Oct. 19, 1882] 



NATURE 



607 



as many as 500 specimens at a single sweep of a small 

 hand-net. The low-lying meadows near the river were 

 covered with the flood waters, and when these subsided 

 the snails were left scattered broadcast over the fields. 

 The snail is almost the smallest species of Limnasus ; the 

 variety which 1 found so abundantly was only a quarter 

 of an inch long when fully grown. Although it is a 

 water-snail it lives much out of water. My observations 

 have convinced me that the individuals left by floods on 

 the fields continue to live out of water so long as the 

 ground is moist. Their numbers are recruited by others 

 which crawl out of neighbouring ditches or streams. If 

 a drought occurs they become dormant, but unless too 

 long continued they revive with the first shower of rain. 



On discovering these snails I immediately started in- 

 fection experiments with them, and was at once successful. 

 The adult fluke in the liver of the sheep or other mam- 

 malian host produces vast quantities of eggs. So prolific 

 is it that I have estimated the number produced by each 

 fluke to be at least several hundred thousand. The eggs 

 pass with the bile into the intestines and are distributed 

 over the fields with the manure. If the eggs fall on to 

 wet ground, or are washed into a ditch, development 

 continues, and after a time, the length of which depends 

 upon the temperature, embryos are hatched out of the 

 eggs. For the purpose of my infection experiments I 

 obtained eggs from the livers of affected sheep, and kept 

 them in water until the embryos were hatched, and then 

 transferred them to vessels containing the snails to be 

 experimented upon. 



The embryo of the liver-fluke has the shape of an 

 elongated cone with rounded apex ; its average length is 

 125 mn., or about 1/200 of an inch ; its breadth at the 

 anterior end about one-fifth of this. The broader end or 

 base of the cone is always directed forwards, and in the 

 centre of this a short retractile head-papilla. The whole 

 of tha surface, with the exception of the head-papilla, is 

 covered with very long cilia, by means of which it swims, 

 with head-papilla drawn in, swiftly and restlessly through 

 the water. It is exceedingly active ; sometimes it goes 

 rapidly 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. The cilia are carried by an outer layer ot flattened 

 ectoderm cells arranged in five or six transverse rings 

 around the body, and are of the same length over the whole 

 of the surface. The first ring is composed of four cells 

 arranged around the papilla, and these are thicker than 

 the other outer cells, often forming projections at the side 

 of the embryo and resembling epaulets. Beneath these 

 ciliated cells is the body wall proper, and within this 

 are a number of delicate vesicular cells — the germinal 

 cells. Behind the head-papilla is a rudimentary digestive 

 tract. The body-wall contains, near the anterior end, a 

 double eye-spot, composed of crescentic masses of dark 

 pigment, placed with their convex sides turned towards 

 each other. 



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 discover its nature, and if not 

 satisfied darts off hastily again. But if the object be a 

 Limnaus truncatulus it at once begins to bore. Under 

 ordinary conditions the head-papilla of the embryo is 

 short and blunt, but as soon as the animal begins to 

 bore it becomes longer, conical, and pointed. The 

 embryo spins round on its axis, the cilia working vigor- 

 ously 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 reaches 

 five times its original length, 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. 



Toe embryo will not bore into all snails alike ; the 

 only other species which I have found it bore into from 

 without is Limnaus pereger, and even here the specimens 

 have always been such as were still very small. I have 

 found embryos enter certain other snails, such -a&Planorbis, 

 but only from eggs which had been swallo.ved by the snail 

 and had been hatched in the digestive tract. This differ- 

 ence seems to be due to an instinctive choice on the part 

 of the embryo, rather than to a greater softness of the 

 tissues in Linuiaus truiicatulus. The tissues of Physa 

 fontinalis, for instance, appear to be equally soft, but I 

 have found that if these two species are placed in a 

 small bulk of water with a very large number of embryos, 

 the Limnaei will be found on dissection to contain 

 fifty or more embryos, whilst the Physce will be entirely- 

 free from them. 



But although the instinct of the embryo seemingly prompts 

 it to enter the right snail, it does not teach it to discrimi- 

 natj between the different parts of the snail's body, for I 

 have found as many as a dozen embryos within the sub- 

 stance of the foot ot a single Limtians truiicatulus. Such 

 a poiition of course is not favourable to further develop- 

 ment of the embryos, which, thus gone astray, soon 

 perish. 



The natural place for the further development of the 

 embryo appears to be the pulmonary chamber, but they 

 may also be found in the body cavity. Once safely/ 

 lodged in the suitable locality, the embryo undergoes a 

 metamorphosis. It loses the external layer of ciliated 

 cells and changes from the conic rl to an elliptical shape. 

 The eye-spots usually become detached, but they, as well 

 as the head-papilla persist, showing the identity of the 

 young sporocyst —for so it must now be called — with the 

 embryo of the liver-fluke. The active embryo has de- 

 generated into a mere brood-sac, in which the next gene- 

 ration is produced. The sporocyst increases rapidly in 

 size, the round, clear cells contained within it increase in 

 number, partly perhaps owing to the division of the 

 germinal cells of the embryo, but also owing to a multi- 

 plication and subsequent detachment of the cells lining 

 the inside of the body wall. As growth proceeds the 

 contents of the sporocyst arrange themselves into round 

 balls of cells, the germs of the second generation These 

 germs increase in size, an 1 assume first an oval and then 

 an oblong shape, whilst a delicate cuticle is formed upon 

 the surface. At one end a number of cells are arranged 

 to form a spherical pharynx, which leads into a blind 

 digestive sac. A little behind the pharynx the surface 

 of the body is raised into a ridge, forming a ring sur- 

 rounding the anterior end, whilst near the opposite end 

 two short processes grow out. The germ has now become 

 a redia, as the brood-sac or nurse-form provided with 

 pharynx and intestine is called. The adult sporocyst 

 is sac-shaped and reaches the length of '6 mm. : it usually 

 contains one or two redia; nearly ready to leave, together 

 with two or three larger and several smaller germs. 

 There is another method of increase during the sporo;yst 

 stage, namely, by the division of a sporocyst into two 

 others by a constriction separating the original one into 

 two smaller ones. This method of multiplication, how- 

 ever, does not appear to be frequent in this species. 



■\Vhen the redia is ready to come forth, it breaks through 

 the wall of the sporocyst, and the wound caused by us 

 forcible exit immediately closes up, and the remaining 

 germs continue to develop. The injury done by the 

 parasites to the snails causes a serious mortality amongst 

 them, especially at the time the rediae begin to leave the 

 sporocysts, for the former are much more active than the 

 almost inert sporocysts, and migrate from the pulmonary 

 chamber into the other organs of the snail, and particu- 

 larly into the liver, upon which they feed. The redi<e 

 can be observed with the microscope, through the trans 



