76 



PROF. AGASSI2 S 



[TLA.TE XXXIV ALTERNATE GENERATIONS OF 



\) I STUM A. | 



animals, but is also often found upon fresh water 

 mollusks in the intestinal cavity,as well as upon their 

 abdominal organs around their liver and upon the 

 anterior portion of the mantle. And it has been 

 recently ascertained by Mr. Steenstrupp that these 

 are free animals at certain seasons of the year, and 

 that they undergo metamorphoses, of which we 

 had no conception before his observations were 

 published. 



Let me give the history of these various changes 

 to some extent. Wherever fresh water shells 

 occur, of the genus Lymneus and Planorbis, we 

 find around them in June a great many little worms 

 of which we have here a figure (Plate XXXIV, fig 

 Z) which has been described as Cercaria. They 

 move with great ease in curved motions describ- 

 ing constantly the figure 8 when moving. Within 

 are various organs whose functions are not fully 

 understood. Whether these branches lead to the 

 alimentary canal, or to one of the glandular ap- 

 pendages belonging to the alimentary system, is 

 not fully ascertained. There is another appara- 

 tus on the side, whose real physiological functions 

 are also not precisely known. But whatever may be 

 the anatomical structure of the&e animals, so much 



is known; that at a certain period of the summer 

 they move around the freshwater shells, and final- 

 ly fix themselves in great numbers upon the mu- 

 cus and burrowing into the mucosity of the ani- 

 mal until they are entirely surrounded in it, they 

 seem to move freely, but cast their tails under vio- 

 lent contortions. They are now surrounded by a 

 cyst of mucus in which they fall as it were into a 

 state of sleep, or into a state similar to that of the 

 pupa of Butterflies remaining motionless in the 

 cyst of mucus. (Plate XXXIV, fig. 1.) During 

 their rest in their little cavities they undergo chan- 

 ges. The part which represents a kind of head in 

 the Cercari^, is uow surrounded by a circle of folds. 

 This part becomes more and more prominent, and 

 when they leave their sacs they come out with a 

 sucker around the mouth, provided with little 

 hooks by which they can attach themselves. The 

 alimentary canal is very distinct, and in this form 

 we recognise a single Distoma. So that such a 

 sucking animal as that of (Plate XXXIV, fig. 2) is 

 finally transformed into a perfect Distoma, (fig. 4) 

 and this Distoma is finally found in the cavities of 

 the animal. After they have left the sac they 

 gradually penetrate into the abdominal cavity. 



The process of the metamorphosis of the Cerca- 

 ria lasts rather long. During the winter it is 

 scarcely perfectly accomplished. But now the 

 question is: How did such a Cercaria arise? 

 Where did it come from ? We have here an Intes- 

 tinal Worm (Plate XXXIV, figs. N, 0,) as it ap- 

 pears in the same fresh-water shell, before the Cer- 

 caria are observed, in one of which (Fig. P.) we 

 however notice small Cercaria. How are these 

 Cercarise formed? In June we find in the Worms 

 before mentioned, (Plate XXXIV, figs. N, 0, P,) 

 a great many little bodies distending them so as 

 nearly to cause their envelope to burst. If we trace 

 many of them, we may find in some which are 

 younger, that there are some with such bodies, 

 (Plate XXXIV, figs. Q, R, S, T, U,) and on close 

 examination these bodies are found to be eggs 

 which develope like those of other animals, and 

 finally give rise to little Worms, which grow to 

 the full size of Cercarias. These Worms (Figs. N. 

 and 0,) are therefore the mothers, or, as they have 

 been called, the nurses of the Cercaria, producing 

 a generation which is freely moveable, while they 

 themselves are constant parasites, and this free 

 generation is changed into Distoma. 



But this is not yet the whole of the process, 

 How were the Worms of figs. N, 0, formed ? Still 

 earlier in the season, another kind of Worm is ob- 

 served in the same animal in which those nurses 

 are noticed, and having some anatomical differ- 

 ences ; for instance, their stomachs being larger, 

 (Plate XXXIV., figs. C, and D.,) and having some 

 other slighter differences ; and in their body we 

 observe in early spring or latter part of the winter 

 a series of transformation of eggs or germs which 

 grow gradually to all the changes of germs (Plate 

 XXXIV, figs. E, F, G, II, I, K, L, M ;) and finally be- 



