It has been known from very early times that the liver-rot of various herbivorous mammals is a parasitic disease due to the presence of very numerous flukes in the liver of the affected animals. Amongst our domesticated animals the sheep is by far the most frequent victim. The fluke disease is always common in certain districts in England and in many parts of the world; but in consequence of a succession of wet seasons there was a serious outbreak of it in the winter 1879-80, and it is estimated that in the United Kingdom 3,000,000 sheep were then destroyed by it. Hence special attention was called to the subject, and the research summarised in the following paper was undertaken on behalf of the Royal Agricultural Society of England.

For the sake of convenience the subject is divided into the following sections: —I. Historical. II. Methods of Investigation. III. Life-history.

From what was known of the course of development amongst the digenetic Trematodes, the nearest allies of Fasciola hepatica, there was reason to believe that here also an alternation of generations existed, and that one or more molluscs served as intermediate host for the asexual forms. Many attempts had been made to discover the intermediate host by various eminent biologists, including Professor Leuckart, von Linstow, Ercolani, &c., but all had proved fruitless, and notwithstanding its important practical bearing the problem remained unsolved.1

Very many suggestions had been made as to the nature of the intermediate host. Moulinié2 had found in Limax cinereus and Arion rufus (ater) sporocysts containing cercariæ with a rudimentary tail, and suggested that these might have some connection with the liver-fluke. I met with this species in Arion ater early in the course of my investigations, and was able to disprove experimentally the conjecture that this was the cercaria of Fasciola hepatica. Willemoes-Suhm3 had drawn attention to the fact that liver-rot was very prevalent in the Faroe Islands, the molluscan fauna of which was restricted to eight species, viz. Arion ater, A.cinctus, LimaXagrestis, L. marginatus, Vitrina pellucida, Hyalina alliaria, Limnæus pereger, and L. truncatulus. Of these Limax agrestis, our common grey slug, was by far the commonest and most injurious, and he suggested that this slug might act as intermediate host. Von Linstow4 had mentioned Planorbis vortex as being possibly the host. Weinland5 had found the liver of L. truncatulus infested with nurse forms. The cercariæ had the habit of throwing off their tails and crawling about by the aid of their suckers, and he thought that the larvæ of the liver-fluke might encyst upon plants. Küchen-meister had suggested certain slugs as possible hosts of Fasciola hepatica. On April 7th, 1880, the ‘Times’ published a letter written by Dr. Cobbold to contradict the statements made by Dr. J. Harley, who denied the existence of any intermediate host. The letter contained the following sentence: —“The investigations of the lamented Willemoes-Suhm render it almost certain that Cercaria cystophora infesting Planorbis marginata is the higher larval state of the small fluke (Distoma lanceolatum), and the still later researches of Leuckart point to the mollusc called Lymnæa truncatula as the bearer of the cercarian stage of Fasciola hepatica”. Seven days later, on April 14th, 1880, the late Dr. Rolleston gave, in a letter to the ‘Times,’ reasons for jegarding the black slug, Arion ater, as the intermediate host of Fasciola. Since that time Ercolani1 has made a similar suggestion for certain terrestrial molluscs. He found that larval trematodes were of far more frequent occurrence in landmolluscs than had hitherto been supposed, and this circumstance, together with the failure of the most illustrious helminthologists to discover the genesis of F. hepatica, led him to think that it was in this direction that success would eventually be obtained.

On the 2nd of the following June the Royal Agricultural Society of England offered Dr. Rolleston a grant for an investigation into the life-history of the liver-fluke. Dr. Rolleston being unable himself to undertake the work, mentioned my name to the Society, and the research was begun by me on June 7th.

On Dec. 22nd, 1880, I found in a Limnæus truncatulus, captured on an infected field at Wytham, near Oxford, on the 24th Sept., and since that time kept in an aquarium in the laboratory, a cercaria, which has since been proved to be the larva of the liver fluke.

The reasons which led me to regard this as the cercaria of Fasciola hepatica need not be explained here, as they are given in another part of this paper. In a report in the Royal Agricultural Society’s ‘Journal’ for April, 1881, I described this cercaria as a new species, and at the end of the description, wrote as follows:—“The structure and habits of this cercaria render it possible that it may prove to be the larva of Fasciola, hepatica, but want of material has prevented my testing the question by giving the cysts to rabbits. I intend, however, td pursue this case further.”

On Dec. 12th, 1881, a paper appeared in the ‘Zoologischer Anzeiger,’ in which Professor Leuckart announced that he had succeeded in infecting young specimens of L. pereger, but had been unable to obtain the development of the expected cercaria. He also made it known, for the first time, that the statement made by Dr. Cobbold in the ‘Times’ for April 7th, 1880, was founded on a private letter from himself, and that the announcement that his researches pointed to L. truncatulus had proved to be premature and incorrect, for on further examination of his snails he had found them to be not L. truncatulus but L. pereger juv.

In the first number of the ‘Archiv für Naturgeschichte’ for 1882, which, however, bears no further date, the same results were given at greater length. Professor Leuckart stated that in a number of specimens of L. truncatulus sent him by a friend he had found three kinds of rediæ. One of these contained tailless distomes, which, he held, probably belonged to the developmental cycle of the liver-fluke. He considered this supposition to be entirely justified until further results were obtained. A second form was not absolutely excluded from all connection with the liver-fluke, but no such statement could be made with respect to the third form. But in this third form I at once recognised the cercaria found by me at Wytham, of which a description had been published some eight months earlier.

During the summer of 1882 I at length obtained L. truncatulus for myexperiments, and succeeded in infecting them with the embryos of F. hepatica. Before the end of August the development of the species was obtained up to the time when the tailed cercariæ were nearly mature, and, as I had by me well-preserved specimens of the rediæ and cercariæ found at Wytham, I was able to compare the two forms step by step, and see that they were identical. A paper was drawn up for the October number of the ‘Journal of the Royal Agricultural Society,’ giving these results, and was sent to the printer on Sept. 1st. A. fortnight later it received a revision, which was merely verbal, and the whole of the copies were printed off by Oct.’2nd.

Separate copies of the paper were received for distribution on the 24th Oct., but the journal was not published until nearly the end of the month.

Inthemean time a paper by Professor Leuckart appeared in the ‘Zoologischer Anzeiger’ for the 9th October. In this he stated that he too had succeeded in rearing the cercaria of F. hepatica in L. truncatulus, “the only true intermediate host,” and that it had proved to be not the tailless form, but, on the contrary, the third form mentioned above, which he had supposed early in 1882 could have no connection with the liver-fluke.

On the 19th Oct. a résumé of my completed researches was published in ‘Nature,’ attention being called to the fact that the cercaria of the liver-fluke was the one already discovered by me in Dec., 1880, and described in April, 1881, as probably belonging to F. hepatica, and that the necessary proof had been furnished by myself, and also independently by Professor Leuckart. ‘

On commencing the investigation into the life-history of the liver-fluke it was felt that where so many different molluscs had been suggested as possible intermediate hosts, it was necessary to examine the question anew, and not to be guided by the numerous conjectures already expressed, some of which had very scanty evidence to support them. Work was begun in two directions, first, by endeavouring to infect many of our commoner molluscs, both terrestrial and fresh-water. In the second place, numerous localities in the country around Oxford, in which sheep were known to have incurred liver-rot, were visited, the whole of the invertebrate fauna was carefully studied, and many specimens brought home to be dissected and searched for larval trematodes. Very numerous places were examined in this way, but mention will only be made here of the one which proves finally to have given the correct solution of the problem. At Wytham, near Oxford, was a clearly circumscribed area of infection. The fields, five in number, were situated on the side of a hill far above the reach of river floods, lying upon the Oxford clay. They were searched thoroughly by day and by night, and the various invertebrate animals found, including snails, slugs, aquatic insect larvæ, Crustacea, worms, &c., .brought home and examined. Freshwater snails were very scarce on the ground; all that were brought to light were two small specimens of Physa fontinalis, a small Cyclas, and Limnæus truncatulus in moderate numbers. The last-named species was found in a boggy spot in one of the fields. -In one of them was discovered, on the 22nd December, 1880, the peculiar and interesting form of cercaria to which allusion has already been made. Its most striking character was due to the presence of very coarsely granular cells arranged in lobed masses along each side of the body. It was very active, but soon came to rest, encysting itself upon surrounding bodies. The cyst was snowy white, from the presence in its substance of the highly refractive granules already seen in the granular cells forming the lateral masses, which were thus shown to be cystogenous organs.

The other points in the structure of the cercaria were all favorable to the supposition that I had here discovered the long-sought cercaria of the liver-fluke. I had already found in a lamb’s liver exceedingly minute flukes, smaller than any yet recorded, one of them being only 1’1 mm. in length, i.e. only l-30th part as long.as the adult, and these immature forms gave me valuable information as to the structures to be expected in the cercaria, the relative size of its suckers, &c. It was usually supposed that the sheep when grazing picked up the parasites whilst they were still within the snail. This view was upheld by so eminent an authority as Professor Leuckart in a paper published in the beginning of 1882.1 But I had already collected evidence from independent sources, which inclined me to the belief that the larvæ were picked up in the encysted condition attached to the.grass. Hence the presence of a special cystogenous organ in the cercaria, and its habit of encysting on grass were highly suspicious. Further, the suspicions raised by the structure and habits of the cercaria were increased by the fact that its nurse-form was the only one found upon the ground, although there was every reason for expecting to find the larva of the liver-fluke, as an infected sheep had been seen a .few months earlier wandering over the boggy spot from which the L. truncatulus was obtained. I can testify that the sheep harboured numerous flukes, for its liver was sent me for examination; and there could be no doubt that large quantities of fluke eggs had been scattered all over the fields. My suspicions were accordingly expressed in a report to the ‘Royal Agricultural Society’s Journal’ for April, 1881.

During the summer of 1881 I was anxious to try infective experiments with Limnæus truncatulus, but was unfortunately unable to obtain any specimens; the localities near Oxford, where I had formerly found it, were searched in vain. I went out repeatedly in quest of this snail, having on several occasions the skilled assistance of my friend and colleague, Mr. W. Hatchett Jackson, but we never found any other trace of this species than the empty shells. It could not be discovered in the localities given for it by Whiteaves in his paper on the “Mollusca inhabiting the Neighbourhood of Oxford.”2 My friends at a distance were appealed to, but were unable to assist me. The comparative freedom from rot of sheep in the neighbourhood of Oxford last year was probably due to the real scarcity of this snail.

In 1882, however, there were floods in July, and the waters of the Isis brought down vast multitudes, probably from its breeding haunts in marshy places up the river. So numerous was it, that a single sweep of a small hand-net repeatedly gave me more than 500 examples, and this was in a ditch where last year not a single L. truncatulus could be found.

On obtaining the snails I had so long been searching for, I exposed a number to infection by placing fluke embryos in the vessel with them. The snails were speedily found to have afforded a suitable place for the further development of the embryos; indeed, infection was too successful, for very large numbers of them died simply from exhaustion owing to the excessive number of parasites they contained.

It may be well to mention here that infection experiments have been tried upon the following species of molluscs, as well as uponL. truncatulus, viz., Limnæus pereger, L. palustris, L. auricularius, L. stagnalis, Physa fontinalis, Planorbis marginatus, P. carinatus, P. vortex, P. spirorbis, Bythinia tentaculata, Paludina vivípara, Succinea amphibia, Li max agrestis, L. ciñere us, Arion ater, A. hortensis. None of these could be infected, with the partial exception of Limnæus pereger. With regard to this last species I can corroborate Professor Leuckart’s statement that the youngest specimens only of this species can be infected, and that even here development does not proceed beyond an early stage.

Although it appears that L. truncatulus is the only English mollusc which can serve as intermediate host to the liverfluke, it is quite possible that elsewhere some other mollusc of similar habits may be victimised. L. truncatulus has a very wide geographical distribution, but so, too, has the liver-fluke; and if the latter has the wider distribution, there must of course be some other intermediate host. Leuckart1 states that Fasciola hepatica is reported from Australia, and a similar assertion has been published in the ‘Veterinarian.’ According to Hutton’ and Wallace,8 the genus Limnæus does not exist in Australia. Hence if both these statements are correct there must be another intermediate host. The liver-fluke is. also found.in North America, where the genus Limnæus occurs indeed, but not the species L. truncatulus. Sheep-rót is also found in the Shetland Islands, where, according to Forbes,3 the genus Limnæus is represented by the species L. pereger alonei It is, however, quite possible that in the last case. L. truncatulus has been overlooked on account of its minute size.

A. First generation—1. Egg.—The eggs of the liverfluke occur in very large numbers in the contents of the bile ducts and gall-bladder of the infected animal. They give a dark brown colour and sandy appearance to the bile, and in some of the smaller terminal ducts often form a stiff brown mass, completely plugging up the lumen. They pass with the bile into the intestines, and may be found abundantly in the droppiugs of animals suffering from liver-rot.

The egg is an oval body, with a smooth, transparent, yellowish-brown chitinous shell. The average size may be said to be 0·13 nim. in length by 0·08 mm. in .breadth, but the dimensions vary greatly, the length from 0·105 to 0·145 mm., and the breadth from 0·066 to 0·09 mm. The anterior end is a little more rounded than the posterior, and a slightly serrated line running around it marks off a circular segment, forming an operculum 0·028 mm. in diameter. The opposite end is frequently a little thicker, and slightly roughened.

The number of eggs produced by a single fluke is exceedingly large, and its fecundity has been underrated. In one case I obtained 7,400,000 from the gall-bladder of a sheep suffering from the rot, and, as the liver contained about 200 flukes, this gives an average of 37,000 eggs to each fluke. And these eggs were found in the gall-bladder alone; the liver must have contained at least as many more, and eggs had been passed copiously by the sheep for several months. The number of eggs produced by a single fluke may be safely estimated at several hundred thousands.

When first formed, the egg includes a single germinal cell, supplied by the germarium, and fertilised by a spermatozoon, and a considerable number of secondary yolk-cells supplied by the vitellaria, which serve as food to the growing embryo. Segmentation of the ovum takes place during the descent of the egg through the oviduct, but no further development takes place so long as the egg remains with the body of the host. Fig. 1 on Plate II represents a fluke egg in the condition in which it is found within the bile ducts of the sheep; the embryo is represented by a pale spherical mass of delicate nucleated cells, and is situated near the opercular end of the shell. It is surrounded by the secondary yolk-cells, which are filled with refractive spherules, both large and small, so that the examination of the embryo is rendered very difficult.

The further development of the embryo can only take place out of the body of the bearer of the adult fluke and at a lower temperature. Eggs kept in an incubator, at the temperature of the mammalian body, do not make any progress, whilst the eggs kept at a lower temperature complete their development in a few weeks. The conditions necessary for development are moisture and a certain moderate degree of warmth. Light -I have found to exert no influence; eggs taken from the gallbladder and placed directly with water into an opaque vessel, develop as soon as similar eggs exposed to light but otherwise kept under the same conditions. A temperature of about 23° C. to 26° C. is most favorable, and with this degree of warmth the embryo is formed in about two or three weeks. At a lower temperature development takes place much more slowly, and with an average warmth of 16° Ç. occupies two or three months. During the winter no progress is made unless artificial heat is supplied.

All the eggs under the same conditions, however, do not produce embryos in the same time, a certain number are hatched out on every successive day for .some weeks or even months, and at the end of this time some of the eggs may remain in the same condition as when just taken from the liver. No explanation can be discovered in the eggs themselves of the very variable time required for the development of the embryo, but the fact is of much practical importance, for eggs scattered over any damp ground may render it dangerous for a long period.

The granular character of the secondary yolk-cells render it very difficult to follow in detail the growth of the embryo whilst still within the egg, and as the matter is one of theoretical importance only, the examination of the development by more elaborate methods has been postponed in favour of matter of more practical interest. I hope, however, to have the opportunity before long of observing the formation of the layers in the embryo. All that can be seen in the egg by direct examination is as follows. The embryo increases in size, being nourished by the absorption of the secondary yolk. The outlines of the yolk-spheres become more distinct and the granules less numerous, whilst some of them appear to coalesce or disintegrate (fig. 2, Pl. II). Within them the outline of the embryo is visible, often showing one or more annular constrictions. As the yolk-cells are gradually used up the body of the embryo becomes larger and more plainly visible, and comes to occupy the whole length of the egg (v. fig. 3). Its surface is somewhat bossy owing to the projection of the cells forming the outermost layer of the body. A papilla appears at the anterior end, which is always directed towards the opercular pole of the shell, and a little way behind a quantity of dark brown pigment is produced, giving rise to a double eye-spot, while the surface loses its bossy appearance. Wave-like peristaltic contractions pass along the body from the anterior towards the opposite end.

In the last stage, when the embryo is ready to emerge from the shell, it lies slightly curved upon itself at one side of the egg (v. fig. 4), the remainder of the space being occupied by the fluid remains and refuse of the yolk-spheres. At the anterior end, just beneath the operculum, is a quantity of viscid mucus, which forms a sort of lining or cushion against which the head-end of the embryo is pressed. Around the body of the embryo may be distinguished a bright border, which is formed by the cilia covering its surface; these cilia, however, can only in exceptional cases be seen in motion before the animal quits the egg.

2. Free Embryo.—The embryo is now ready to come forth; its movements become more marked, and at length a vigorous extension of the body causes the operculum to fly open, as if moved by a spring. The cushion of mucus pours out, the embryo thrusts the fore part of its body out of the shell, the cilia begin to move instantly the water touches them, and the animal, after a short struggle, succeeds in drawing the whole of its body through the narrow opening of the shell, and glides away with ease and rapidity through the water. Although light has no influence in accelerating development, the embryo itself is very sensitive to it. Thus they congregate especially on the light side of a vessel containing them; and I have repeatedly observed that, although on removing a vessel of eggs from the darkened incubator in which they were being hatched, not more than two or three embryos could be seen, yet after it had stood in a window for twenty minutes the water was quite nebulous from their presence.

The form of the free-swimming embryo is an elongated cone, with rounded apex (fig. 5, Plate II), its average length 0·13 mm., its breadth at the anterior end 0·027. The broader end or base of the cone is directed forwards, and in its centre is a short retractile head-papilla, The whole of the surface, with the exception of the head-papilla, is covered with long cilia, which are borne by an outer layer of flattened ectoderm cells. These cells are arranged around the body in transverse rings, usually five in number, though occasionally six may be counted; they vary in length from ·025—0·35 mm.; each has a very small nucleus, 0·003 mm. in diameter. The cilia are of the same length (·012 mm.) over the whole of the surface, but on the cells of the anterior ring they are more numerous, and hence more conspicuous. This first row is composed of four or sometimes five cells, arranged round the papilla, and these are thicker than the other cells belonging to the same layer, often forming ear-like projections at the side of the embryo, and resembling epaulets. The second ring contains five or six cells, the next two rows, each four, as a rule, whilst the last ring is formed by two cells only. In the last two rings the cells are of greater length than in the others. Seen in a surface view the cells of this outer layer are polygonal and sometimes hexagonal. They overlap one another at their edges, and it is probably owing to this fact that the outlines appear double in silver nitrate preparations. In the small number of examples possessing six rows of cells, the second and third rows are formed by smaller cells..

Beneath the ciliated cells the body wall is formed by a granular layer, the cellular nature of which is not easily made out. In favorable preparations, however, nucleated cells can be seen slightly projecting on the inner surface. In the outer parts of the layer are situated both transverse and longitudinal muscle-fibres. The longitudinal are more feebly developed than the transverse, and are only seen with difficulty. The double eye-spot belongs to this deeper layer; it has been figured as having a form of the sign of multiplication. This, however, is not the case, for it is really double, and has commonly the form of two crescentic masses of dark pigment, placed with their convex sides turned towards each other, and in contact near the anterior horns. On closer examination it is seen that each eye-spot is composed of a cell in which the pigment is arranged at one side in a crescent, the hollow of which is filled up by refractive material which will act as a rudimentary lens. The body wall also contains numerous yellowish refractive granules, especially just behind the eye-spots, and to it belong the two ciliated funnel-shaped spaces of the excretory system. These are situated one on either side of the middle of the body, in each is a large cilium carried by a nucleated cell, and usually directed forwards. The cilium is connected with the cell by a disc at its base, it is tongue- or flame-shaped, and is constantly in motion, waves passing along the cilium from the base to the tip, and hence towards the apex of the rather narrow infundibulum. Just behind the head-papilla is a granular mass, which reacts with staining fluids differently from the adjacent tissues. This, from comparison with other trematode embryos, would seem to be a rudimentary digestive tract. Behind, the rest of the body cavity is occupied by delicate round nucleated cells—the germinal cells.

‘The embryo is exceedingly active, and with head-papilla retracted swims swiftly and restlessly through the water, not unlike some of the larger infusoria, though more rapidly. Sometimes it goes directly forwards, and then rotates on its longitudinal axis, just turning a little from side to side, as if searching for something. At other times, by curving its body, it sweeps round in circles, or, curving itself still more strongly, spins round and round without moving from the spot. When the embryo, in moving through the water, comes in contact with any object, it pauses for a moment, and feels about as if trying to test its nature, and, if not satisfied, darts off hastily again. But if the object be a Limnæus truncatulus it at once begins to bore. Prof. Leuckart has said of the headpapilla of the embryo, that “it seems to ‘have the function of a tactile organ.” But I have no doubt that it has the function assigned to it in my former papers, viz. that it is a boringorgan.1 The papilla is ordinarily short (about ·006 m. in length), and the end is quite blunt, or may have a slight depression in the middle. A differentiation in the tissue of the head-papilla is visible in the form of a delicate rod-shaped structure, occuping the axis, certainly not distinct enough to be called a spine, though the papilla seems to possess considerable rigidity. It is particularly evident in preparations of embryos killed with osmic acid and stained with picro-carmine.

As soon as the embryo begins to bore the head-papilla becomes longer, conical, and pointed. The embryo spins round on its axis, the cilia working vigorously and pressing the embryo against the surface of the snail. This pressure is increased by the body of the embryo being alternately drawn up and then suddenly extended. As the papilla sinks further into the tissues of the snail, it becomes longer and longer until it may reach five times its original length (Plate II, fig. 6), and the tissues of the snail are forced apart, as if by a wedge, leaving a gap through which the embryo squeezes its way into the snail.

The embryo appears to exert an instinctive choice in selecting the host into which it enters. It is conceivable that the tissues of Limnæus truncatulus are softer than those of other molluscs, and that the embryo is able to bore its way into the former whilst it cannot do so into the latter. But from the greater eagerness which the embryo exhibits when placed on a slide with Limnæus truncatulus I do not think that this is the correct explanation. Moreover, the tissues of such snails as young specimens of Physa fontinalis or Lim-naeus palustris appear to be quite as soft as those of L. truncatulus, and yet if a quantity of embryos are placed in a vessel containing equal sized examples of the three species just named, it is found, on subsequent examination, that whereas each L. truncatulus may contain fifty or more intruders, the other snails are quite free from them. The most probable explanation seems to be that there is some difference in the nature of the secretion of the surface of the body in these snails, which is sufficient to serve as a guide to the instinct of the embryos.

But, although the embryo instinctively chooses the snail in which its further development is possible, it does not always make an equally happy selection of the part of the snail into which it enters. I have found as many as a dozen embryos embedded in the substance of the foot of a L. truncatulus, such a place of course is unfavorable to further development, but they may remain alive there for two or three days. Once only I found a full-grown sporocyst in the foot of a snail; the survival of this one in so unsuitable a place was probably owing to its having accidently forced its way into a connective tissue space or into a venous sinus. The most natural situation for the development of the embryo seems to be the pulmonary chamber, and this organ is, of course, from its position and the thinness of its walls, most easily accessible to the embryo. Other embryos, however, may be found in the body cavity of the snail.

The average maximum duration of the embryo’s free and active life in water is only about eight hours, though occasionally one may live over night. During the last portion of the time its movements become slower, and it will then in desperation often endeavour to bore into any object which presents itself, even into its own empty egg-shell. If an embryo has not succeeded in finding a host, its motion becomes gradually feebler, and at length ceasing the body assumes an oval or elliptical shape; the outer ciliated cells absorb water and swell up into round vesicles, and the whole body disintegrates. In a feebly alkaline solution of peptone I have kept them alive for three days. The cilia were not lost until the third day, though their motion became very sluggish; the embryos increased a little in size and remained alive even after a number of the ciliated cells were detached.

3. Sporocyst.—Arrived within the suitable snail the embryo undergoes a metamorphosis, loses its organs of locomotion, and degenerates into an inactive sporocyst. The outer layer of ciliated cells is lost, whilst the embryo changes in form. The ciliated cells absorb water and appear as round or hemispherical vesicles with the cilia standing out perpendicularly from their surface (fig. 7). During the metamorphosis embryos may have various irregular shapes, but sometimes retain a less elongated conical form, even after they have lost the ciliated cells. The conical form is, however, soon lost, and the embryos take an elliptical shape such as is shown in fig. 8. The eyespots of the embryo become detached from one another and lose their crescentic form; but they, as well as the head papilla, persist, showing the identity of the young sporocyst with the embryo of the liver-fluke. After the change in form has taken place the length is only about ·07 mm. The rudimentary digestive tract remains for a time, but later on is no longer distinguishable. The growth of the various larval forms of trematodes depends very much on the temperature to which they are exposed. During warm summer weather it is very rapid, and in the case of Fasciola hepatica, the sporocyst may reach its full size before the end of a fortnight; in autumn development to the same stage takes a period of double the length. The sporocyst commonly preserves the elliptical shape until it reaches the length of 15 mm., after this time the growth is most rapid in the longitudinal direction and the form becomes sac-shaped. The contents of the sporocyst are formed by a number of very clear rounded cells, some of which are the germinal cells of the embryo or cells derived from them by division, others are formed by a proliferation of the epithelium lining the cavity of the sporocyst. If the sporocyst be contracted, these cells sėem to fill up the whole of the space, and the cells which are still attached to the body wall, and form part of its inner surface, cannot be properly distinguished from those which are lying free. But if a sporocyst be chosen for examination which is not in a state of contraction, cells of various sizes, with very large nuclei, may be seen projecting here and there from the inner surface; sometimes in a single layer, at other times in rounded heaps, two or three cells deep. It is very difficult to follow the earliest stages in the formation of the spores within the sporocyst, but by the time the sporocyst has reached the size of ·2 mm., there are always indications that the contents are becoming arranged in separate balls of cells—the germs of the next generation.

The sporocyst continues to increase in size, and ultimately reaches the length of ·5— ·7 mm. (Plate III, fig. 10). On the outer surface is a structureless cuticle, and beneath this is the thin layer in which the external circular and internal longitudinal muscle-fibres are often the only structural elements which can be distinctly observed; but in some cases, though not in all, there is visible beneath the cuticle a finely granular layer in which the muscle-fibres appear to be embedded.

It appears probable that some of the most superficial cells of the body, or at least portions of them, are converted into muscle-fibres, whilst others undergo more or less degeneration. These muscle-fibres are more feebly developed in the sporocysts than in the rediæ which form the next asexual generation, and in accordance with this feeble development the sporocysts are exceedingly inert, and rarely show any movements. In the redia active movements are necessary in order that the digestive tract, which is present, may be filled with food, and the muscular system accordingly reaches a greater development. In the sporocyst the digestive tract is altogether rudimentary, and as no exertion is required to procure the nourishment, the degeneration of the muscle-fibres has to some extent followed that of the enteron. In those sporocysts, however, in which the power of performing active movements is useful for some other reason, the muscles may retain a high degree of development; as is, for instance, the case in the sporocysts of Cercaria limacis, which, as soon as their included cercariæ are sufficiently matured for transference to the ultimate host, bore their way out, through the thick integument of the slugs (Arionater and Limax cinereus), which serve as intermediate hosts, and are then left behind in the mucous track of the slug.

Immediately following the layer of muscle-fibres is an epithelium, which lines the cavity of the sporocyst, and forms the greater part of the thickness of the body wall. It is composed of cells of very various sizes, round or polygonal in form, and containing large nuclei (fig. 11). The layer in most places is only one cell deep, though adjacent cells may overlap one another; but in places, and especially in the less mature sporocysts, it is two or three cells deep. The excretory system is lodged in the body wall of the sporocyst; on each side may be distinguished an irregular group of about half-a-dozen ciliated infundibula. They have the same structure as the two described above as being present in the embryo, and are always found in the middle third of the length of the body. No clearly defined or regular canals can be distinguished, but the ciliated infundibula appear to communicate with an extensive system of irregular lacunæ between the cells of the body wall. Numerous yellowish refractive granules occur in the tissues of the sporocyst, or in the body cavity; they are found within the cells, but are especially numerous between them, or on their surface. They are also present in large numbers in the lacunæ, and there frequently exhibit molecular motion, thus showing that the lacunæ contain a fluid of some tenuity; occasionally a whole group of them may be seen to move en masse, or by careful pressure on the cover-glass may be made to travel for a short distance along the lacunar passages. No external opening of this system of passages can be seen, nor any communication with the body cavity be clearly proved. In the ciliated infundibula, however, there is present in the lower wall of the space and close to the base of the flameshaped cilium, an elliptical structure, closely resembling that which Fraipont has described in the ciliated infundibula of certain other trematodes, as an opening into the body cavity. This interpretation has more recently been called in question, but whatever the real nature of this structure may be, and it is difficult to see what else it can be, there can be no doubt of its presence in the sporocysts. The yellowish granules described appear to be excretory products formed within the cells of the sporocyst and then ejected. They are partially soluble in acids, leaving an organic basis.

Wagener1 found in the sporocysts of Cercaria macrocerca (the larva of Distoma cygnoides) vibratile lobules (Flim-merliippchen) or ciliated spots, which he did not describe in detail. Thiry3 described in the same sporocysts a system of vessels, the branches of which had ciliated terminations opening into the body-cavity. The vessels, however, were very pale and difficult to follow, and only in one instance, where the animal was exceptionally transparent, did he plainly see the whole system with its branches. The ciliated ends, on the other hand, were present in almost all fairly developed examples; in form they most closely resembled the ciliated openings in Clepsine complanata, as figured by Leydig. The vessel was opened on one side and expanded into a two-horned lobe, covered on its inner surface with cilia. In short, the ciliated opening was described as similar to the inner ends of the segmental organs of various annelids. I have never found the sporocysts which Thiry studied, but it seems improbable that there should be any great difference in the structure of the ciliated ends in question in the various species of sporocysts. The isolated cilia really present are very large and their motion peculiar, so that it is not difficult to understand that any one who had before his mind the segmental organ of the earthworm, and was not prepared to see a large isolated cilium within an infundibulum, might take the waves passing along the cilium for waves travelling over a series of small cilia. There can be no doubt, therefore, that the ciliated infundibula have essentially the same structure in the asexual generations (sporocysts and rediæ) as in the adult sexual trematodes.

Amongst the digenetic trematodes the reproduction of sporocysts by sporocysts takes place, either by transverse fission, which may be continued through several generations, as in the case of Cercaria limacis, or by the formation of sporocysts within the parent, or both methods may occur in the same species (e. g. Cercaria chlorotica, &c.). But the only way in which the sporocysts are multiplied in the case of Fasciola hepatica is by transverse division, and this is of far less frequent occurrence than in some other species of trematodes. There appears to be a great and invariable increase of the nurse-forms amongst the Distomidæ, and in Fasciola hepatica the multiplication is effected by the production of numerous broods of the more highly organised rediæ. Fission does, however, sometimes occur, and usually at an early stage in the growth of the original sporocyst. A constriction appears about the middle of the body, and becomes deeper and deeper (fig. 9), and finally the two halves are completely severed. One of these contains the remains of the head-papilla and the two separated eye-spots, whilst the other is, of course, without the signs of any such structures. Hence sporocysts can be found, which even at an early stage show no trace of head-papilla or eye-spots; and in the majority of adult sporocysts these organs have degenerated entirely.

B. 1. Development of Redia within Sporocyst

It has been mentioned above that the germinal cells which give rise to the rediæ are in part already present in the embryo, but that they gain an increase in their numbers by the proliferation of cells lining the body cavity. The earliest stages in the development of the spores cannot be so well distinguished in the sporocyst as in the redia. The cells within a sporocyst having the length of about ‘2 mtn., begin to show an arrangement into rounded masses or solid morulæ. One side of the morula then becomes flattened, and the cells here then appear to be invaginated, producing a gastrula, whilst the surface becomes smooth, and its outline first round and then oval. The cells forming the opposite sides of the archenteron are in contact, so that there is no archenteric cavity. Each spore may now be seen to be surrounded by a delicate membrane, and as it increases in size its form becomes more nearly quadrate. At one end a number of cells are separated to form a spherical pharynx leading into the blind digestive tract, which now extends a little beyond the middle of the body. A little behind the pharynx, the body shows a slightly raised annular ridge, whilst more posteriorly two short blunt processes are formed. Germs, as described above, and in various stages of development, are found in each mature sporocyst; there is usually one redia (or less frequently two), nearly ready to leave the sporocyst, with two or three germs of medium size, and several small ones. Owing to the varying size and shape of the included germs, the .sporocysts have frequently a very irregular outline.

As soon as the redia is ready to issue from the sporocyst, which is usually the case by the time it has reached the length of ‘26 mm., it shows active movements, which increase in strength until at length it succeeds in rupturing the wall of the sporocyst, and as this state of contraction is continued, the wound produced by the forcible exit of the redia is kept closed until it has healed up. Meanwhile the development of the remaining germs proceeds. Many of the nurse-forms of trematodes are known to possess a special birth opening for the escape of the brood, and even amongst the sporocysts such an opening is present in those possessing-a filiform shape; and I have myself observed this structure in the sporocysts of Cercaria gracilis. But in F. hepatica no such definite opening can be detected in the sporocyst.

2. The Free Redia

The sporocysts of the liver-fluke are found in the pulmonary chamber of the snail, or less abundantly in the body cavity. But the free rediæ force their way through the tissues of the host, and wander into the other organs, and especially into the liver. They are usually found, with the digestive tract quite yellow from the remains of the snail’s liver-cells, with which it is filled. In thus forcing their way through the tissues they necessarily inflict much injury on their host, so much, in fact, that comparatively few snails survive three weeks from artificial infection, and the majority, even of these, die before the time when the cercariæ are completely mature. Thus, in the laboratory at any rate, the fluke-disease is more fatal to the snail than it is subsequently to the sheep.

The redia increases in size until it may reach the length of l·3 mm. to 1·6 mm. It has an elongated cylindrical form (Pl. Ill, figs. 12, 13), and at a little distance behind the pharynx there is present an annular ridge or collar projecting from the surface, the use of which will be explained below. From this ring or collar the body tapers gently towards the anterior end, which is abruptly truncated, and includes in its centre the mouth. Behind the collar the body becomes a little narrower, but then swells out gently again until it reaches the middle of its length, from which point it tapers, at first almost insensibly, and then more rapidly, the extremity being conical with rounded apex. At a distance from the posterior end, equal to about one fourth of the total length of the body, are situated two short and bluntly conical processes, which serve as rudimentary feet, and are no doubt of much service in steadying the redia and preventing it from slipping backwards whilst wandering through, the tissues of the host. They are not situated on opposite sides of the body, but are close together on the same surface, and their bases may even be connected by a low transverse ridge. They are directed outwards and somewhat backwards, their axes being usually inclined at an angle equal to or rather less than a right angle.

The body-wall has a similar structure in both redia and sporocyst, so that it will only be necessary to describe the points in which a difference exists. The muscle-fibres are far more strongly developed, especially in the anterior part of the body, so that the rediæ show considerable activity as compared with the sporocyst. When the body is fully extended it may have a length twice as great as when in a state of contraction. If an example of the host be chosen, which has a clear and transparent shell, and has had the greater part of its liver consumed by the parasites, the rediæ may be observed performing movements of elongation and contraction whilst still within the living snail.

In the collar or ring mentioned above the muscle-fibres are strongly marked, and have a peculiar arrangement. The transverse muscle-fibres appear to lie directly under the cuticle, and are closer together at the sides of the ridge then near its most convex part. The longitudinal muscle-fibres, however, do not follow the curve of the surface, but stretch across from one to the other side of the base of the ridge. Sometimes, when the ring is strongly marked, the longitudinal muscle-fibres as they pass forwards may spread out in a fan-shaped fashion before they are finally inserted in the cuticle (fig. 16). The extent to which the ring projects above the rest of the surface of the body varies very greatly according to the size and condition of the redia, and may be altered from time to time by the contraction of the muscle-fibres. It is greatest in those which show the most active movements, least in those which are the most passive. The smaller or half-grown rediæ commonly show the greatest activity, and in one of these I have observed the ring so enormously developed that the diameter of the body was almost doubled at this point. Those rediæ in which fully-developed cercariæ are present are frequently very sluggish in their behaviour, and the ring may then be relatively inconspicuous. In very young rediæ the outline of the body appears to present a slight process on each side anteriorly, and without the most careful focussing it is often impossible to see that these are simply the optical expression of the collar, the tissues of which are still so delicate that the ridge is flattened above, and therefore, owing to the transparence of its substance, not readily recognised.1 The function of the collar is to maintain the shape of the body and to produce a firm basis upon which the neck of the redia can be moved. I have observed a redia, whilst the whole of its body behind the ring was at rest, stretch forth its neck in such a way as to sweep a considerable area in front, and thus be enabled to reach conveniently the tissues of the snail upon which it was browsing. When disturbed the neck was retracted and the pharynx drawn back close to the collar. But although the collar has thus a supporting function, there is no thickening of the structureless cuticle in it, such as could be termed a definite skeletal structure.

The excretory system is better marked in the redia than in the sporocyst, and definite canals can be distinguished in the body-wall. Sinuous longitudinal vessels, one on each side, have been described in the rediæ of several other Trematodes, and may also be distinguished here, though the main trunks are less distinctly visible than their ramifications, and can rarely be followed for any great distance. Hence it is impossible to discover whether the system of vessels opens externally. The branches begin with a long narrow infundibulum, in which a flame-shaped cilium is constantly working, as described in the sporocyst. The ciliated infundibula are arranged in two groups on each side of the body; the anterior group on each side lies a short distance behind the collar, the posterior close to the processes which serve as feet (fig. 15). The ciliated cells do not all lie at the same level beneath the surface, so that occasionally two of the infundibula may be seen lying across one another, and sometimes the cells may lie free within the body-cavity, the end of the cell opposite the cilium being connected with the wall by one or more processes (fig. 14).

The digestive tract is the characteristic structure of the redia, and at once differentiates it from the simple sporocyst. Quite at the anterior end of the body is the mouth surrounded by projecting folds, which may be termed the lips. The transverse muscle-fibres are especially well developed in the lips, and assisted by the transverse muscles of the following part of the body-wall, serve as a sphincter muscle in closing the orifice of the mouth. The space within the lips is very small, and leads almost directly into the pharynx, an elliptical muscular organ by means of which the animal draws in and crushes the tissues which serve as food. Its outer surface is formed by a clearly marked limiting membrane, so that it is everywhere distinguished with readiness from the mass of ill-defined cells in which it is embedded, and its cavity is lined by a thickened cuticle. To the pharynx immediately succeeds the digestive sac, a blind tube of very simple structure. Its wall is composed of a single layer of clear nucleated cells (fig. 12), supported by a basement membrane, and when it is distended the cells are flattened out till they are little more than discs, in which the nucleus causes a distinct swelling. The digestive sac is seldom more than ·3— ·4 mm. long, and may be less than this, but its length differs a good deal, not only in different individuals, but also according to the amount of food contained in it. It reaches its full size early; indeed, in a redia not half grown it may be as long as in a full-grown example.

The body-cavity is traversed in different directions by bridges or trabeculæ of tissue, in which cells of various shapes, some of them with long processes, can be distinguished. This tissue is most abundant in the anterior part of the redia around the pharynx and digestive tract, and here often contains fibres, probably contractile. Its amount varies very greatly in different specimens, and behind the digestive tract is sometimes altogether absent. At other times it is so extensively developed that the cavity of the redia appears to be divided up into a number of imperfect compartments, in which the germs lie loosely. De Filippi appears to have observed similar trabeculæ in the redia of Cercaria coronata, of which he says,1 “La cavité du corps est traversée sans ordre par des brides.”

There is always a good deal of tissue around the pharynx and beginning of the digestive tract, aud embedded in it may be seen, in favorable specimens, a few large round cells with clear protoplasm and large nucleus; each has a process or duct (?) passing towards the angle formed by the junction of the digestive sac with the pharynx. They are probably glandular in function. At the side of the redia, a little behind the collar, there is present a birth-opening (Pl. Ill, fig. 13, v), which permits the exit of the brood when ready to leave the parent. Such an opening has been seen in a number of rediæ of different types, and probably exists in all.

The germs produced within the redia develop either into daughter-rediæ or into cercariæ, and it appears to me that slight differences exist between the individuals giving rise to one or the other of these generations. A redia producing rediæ is usually smaller, but its pharynx and digestive sac are larger; for example, two rediæ were taken from the same snail, one producing rediæ measured less than 1 mm. long, with a pharynx ‘117 mm. and an intestine ·44 mm. in length, whereas in the slightly larger redia containing cercariæ the pharynx measured ·078 mm. and the digestive sac ·24 mm. A further distinction lies in the number of the progeny; a mother-redia may contain from one to three well-formed daughter-rediæ with a few germs in various stages of growth ; the highest total observed was ten. On the other hand, in a well-grown redia producing cercariæ, I have counted a total of twenty-three.

The early stages in the development of the spores is the same, whether they are destined to become rediæ or cercariæ. Some of them may be formed from the cells which fill the body cavity in the very young rediæ, but the majority seem to be formedin the following way:—Some of the cells lining the body-cavity of the parent, especially those at the posterior end, are greatly enlarged, and each of these germinal cells undergoes segmentation, giving rise to a morula. Fig. 18 represents a large number of germinal cells in the hind end of a young redia, in which no morulæ were yet present. Similar cells may be found in the mature rediæ (fig. 13 k′), for they retain the power of producing more spores as the older ones reach their full development and quit the parent. Hence we find in the adult redia germs in all the successive stages of growth. Each morula or germ is enclosed by a delicate membrane forming a loose envelope. The germs are usually detached from the bodywall whilst still small and lie free in the cavity of the parent, but occasionally they may remain in sitû in the body-wall until they have attained a considerable size (fig. 13 w′). The morula soon becomes flattened on one side (fig. 12 s), and the cells of this area are then invaginated, giving rise to a gastrula (fig. 12 m), whilst the germ again becomes round. The opposite sides of the archenteron are in contact, so that there is rarely any archenteric cavity, and as growth proceeds and the cells become more numerous it is no longer possible to distinguish the cells of the endoderm, for the cells have the same size and appearance. Nevertheless it appears to me probable that the cells invaginated form the digestive tract, which becomes visible at a later period in the development, rather than any other cells in the germ. As the germ continues to increase in size the surface becomes smooth and the outline oval.

Further growth in size is accompanied by a change in shape, and it then becomes possible to distinguish between the germs destined to become rediæ or cercariæ. The growth of the young redia within the redia agrees in every respect with the development of the mother-redia within the sporocyst. The growth of the cercaria follows a different line.

It may be asked what determines the character of the progeny, whether the germ shall become redia or cercaria. My observations are not sufficiently extensive to definitively decide the question, but it appears to me that the season of the year is one of the principal determining causes. Rediæ producing rediæ were only found during warm weather, in the cold months cercariæ were always produced directly. Further, it is a noteworthy fact that I found at the beginning of the autumn a redia, containing a single daughter-redia in addition to numerous cercariæ and their germs (fig. 13). I am inclined to think that the redia was producing rediæ but that a fall of temperature induced the formation of cercariæ instead. The explanation suggested is the more likely to be correct, since such an arrangement would be highly advantageous to the species.

C. 1. The developmentof the cercaria within the redia

The earliest stages of the development have already been described up to the time when the germ is an oval mass of cells. As this continues to increase in size it assumes a more elongated shape, whilst one end becomes rather more attenuated than the other. The more slender end becomes slightly constricted off to form the rudiment of the tail, which as yet is very stumpy. The remainder of the germ forms the body of the cercaria, it becomes more depressed in shape, whilst cells are separated at the anterior end to form an oral sucker, in the midst of which opens the mouth, and in the centre of the inferior surface to form a ventral sucker equal in size to the oral. The digestive tract is now visible as a solid mass of cells. Immediately following the oral sucker is the rounded pharyngeal bulb. Then comes a narrow oesophagus ascending slightly towards the dorsal surface, and at a short distance in front of the ventral sucker it bifurcates to form the two limbs of the intestine, which reach, one on each side of the ventral sucker, to nearly the end of the body. The limbs of the digestive tract are solid, bein’g formed for the most part by single rows of thick disc-shaped cells (fig. 13). The cells are finely granular at this stage, and show out distinctly against the clear spheroidal cells which surround the limbs and produce concave impressions on the surface. At the sides of the body refractive granules begin to collect in certain of the cells, which are destined to assist in the formation of the cyst of the cercaria, and may conveniently be termed cystogenous. At first the granules are few and inconspicuous, but gradually become more and more numerous until at length they may obscure the nuclei, and render the cells opaque. Many of the cells in the body of the cercaria are crowded with most remarkable rodshaped bodies closely resembling bacteria in size and shape (fig. 20). They reach the length of ·006 mm., and are often arranged in rows side by side, whilst the long axes of nearly all the rods in each cell have approximately the same direction. Both Wagener and De Filippi appear to have observed similar structures in the cercaria of Amphistoma subclavatum. The former speaks of them as “rod-shaped corpuscles,” and the latter says that “their form may not inaptly be compared to that of a shuttle or spindle, with thick walls, and truncated at both ends. They are destined to disappear later.” These bodies are not precisely like the narrower ones found in the cercaria of the liver-fluke, but they are probably corresponding structures.1

An adult redia generally contains a brood numbering about a score; amongst these there will be one, two, or three cercariæ approaching complete development. On one occasion I counted as many as six.

2. Free Cercaria

As soon as the cercaria has reached the limit of development within the redia, it escapes from the parent by the birth-opening (fig. 13, v r) and then by the aid of suckers and tail, crawls or wriggles its way out of the host. The free cercaria is very active, and its tissues so contractile that the form and dimensions of the body are constantly changing. When in a relatively quiescent condition, the body has a depressed oval form (Plate III, fig. 19), its average size is ·28 mm. long and ·23 mm. broad, though the largest may measure over ·3 mm. in length. The tail is more than double the length of the body, and is exceedingly contractile. The oral sucker is subterminal, the opening of the mouth being directed downwards and forwards, and has a diameter of ·06 mm.; the pharynx is ·034 mm. in diameter. The ventral sucker is situated slightly behind the centre of the ventral surface, and is equal in size to the oral, or is sometimes a little larger. As is the case with all the cercariæ produced in rediæ (with the partial exception of Distoma Paludinæ impuræ armatum) the cercaria has no head spine. In the most mature specimens, and especially in such as have left the redia in the natural course, and have not been disturbed by the dissection of their host, the surface of the body is beset anteriorly with exceedingly minute spines. But the most striking character is due to the presence of the cystogenous cells, large nucleated cells so crowded with coarse, highly refractive granules as to be rendered quite opaque. They are arranged in two lobed masses extending along each side of the body (Plate III, figs. 19 and 21), from the level of the pharynx to the posterior end of the body. Just in front of the ventral sucker is another group of these cells, which is often large enough to connect the two lateral masses, and behind the ventral sucker others are scattered. Cells of the same kind, and showing a similar arrangement, are found in Cercaria tuberculata (inhabiting Bythinia tentaculata), a species which shows at first sight a remarkable resemblance to the cercaria of Fasciola hepatica. I have, however, myself met with C. tuberculata, and from comparative measurements, as well as the difference in the host, can state with confidence that the species are quite distinct. And even in an armed cercaria, recently found in Limnæus pereger, I found cells, showing a similar arrangement, distinguished from the remaining cells of the body, not, indeed, by coarsely granular contents, but by the possession of a protoplasm of a finely granular nature. In this case also the more granular cells are probably cystogenous.

The other organs of the body are much obscured by the presence of the opaque cystogenous cells, but the contractile vesicle of the excretory system, together with the principal lateral vessels, one on each side, which contain small highly refractive concretions, can be made out.

3. The Cyst

When the snails infested with the larval forms of Fasciola hepatica are kept in an aquarium, the cercariæ may occasionally be found swimming about in the water, for the granular cells which render the body nearly opaque when viewed under the microscope by transmitted light, give it a snow-white appearance by reflected light, and it is thus rendered conspicuous for its size. The life as a free-swimming animal, however, never seems to last long, for, on coming in contact with the side of the aquarium or the waterplants contained in it, the cercaria proceeds to encyst itself. Numbers of minute snow-white cysts may thus be seen adhering to the walls of the aquarium or to the dark-green leaves of the water-plants. The way in which the cyst is formed can be readily observed under the microscope, for when examined on the glass slide the cercaria soon comes to rest, and assumes a rounded form, whilst a mucous substance is poured forth all over the body, together with the granules forming the contents of the cystogenous cells already mentioned. The tail is sometimes shaken off before the encystation begins, but, as a rule, the tail remains in connection with the body during the process, and continues to be energetically lashed from side to side, until finally a more vigorous movement detaches it. The whole process of forming the cyst is very rapid, and in a few minutes a layer of considerable thickness is formed, whilst its substance begins to harden. The cysts, as already remarked, are snowy-white, but the body of the included Fasciola is quite transparent.

The habits of the intermediate host (Limnæus truncatulus) are of much importance, as showing the manner in which the cysts are distributed in places where they are likely to be picked up by some herbivorous mammal, within which they can attain the adult state. Limnæus truncatulus belongs to the group of fresh-water Pulmonata; it is a common snail, but one which is often very difficult to find on account of its small size and peculiar habits. It has a very wide geographical distribution, being found, according to Dr. Gwyn Jeffreys, throughout Europe, in North Asia, Morocco, Algeria, Madeira, and (doubtfully) in Guatemala. Several species belonging to the genus Limnæus occasionally crawl for short distances out of the water, but in L. truncatulus this habit is so much more strongly developed that the snail should be termed amphibious. Indeed, it is oftener found out of the water than in it. When kept in an aquarium it quits the water, and as often as it is put back crawls forth again so long as the necessary strength remains. It is said to breed on the mud at the sides of ditches. To show how much it lives out of the water I may briefly relate my own experience. There were floods on the Isis in July last, and the waters brought it down in vast multitudes, probably from its breeding haunts in marshy places far up the river. It was extremely abundant, and a single sweep of a small hand-net repeatedly gave me more than 500 examples, and this was in a ditch where previously I could not obtain a single L. truncatulus.

All along the margins of the ditches the ground was covered by them, and they were found in numbers on the flooded ground when the flood waters had retired. On returning a month later to the same ditches I was unable to find a single example alive in the water. There had been dry weather since the flood, but early that morning heavy rain had fallen, and I found numbers of specimens of L. truncatulus out on the gravel of a path near the ditch, and these seemed to have crawled out of the grass when revived by the rain. At the roots of the grass, along the margin of the ditch, others were found in abundance. Some few shells were quite empty, but the majority contained the dried remains of the snail, which had shrunk far back into the spire of the shell. Most of these appeared to be quite dead, but were, however, merely dormant, for on placing them in water the tissues imbibed moisture and assumed their normal bulk, and after a few hours the snails had regained their full activity, and were seemingly none the worse for their prolonged desiccation. To test their power of resisting drought I collected specimens of L. truncatulus and placed them in an open vessel on a shelf in a dry laboratory, iu a position where the sunshine fell on them for an hour or so daily. I found that rather more than 50 per cent, withstood twenty-six days of this treatment, and some few revived after more than six weeks. That the snails can live on moist ground quite away from any quantity of water for considerable periods, is sufficiently proved by the fact that I have kept them alive for eleven weeks on moist grass and moss, even when infested with Fasciola hepatica.1

It is clear, therefore, that the species of snail under consideration, when left on the fields by the passing away of a flood, continues to wander and feed so long as the bottom of the grass remains moist. It is equally clear that the numbers so left are recruited from surrounding ditches and streams. A drought may render the snail dormant, but, unless too long continued, it revives at the first shower of rain. If there are fluke-eggs on the ground and water in puddles or ditches for them to develop in, the L. truncatulus will most certainly be infected with the larval forms of the liver-fluke; and owing to the habit this particular snail has of living so much out of water, either on the banks of ditches or further away towards the centre of the fields, if they are damp enough, the cercariæ will, on leaving their host, encyst on the grass in the places where they have the best chance of being transferred to the herbivorous mammals grazing on the ground. Having thus gained a suitable home they will attain the mature sexual condition, and reproduce their species by means of ova, thus completing the developmental cycle.

Man himself sometimes serves as host to the liver-fluke, and in this case the cysts are probably eaten with water-cress.

4. Growth of Sexual Fluke

From observations,1 which I need not describe here, it appears probable that six weeks elapse from the time of the entrance into the ultimate host before the fluke begins to produce eggs. During growth the body undergoes a very great change in form; the posterior part, which contains the reproductive organs, far outstrips the anterior part (figs. 24—26). The ventral sucker shares in some degree the greater growth of the hinder portion of the body; in the cercaria the suckers are of nearly equal size, and the same was the case in a young fluke 1·1 mm. in length. But in specimens 2—3 mm. long, the diameters of the oral and ventral suckers have usually the ratio of 1: 1·1, and in still larger examples 6—8 mm. long, the ratio is 1: 1·2, whilst in the adult the ratio is 1: 1·35, though there is much individual variety.

The smallest fluke I have yet found in the liver of a sheep is represented in fig. 23; the digestive tract, which in the cercaria was simply forked, has already acquired a large number of branches, though they are comparatively simple as yet. They subsequently attain a much more complex form, owing to the number of secondary branches. This branched intestine is highly characteristic, and affords the principal reason for separating the three species which constitute the genus Fasciola from the species forming the distinct genus Distoma, none of which have a branched digestive tract. It is usually supposed that the liver-fluke passes out of the sheep at the beginning of the summer, i.e., life lasts only about three quarters of a year. But I have shown elsewhere1 that the life of the liver-fluke may extend beyond one year, and have found both digestive and reproductive organ in full functional vigour in flukes at least thirteen months old; the oviduct was filled with eggs, and there was no indication of any exhaustion of the supply.

For an account of the economic aspects of the subject, including the discussion of preventive measures, I may refer to a paper in the forthcoming number of the ‘Journal of the Royal Agricultural Society.’

It gives me much pleasure to take this opportunity of thanking Dr. Acland for kindly permitting me to use the Sanitary Laboratory of the Oxford Museum for my experiments, and Professor Moseley for kindly placing apparatus, &c., in the Anatomical Department at my disposal.

1

A statement lias been published in several text-books, English and American, to the effect that Cercaria cystophora inhabiting Planorbis marginatus is the larva of Fasciola hepatica. This, of course, is erroneous, and the mistake appears to have been copied from an abstract in the ‘Zoological Record ‘for 1872 of a paper by Willemoes-Suhm. The suggestion really made in the original paper was that C. cystophora is the’larval form of Distoma lanceolatum. This specieá is known on the Continent as the small liver-fluke, and is far less formidable than the larger, F. hepatica, tlie true liver-fluke. It appears not to exist in England.

2

‘Mémoires de I’institut Genevois,’ vol. iii, p. 267.

3

‘Zeitschrift für wissentschafdiche Zoologie,’ 1873, vol. xxiii, p. 339.

4

‘Arch, für Naturgeschichte,’ 1875, p 191.

5

Abstract in ‘Archiv für Naturgeschichte,’ 1874, vol. ii, p. 423.

1

“Dell’ Adattamento delle specie all’ ambiente,” ‘Memorie dell’ Accademia delle Scienze dell’ Istituto di Bologna,’ serie iv, tomo ii, 1881, pp. 241, 327.

1

‘Archiv für Naturgeschichte,’ 1882, p. 80.

2

‘Proceedings of the Ashmoleau Society,’ 1857.

1

‘Die menschliclien Parasiteu,’ p. 531.

1

‘Transactions ‘of the New Zealand Institute, vol. v, p. 18.

2

‘Geographical Distribution of Animals,’ vol. i.

3

‘Brit. Ass. lieport,’ 1859, p. 127.

1

‘Roy. Agrio. Soo. Journ.,’ 1881, p. 7.

1

‘Beiträge z. Entw. d. Eiogeweidewürmer,’ p. 65.

2

‘Zeitschrift für wissentscbaftliche Zoologie,’ x, p.272.

1

Diesing (‘Wien. Sitzungsberichte,’ vol. xxxi, p. 248) has described the redia of Cercaria fallax as having two short processes situated anteriorly, and two, pf thrice the length, posteriorly. De Filippi (‘Memorie della Reale Accademia delle Scienze di Torino,’ Ser. ii, tomo xviii, p. 207) has described the redia of Cercaria tuberculata as having four lateral processes, two anteriorly and two posteriorly. I have met with a species which appears to be identical with Cercaria tuberculata, and in the redia recognised a collar. The same writer has figured (ibid., vol. xvi, pl. i, fig. 13) in the young redia of Cercaria coronata four processes; the two placed in front are slightly smaller than the two posterior, but otherwise they are drawn as if exactly alike. There can be no doubt that in all these cases the structures described as anterior lateral processes are simply the projecting borders of the transparent collar, seen perhaps in the flattened redia. From comparison with the descriptions given by these distinguished observers 1 was led in my first paper (‘Roy. Agricult. Soc. Journ.,’ 1881, p. 19) to similarly misinterpret the corresponding projections in the young rediæ of Fasciola hepatica.

1

Prof. Leuckart (‘Zool. Anz.,’ Oct. 9th, 1882) has also found these curious bodies (which had already been described by me in the ‘Journ. Roy. Agrie. Soc.,’ for April, 1881), and as he was unable to find any spines on the cuticle of his cercariæ, he suggests that the rod-like bodies are subsequently arranged in bnndles to form the spines of the adult fluke. But I have found the spines in the most mature cercariæ, and can say that these rod-shaped bodies have no connection with them, though I am unable to suggest any probable explanation as to their nature.

1

Sir Charles Lyell (‘Life,’ vol. ii, p. 212), in speaking of Madeira, says that Limnaeus truncatulus was unintentionally introduced by the Portuguese thirty years before, and has spread so widely that it is now found even in the pools and ruts in the roads, so that it must have a mode of distribution which needs investigation. It will be seen from the above account that the terrestrial habits of this snail, and its power of withstanding drought, are amply sufficient to explain its spread in Madeira.

1

‘Journ. R. A. S.’ 1881, p. 25.

1

Ibid., p. 26.