Journal.

1

It must be remembered that the terms superior, anterior, and posterior, when applied to the embryos, have entirely different meanings from those when applied to the adults. This is owing to the doubling of the cerebral lobes on to the back of the adult, so that the anterior border of the cephalic lobes becomes the posterior border.

1

It seems to me now very probable that the mandibles on the walking appendages of Limulus and the coxal spur of Scorpions, with its rudimentary sense organ, represent much-shortened endopodites of the thoracic appendages. In Scorpions the sense organ comes in about the same position as the rudimentary appendages described by Jaworowski (‘Zool. Anz.,’ xiv Jr., 363) in Trochosa singoriensis, and regarded by him—rightly, I believe—as the endopodites of the thoracic appendages. But he has not, in my judgment, produced satisfactory evidence that the structure described by him as corresponding to the antennæ Of Insects is really an appendage ; it may be either a sense organ, or the endopodite or exopodite of the cheliceral segment.