About two years ago, while examining some sections of a Nereis (probably N. diversicolor, O. F. Müll.) which I had cut in the Zoological Laboratory at University College, London, I discovered an interesting organ, which appears to have hitherto escaped the notice of those naturalists who have studied the anatomy of the Nereids. Since then I have from time to time continued my observations, both in London and in the Oxford Museum, always hoping to be able to obtain the stages in the development of Nereis, the study of which alone can give a real clue to the homology and functions of these organs. This unfortunately has not been possible, owing chiefly to the fact that I have not been able to go myself to the sea-side to pursue my researches. It has therefore been thought advisable to publish the facts as they stand, and this paper amounts to little more than a mere description of this new organ. Some notes on the minute structure of the nephridium of the Nereids have also been added.

Various genera and species were examined, but most of the observations were made on Nereis diversicolor, which occurs in abundance at Plymouth, whence I have obtained my specimens alive. A detailed description will, therefore, be first given of the organ in this species, which will then be compared with some other members of the family.

If the fresh worm be killed, laid out, and slit up on the dorsal surface on each side along the bases of the parapodia, and a piece of the strip so obtained be placed with the inner surface upwards on a slide and examined under the microscope, there will be seen running transversely on either side, and curving round the outer edge of the great longitudinal muscles, faint lines made by a folded area of the coelomic epithelium, bounded in front by a lateral blood-vessel coming from the dorsal vessel. This folded and thickened patch of epithelium extends over the dorsal longitudinal muscles in the posterior half of each segment, and its surface is covered with delicate cilia. If now the strip be examined from the upper surface the folded ciliated area is seen to extend transversely across the middle of each segment, becoming narrower and narrower, and reaching to near the mid-dorsal line. I propose to call this organ the dorsal ciliated organ until we know more about its functions; the portion which lies on the outer surface of the muscles may be called the outer limb, and the portion which lies on the inside of the muscles the inner limb. Little more can be seen in the fresh condition; the exact position and structure of the organ can only be ascertained by means of sections.

For the preservation of Nereis diversicolor hot corrosive has, on the whole, given the best results. The worms were chloroformed, then opened up here and there, and the hot preservative poured on them. Hæmatoxylin and borax carmine were used as stains.

In a transverse section through the middle of a segment of an adult N. diversicolor (Pl. XXXIV, fig. 5) we see a covering of ciliated epithelium extending on the outer surface of the dorsal longitudinal muscle (figs. 1 and 5, dors. long, m.), and round its ventral border on to the lateral tegumentary blood-vessel (figs. 1 and 5, dors. teg. v.) which communicates with the dorsal vessel (figs. 1 and 5, dors. v.). This is the outer limb of the dorsal ciliated organ (Pl. XXXIV, figs. 1, 2, 5, o. limb; Pl. XXXV, figs. 15,17, o.limb). Fig. 15 represents a transverse section through the dorsal muscle of another specimen, taken rather farther back in the segment than that in fig. 5, thus avoiding the blood-vessel. The ciliated organ curls round the edge of the muscle, and ascends on its inner side nearly as high as the slight angle formed where this muscle leaves the body-wall. It will be seen that the inner limb (fig. 1, and figs. 15, 17, i. limb) does not adhere closely to the muscle, but to a fold of the septum which is bounded anteriorly by the dorsal tegumentary vessel (figs. 1, 4, 15, 17). The septum, in fact, is not attached to the inner face of the longitudinal muscle, and its free edge bends forward at right angles (fig. 4, fl. sept.), and proceeds upwards, leaving a space between itself and the muscle (figs. 1, 4, 5, 15, 17); it is on the inner side of this flap, which faces slightly backwards, that is situated the inner limb of the dorsal ciliated organ (figs. 1, 4, 5, and figs. 15, 17, i. limb). The outer and dorsal limb varies somewhat in its extension, sometimes reaching quite near the median dorsal line, as I have already mentioned. In this region blind prolongations of the coelom run up between the longitudinal muscles and the body-wall (figs. 1,5,7, dors, cœl.); they are separated from each other by connectivetissue walls. Small blood-vessels (fig. 7, bl. ves.) and bundles of circular muscle-fibres run along the coelomic spaces (fig. 7, circ. m.). The end of the outer limb of the dorsal ciliated organ (fig. 7, o. limb) extends up the largest of these spaces (which is situated almost exactly in the middle of each segment), being continuous with the coelomic epithelium which lines the space and covers its anterior side. A branch of the tegumentary vessel follows it closely during its course. I first expected that this dorsal tubular prolongation of the cœiom would, with the outer limb of the ciliated organ, lead to an external aperture; but I have looked in vain for such a pore in numerous sections—transverse, sagittal, and horizontal. Both coelomic space and ciliated organ gradually dwindle away until they disappear altogether some little way from the median dorsal line. As for the extension of the organ at its ventral border, fig. 4, which represents the middle portion of the sagittal section through two segments, shows that it reaches halfway across the segment from near its posterior limit (figs. 2, 4, dors. cil. org.). The surface of the organ is much folded, especially the outer limb.

The finer structure is shown in figs. 4 and 6. In the former, which was taken from a section stained in borax carmine, the cells are seen to have indistinct walls and to be somewhat striated on the outer edge, which is very darkly staining. The nuclei are large and more or less oval in shape. Fig. 6 represents a small portion of the organ, taken from a section very similar to the right side of that shown in fig. 5. This section has been stained with hæmatoxylin, and shows no distinct cell outlines. The protoplasm in which the nuclei are embedded is slightly granular within, and under a high power (Z. 2 mm. horn, oil im. and oc. 8) appears to be divided up into chambers, as it were, by the minutest ramifications of a network of darkly staining substance very similar to that described below in the nephridium. Towards the inside this substance forms the thin limiting membrane (Pl. XXXIV, fig. 6, l. mb.) which lies on the muscle, while towards the free surface it forms a dark zone (fig. 6, d. z.), and then the dark lines which give the outer edge of the organ its striated appearance. Outside this striated layer, at the very surface, the darkly staining substance forms a cuticular layer (fig. 6, cut.). The surface is beset with fine cilia (fig. 6, cil.), the bases of which, perhaps, are continuous with the light portions of the striae in the underlying substance. On every side the epithelium forming the organ is continuous with the coelomic epithelium, of which it is, in the adult at all events, a highly modified patch. It is quite clear from these descriptions, I think, that the dorsal ciliated organ shows no trace of a glandular structure.

In conclusion, I may say that this organ is present in every segment of the body from the tenth, except the last two, in which the nephridia appear to be also absent. It occurs in both sexes, and I have found it in every member of the family which I have had the opportunity of examining—namely, Nereis diversicolor, N. macropus, Clp., 1 N. cultrifera, Gr., Alitta virens, Sars, and Nereilepas fucata, Sav. In all these species the organ resembles that described in N. diversicolor in every particular, as far as I was able to ascertain; but in some cases the specimens were not very well preserved. It is a somewhat remarkable fact that I have failed to find it in two Heteronereid forms (the ♀ Heteronereid of N. pelagica, and a small Heteronereid which I have not yet been able to identify).

Of other genera, both nearly related to, and far removed from, the Lycoridea, I have examined the following species: Aricia foetida, Nephthys scolopendroides, Eunice sp., Lumbriconereis impatiens, Staurocephalus Ru-dolphii, Sthenelais dendrolepis, Polynoë elegans, Rhynchobolus convolutus, and R. siphonostoma. In none of these forms has any trace of the dorsal ciliated organ been found, and it is to be noticed that all these species have nephridia provided with large and widely open funnels, and short, more or less straight, ducts. 2

A short time ago Professor Lankester directed my attention to a very interesting paper by Professor A. Kowalevsky (7) on excretory organs, in which certain segmentally arranged organs, occurring in N. cultrifera, are shortly described. I shall quote the passage in full from p. 71: “Das Indigokarmin [which the animals had taken in] wird meistens von den Blutkörperchen aufgenommen, und zum Teil färbt es in jedem Segmente besondere Organe, welche an der Ruckenseite der Segmente liegen und aus drüsenartigen Zellen der Haut bestehen, unter denen man immer eine grosse Anhäufung der Blutkörperchen sieht. Die Stellen, welche so das Indigokarmin bei Nereis aufspeichern, sind auch an normalen Tieren zu sehen; dies sind Stellen mit Anhäufungen von braunen oder gelben Körpern, die in jedem Segmente und besonders in den hintern liegen. Diese Stellen nun speichern auch das Indigokarmin auf. Tötet man mit Alkohol eine so bearbeitete Nereide und spannt sie dann auf oder nimmt nur die Haut der Rückenseite, so gewinnt man ein Präparat wo diese blauen Organe in jedem Segmente sich metamerisch wilderholen und den Eindruck besonderer regelmässiger Segmentarbildung machen.” It seems evident that this excretory organ is not the same as the dorsal organ I have described, which is not glandular, and is not situated in the skin. Also Kowalevsky makes no mention of cilia. On the other hand, it appears not improbable that the “grosse Anhäufungender Blutkörperchen” seen below the excretory organ are the accumulations of coelomic corpuscles, which seem to be collected together by the cilia of the dorsal organ near the top of the outer limb (Pl. XXXIV, figs. 5, 7, cœl. corp.); it also frequently happens that a large number of ova are collected in this region.

When I first discovered the dorsal ciliated organ in Nereis, I was immediately struck with its remarkable resemblance to the genital ducts (Genitalschläuche) described in the Capitellids in Dr. Eisig’s beautiful monograph (6). I have obtained sections of Nereis showing the organ in question almost exactly like Eisig’s figs. 1, 3, and 14, pl. xiv. It seems to me, therefore, very likely that we have here a “genital duct,” which has not yet acquired an opening to the exterior. My inability to show such an opening may, perhaps, be explained by the fact that it has never been my good fortune to obtain a perfectly mature Nereid. The acquiring at maturity by the duct of an opening would be a fact by no means unparalleled in nature; in the Capitellids themselves it only occurs late (6). It may be remarked, also, that the dorsal organ of Nereis and the genital ducts of the Capitellids show many points of resemblance in histological detail.

Mr. Cunningham (4) argues that it would be quite impossible for the genital products in Nereids to be discharged by means of the nephridia; in this I entirely agree with him. Not only would it be impossible for the ova to escape by the nephridial canal, owing to their size; but their nuclei alone could not pass down its tortuous and narrow lumen. The genital products must therefore escape either by dehiscence, as held by Mr. Cunningham (4), or by means of genital ducts (ducts such as we know to be formed at maturity in the Nemertines).3 The former method would be neither agreeable nor advantageous to the parent; the latter, on the contrary, would seem to be the natural one.

The fact that no dorsal organs were found in the two Heteronereids I have examined, does not appear unnatural when we consider that these forms are much modified for the purpose of dispersing the ova, and are in many respects degenerate and unfitted for a long life. 4 Probably, in this case, dehiscence takes place, followed by the death of the individual.

Although many naturalists have examined the nephridia of various members of the family Lycoridea, these organs are still but incompletely known.

One of the chief difficulties we meet with in studying the nephridia of the Nereids, is owing to the small size of most of these worms (except Alitta virens and Nereis pelagica, which unfortunately I have not been able to obtain alive). However, if a N. diversicolor be carefully dissected under a powerful lens, the nephridia may sometimes be seen as small yellowish or whitish bodies at the base of each parapodium. On removing these with the septa, the whole organ, both body and funnel, may occasionally be obtained. It is then seen to consist, as described by Mr. Cunningham (4), of an opaque, massive body, containing a convoluted tube, which leads to a long canal terminating by a small funnel, the internal opening.

The whole nephridium is covered with coelomic epithelium. I have not attempted to follow out the complicated coils of the nephridial tube in series of sections; but fig. 8, Pl. XXXIV, is a diagram showing the main features of the system greatly simplified. The nephridial canal may, for convenience’ sake, in description be divided into four regions; it will be understood that the transition in structure between the regions is gradual. The ventral tegumentary vessel (fig. 1, vent, teg. v.) sends branches over the nephridium. The small nephrostome is attached to the lower edge of the septum (figs. 1, 3, neph. fun.; fig. 9 a), which is in this region free from the ventral longitudinal muscle. Cunningham (4) was unable to ascertain definitely whether it opens into the segment in front of that containing the main body of the nephridium and the external pore; while Cosmovici (3) says, “Pour moi, chaque paire d’organes segmentaires n’est en communication qu’avec l’anneau qui la contient.” Claparède, on the other hand, figured the funnel opening into the next segment (2, pl. vi, fig. 5); horizontal and sagittal sections clearly show that he was right. Frequently, however, being only suspended to the border of the septum, the nephrostome seems to swing round and open into the segment which contains the nephridium; but this is not its normal position. In its general structure it is very similar to the nephrostome of Alitta virens (4); it has, however, a nearly straight margin to its internal opening, and possesses no trace of an internal septum, as described by Cunningham in the above species. In N. diversicolor the edge or lip of the funnel is closely folded back over the outside of the organ (fig. 4, lip of fun.; fig. 9, dors, lip, vent, lip), and from this reflected surface and the margin of the internal opening (fig. 9, int. op.) spring long delicate processes (fig. 9, cil. proc.), just as in A. virens. These processes contain no nuclei, are formed of very granular protoplasm covered with long waving cilia, and must be regarded as mere outgrowths from the underlying cells. Occasionally the processes branch (fig. 3). In the nephrostome of Nereilepas fucata (fig. 9 a) the lip of the funnel is not reflected, and the processes, which are longer and more delicate, spring from the margin only. The inner wall of the funnel is formed of large vesicular cells (figs. 9 and 9 a, w.fun.) with round nuclei and deeply staining inner borders; they bear numerous strong cilia (figs. 9 and 9 a, int. cil.), which beat very rapidly.

From the funnel we pass to a short, narrow, glandular region, which is not ciliated (fig. 9 a, n. can.; not shown in fig. 9), leading into the first region, the long post-septal canal (fig. 8,p. s. can.). The wall of this canal is composed of many glandular cells, and becomes ciliated towards the nephridial body. On entering the main compact body of the nephridium the second region of the nephridial canal becomes much convoluted, but scarcely differs in structure from the next region, except in having more nuclei round the tube, and a few cilia distributed all round the lumen.

The third region is chiefly distinguished by having a very narrow lumen, and the cilia situated only on one side of the tube (figs. 11, 16, cil.). The cilia may not be in a single row, but the arrangement is essentially similar to that described by Dr. Benham in Lumbricus (1); and the peculiar sinuous, wave-like effect produced by the motion of the cilia in the nephridia of most Oligochætes is visible here also.

In a transverse section of this region of the canal, one (fig. IL n.) or two nuclei are generally present; I cannot say for certain whether the tube should be considered as inter- or intra-cellular. No doubt a hard and fast line of distinction cannot be drawn between the two varieties. The narrow tube which constitutes the third region of the nephridial canal is chiefly situated in the distal half of the nephridium, and leads into the fourth and last region (fig. 8).

This region is shorter and less convoluted than the last, and is situated on the under surface of the nephridium (fig. 8). Its lumen is moderately wide, and not provided with cilia (figs. 12—14); at more or less regular distances along the tube are two nuclei, situated opposite each other on either side of the lumen (figs. 12 and 15, n.). The nuclei in this and other regions possess, as a rule, one large nucleolus, and sometimes some small ones in addition.

The fourth region of the nephridial canal leads directly to the exterior by means of the nephridiopore (figs. 1 and 2; fig. 10, neph. p.), which opens on the ventral surface of each segment, about halfway between the ventral cirrhus and the longitudinal muscles. In fig. 10 it is seen that the wall of the tube pierces the epidermis (fig. 10, ep.), and reaches the cuticle itself: near the pore are a pair of nuclei, one on either side of the canal.

The sections through the wall of the nephridial canal show that it is lined by a more or less definite layer (figs. 11—14, b. I.), which is formed of an apparently denser material than that which lies outside. This boundary layer is very well developed in the last region (fig. 14, b. I.). Running throughout the substance of the nephridial mass is a delicate network of some denser cuticular substance, which stains darkly with hæmatoxylin (figs. 11, 12, 12a, 6, 13, 14, 16, net.). The boundary layer is pierced by the network, which thus reaches the lumen of the tube, and forms a close network round it. The cut ends of the threads of this network can be seen in the longitudinal section of the lumen drawn in fig. 14. Dr. Eisig describes in the Capitellids a somewhat similar though coarser mesh-work, as continuous with a boundary layer which he believes to be cuticular. In Nereis there appears to be a supporting network, and a distinct boundary layer, which is probably formed, not of cuticle, but of dense protoplasm, filled with minute granules. In some sections of a specimen preserved in Hermann’s fluid (fig. 12 a, ves.) little vesicles or vacuoles can be seen piercing this layer and opening into the lumen, presumably between the meshes of the network; these vesicles probably contain some excretory products.

Fig. 12 b represents a small portion of the supporting network, situated between the coils of the canal, in which are seen some connective-tissue cells. These cells do not belong to the wall of the canal, and their nuclei are smaller than those of the lining cells.

The nephridium may, therefore, be described as a mass of connective tissue, in which is embedded an excretory canal, the wall of which has a small number of nuclei in it, but no distinct cell outlines.

It is well known that Ehlers (5) described the nephridium of N. cultrifera as having a long duct leading to the exterior, and a crescent-shaped internal opening. Schröder (9), who seems to have copied Ehlers’ errors as carefully as his correct observations, gives a similar figure and description of the nephridium in N. diversicolor. I feel convinced that these observers, examining the nephridium after its removal from the worm, have simply mistaken the post-septal canal for the duct to the exterior, and that their internal opening is the scar left on tearing the organ away from its attachment to the body-wall at the nephridiopore. This interpretation is confirmed by the fact that Ehlers states that “die Richtung der Flimmerbewegung ging an der äusseren Mündung nach innen.”

We may sum up the main facts set forth above as follows:

Nereis diversicolor has in every segment, except the first and last few, a pair of large, highly differentiated, ciliated patches of coelomic epithelium—the dorsal ciliated organs. These organs seem to occur throughout the Lycoridea, as they have been found in all the genera of that family which I have examined.

The nephridium of Nereis diversicolor consists of a compact mass, perforated by a convoluted canal, which can be distinguished into several regions, and communicating with the exterior by a short duct leading to a nephridiopore on the ventral surface. A long canal springs from the main body of the nephridium, and ends in front by a nephrostome which opens into the next segment. The nephrostome is provided with long ciliated processes. The nephridium of Nereilepas fucata is of essentially similar structure.

Reasons have been given above for considering the dorsal ciliated organ as a genital duct not fully developed. It would be premature, however, to try to come to any definite conclusion on the subject of its functions and homology until we know both its development and its fate. But I should like to point out that the Capitellidse, the only other Polychaetes possessing a nephridium described in detail which resembles that of Nereis, also have in many segments a large ciliated patch of cœlomic epithelium, which becomes funnel-shaped and functions as a genital duct (Eisig, 6). Most of the Polychœta, however, have large-funnelled nephridia which may function as carriers of the genital products. On the other hand, the latest researches on the development of the nephridia in the Polychætes (Meyer, 8) and in the Oligochætes (Vejdov-sky, 10) seem to show most conclusively that these organs were developed phylogenetically from nephridia similar to those of the Planarians, which were provided with flame-cells, and did not open internally. Now Meyer has shown (8) that the large funnels in the tubicolous worms which he studied are developed independently and as outpushings of the cœlomic epithelium which join and fuse with the blind end of the nephridium. May not this funnel be the same as the genital ducts of the Capitellids and other worms (Oligochætes) ? I should consider, then, that in most Polychaetes the nephridium is a compound organ formed from the fusion of a tube with an outpushing of the cœlomic epithelium, as Meyer holds; and further, that in the Capitellids this funnel is partly nipped off as the genital duct, while in the Oligochætes, and perhaps in the Lycoridea also, this cœlomic funnel either has never joined the nephridium at all (most of the lower Oligochæta, Enchy-træids, Naids, &c.), or that it is afterwards mostly or entirely separated off as a genital duct.

It must be remembered that if we assume that in the Chæto-pods the genital ducts are simply modified nephridia, and at the same time consider that their nephridia, which open into the cœlom, were derived from nephridia without internal nephrostome, but provided with flame-cells;—if we assume this, then we must be prepared to explain what has become of the genital ducts with which the ancestors of these Chætopods were provided before their nephridia opened inwards, and why these ducts have disappeared leaving no trace behind them. On the other hand, I think that this difficulty may be avoided if we consider the original genital duct to have been a simple outpushing of the cœlomic epithelium which opened to the exterior, and afterwards became more or less grafted on to the nephridium. 5 Whether the cœiom in the Chætopods was primitively ciliated all over—as, for instance, in Rhynchobolus—is a very interesting question suggested to me by Professor Lankester. If this was the case the ciliated organs might have been formed by the restriction of the cilia to a definite area, bringing with it the specialisation of the epithelium and the development of a definite organ for a particular function.

In conclusion, I wish to express my thanks to Professor Lankester and Dr. Benham for their constant help and advice throughout my investigations.

1.
Benham
. —“
The Nephridium of Lumbricus
,”
‘Quart. Journ. Mier. Sci.,’
xxxii
,
1891
.
2.
Claparède
. —
’Les Annélides Chétopodcs du Golfe de Naples,’
Suppl
.,
1870
.
3.
CosMOVici
. —“
Glandes génitales et organes segmentaires des Annélides Polychétes
,”
‘Arch, de Zool. exp. et gén.,’
viii
,
1879-80
.
4.
Cunningham
. —“
Some Points in the Anatomy of Polychæta
,”
‘Quart. Journ. Mior. Sei.,’
xxviii
,
1887
.
5.
Ehlers
. —
“Die Borstenwürmer,”
Leipzig
,
1864-8
.
6.
Eisig
. —
‘Die Capitelliden des Golfes von Neapel,’
Berlin
,
1887
.
7.
Kowalevsky
. —“
Ein Beitrag zur Kenntniss der Exkretionsorgane
,”
‘Biol. Centralblatt,’
ix
,
1889
.
8.
Meyer
. —“
Studien über den Körperbau der Anneliden
,”
‘Mittheil. Zool, Station Neapel,”
vii and viii
,
1887-9
.
9.
Schröder
. —“
Anatomisch-histologische Untersuchung vonNereis diversicolor, O. Fr. Müll
.,”
‘Inaug.-Diss. Kiel,’
1886
.
10.
Vejdovsky
. —
“Entwickelungsgeschichtliche Untersuchungen,” 3rd part, Prague
,
1886
.

Illustrating Mr. E. S. Goodrich’s paper “On a New Organ in the Lycoridea, and the Nephridium in Nereis diversicolor, 0. F. Mull.”

All the figures, except Fig. 9a, are from Nereis diversicolor, O. F. Müll.

Lisi of Reference Letters

ap. Aperture through the septum leading from one segment to the other. b. l. Boundary layer, bl. v. Blood-vessel, c. b. w. Cut body-wall. cil. Cilia, cil. proc. Ciliated processes, circ. m. Circular muscles, cœl. Cœlom. cal. ep. Cœlomic epithelium, cal. corp. CœLomic corpuscles, cut. Cuticle. A. z. Dark zone. Aors. cil. org. Dorsal ciliated organ. Aors. cal. Dorsal extension of the coelom, dors. long. m. Dorsal longitudinal muscle, dors, lip. Dorsal lip of the funnel, dors. marg. fun. Dorsal margin of the funnel, dors. teg. v. Dorsal tegumentary vessel, dors. ". Dorsal vessel, ep. Epidermis. excr. Excretory products, ext. cil. External cilia. JI. sept. Flap of the septum, i. limb. Inner limb of the dorsal ciliated organ, int. Intestine. ini. cil. Internal cilia, ini. op. Internal opening. I. mb. Limiting membrane. I. neph. can. Lumen of the nephridial canal, lip of fun. Lip of the funnel, lu.fun. Lumen of the funnel, n. Nucleus, ti. Nucleus of a connective-tissue cell. n. can. Narrow canal, net. Network of supporting substance, neph. Nephridium, neph. fun. Nephridial funnel, neph. p. Nephridiopore, o. limb. Outer limb of the dorsal ciliated organ, obi. m. Oblique muscles, p. s. can. Post-septal canal, par. Parapodium, sept. Septum, sp. m. Sperm-morul®. v. marg. fun. Ventral margin of the funnel. vent. lip. Ventral lip of the funnel, vent. long. m. Ventral longitudinal muscle, vent. teg. v. Ventral tegumentary vessel, vent. v. Ventral vessel. ves. Vesicles of excretory fluid, to. fun. Wall of the funnel, w. neph. can. Wall of the nephridial canal. Ist, 2nd, 3rd, reg. First, second, third, and fourth region of the nephridial canal.

FIG. 1.

—Diagram of a transverse section, showing the posterior half of a segment as seen from in front. For the sake of clearness the smaller bloodvessels, setæ, and all the muscles except the four main longitudinal muscles have been omitted. The larger blood-vessels are shown on the left-hand side of the figure.

FIG. 2.—Diagrammatic side view of three segments from which the bodywall and parapodia have been removed from the left side. Only the dorsal and ventral blood-vessels are indicated.

FIG. 3.—A branching ciliated process of the nephrostome.

FIG. 4.—Portion of sagittal section through two segments—above the ventral edge of the dorsal muscle, below the dorsal edge of the ventral muscle, between the two the cœlomic cavity. The mass of fatty tissue which fills the cœlom is not represented. The anterior segment is to the right. Corrosive borax carm. Zeiss B., detail with high power.

FIG. 5.—Upper part of a slightly oblique transverse section through the middle of a segment. Corr. bor. car. Zeiss A., oc. 2, camera.

FIG. 6.—Transverse section through the edge of the dorsal ciliated organ. Corr, hæmat. Zeiss apochr. 4mm., oc. 3, camera.

FIG. 7.—Sagittal section cutting across the dorsal extension of the coelom. The cuticle is not represented. Corr. bor. car. Zeiss D., oc. 3, camera.

FIG. 8.—Diagrammatic sketch of the nephridium and its canal, which is subdivided into four regions.

FIG. 9.—Optical section through the nephridial funnel, drawn from the fresh, and completed from a stained specimen. Zeiss F., oc. 4.

FIG. 9a.—Optical section through the nephridial funnel of Nereilepas fucata, Sav., drawn from the fresh, and completed after staining with alum carmine. Zeiss apochr. 4 mm., oc. 4, camera.

FIG. 10.—Portion of a transverse section, showing the nephridiopore. Corr, bor. car. Zeiss apochr. 4 mm., oc. 8, camera.

FIG. 11.—Transverse section through the third region of the nephridial canal. Corr, htemat. Zeiss 2 mm. oil im., oc. 8.

FIG. 12.—Transverse section through the fourth region of the nephridial canal. Corr, haemat. Zeiss apochr. 4 mm., oc. 8.

FIG. 12a.—Transverse section through the third region of the nephridial canal. Hermann, htemat. Zeiss 2 mm. oil im., comp. oc. 4.

FIG. 12b.Section through the tissue between the coils of the nephridium, showing network and connective-tissue cells. Hermann, htemat, Zeiss 2 mm. oil im., comp. oc. 4.

FIG. 13.—Transverse section, slightly oblique, through the fourth region of the nephridial canal. Corr, htemat. Zeiss 2 mm. oil im., oc. 8.

FIG. 14.—Section through the fourth region of the nephridial canal. The lumen not being straight, the wall of the canal is cut through here and there. Corr, hsemat. Zeiss apochr. 4 mm., oc. 4, camera.

FIG. 15.—Portion of a transverse section, showing the dorsal muscle and dorsal ciliated organ cut through. Corr, hsemat. Zeiss B., oc. 4, camera.

FIG. 16.—Longitudinal section through a piece of the third region of the nephridial canal. Corr, hsemat. Zeiss apochr. 4 mm., oc. 4, camera.

FIG. 17.—Portion of a horizontal section, about midway through the intestine. Corr. bor. car. Zeiss A., oc. 3, camera.

FIG. 1.

—Diagram of a transverse section, showing the posterior half of a segment as seen from in front. For the sake of clearness the smaller bloodvessels, setæ, and all the muscles except the four main longitudinal muscles have been omitted. The larger blood-vessels are shown on the left-hand side of the figure.

FIG. 2.—Diagrammatic side view of three segments from which the bodywall and parapodia have been removed from the left side. Only the dorsal and ventral blood-vessels are indicated.

FIG. 3.—A branching ciliated process of the nephrostome.

FIG. 4.—Portion of sagittal section through two segments—above the ventral edge of the dorsal muscle, below the dorsal edge of the ventral muscle, between the two the cœlomic cavity. The mass of fatty tissue which fills the cœlom is not represented. The anterior segment is to the right. Corrosive borax carm. Zeiss B., detail with high power.

FIG. 5.—Upper part of a slightly oblique transverse section through the middle of a segment. Corr. bor. car. Zeiss A., oc. 2, camera.

FIG. 6.—Transverse section through the edge of the dorsal ciliated organ. Corr, hæmat. Zeiss apochr. 4mm., oc. 3, camera.

FIG. 7.—Sagittal section cutting across the dorsal extension of the coelom. The cuticle is not represented. Corr. bor. car. Zeiss D., oc. 3, camera.

FIG. 8.—Diagrammatic sketch of the nephridium and its canal, which is subdivided into four regions.

FIG. 9.—Optical section through the nephridial funnel, drawn from the fresh, and completed from a stained specimen. Zeiss F., oc. 4.

FIG. 9a.—Optical section through the nephridial funnel of Nereilepas fucata, Sav., drawn from the fresh, and completed after staining with alum carmine. Zeiss apochr. 4 mm., oc. 4, camera.

FIG. 10.—Portion of a transverse section, showing the nephridiopore. Corr, bor. car. Zeiss apochr. 4 mm., oc. 8, camera.

FIG. 11.—Transverse section through the third region of the nephridial canal. Corr, htemat. Zeiss 2 mm. oil im., oc. 8.

FIG. 12.—Transverse section through the fourth region of the nephridial canal. Corr, haemat. Zeiss apochr. 4 mm., oc. 8.

FIG. 12a.—Transverse section through the third region of the nephridial canal. Hermann, htemat. Zeiss 2 mm. oil im., comp. oc. 4.

FIG. 12b.Section through the tissue between the coils of the nephridium, showing network and connective-tissue cells. Hermann, htemat, Zeiss 2 mm. oil im., comp. oc. 4.

FIG. 13.—Transverse section, slightly oblique, through the fourth region of the nephridial canal. Corr, htemat. Zeiss 2 mm. oil im., oc. 8.

FIG. 14.—Section through the fourth region of the nephridial canal. The lumen not being straight, the wall of the canal is cut through here and there. Corr, hsemat. Zeiss apochr. 4 mm., oc. 4, camera.

FIG. 15.—Portion of a transverse section, showing the dorsal muscle and dorsal ciliated organ cut through. Corr, hsemat. Zeiss B., oc. 4, camera.

FIG. 16.—Longitudinal section through a piece of the third region of the nephridial canal. Corr, hsemat. Zeiss apochr. 4 mm., oc. 4, camera.

FIG. 17.—Portion of a horizontal section, about midway through the intestine. Corr. bor. car. Zeiss A., oc. 3, camera.

1

I am much indebted to Signor Lo Bianco, of the Naples Zool. Station, for collecting and sending me several species of Nereis.

2

I hope soon to be able to give a description of the minute structure of the nephridia of some of the genera mentioned above.

1

Should future observations establish the fact that the dorsal ciliated organ is a genital duet, it might well be compared with the metameric genital canals of the Nemertines.

2

Not only is the alimentary canal much reduced (according to Claparéde [2] the Heteronereids take no food), but the nephridia also seem to be degenerate and in some cases even absent. I have not, however, yet been able to examine sufficient material to say anything definite on this latter point.

1

Since this was printed I have found that E. Meyer put forth very much the same theory in 1890 (“Die Abstammung der Anneliden,” ‘Biol. Central.,’ x).