170 COMPARATIVE ANATOMY CHAP. 



in its interior a certain number of cell spheres which have developed 

 out of the germ cells or parthenogenetic eggs. Instead therefore of 

 the young animals, which are called Sporocyst s (C), developing further 

 into new Distonw, they not only remain at a low stage of development, 

 but they even suffer a considerable degeneration. It seems as if early 

 reproduction were the only function of this Sporocyst. The cell spheres 

 which they contain actually develop again into new germs, which 

 leave the Sporocysfs body as Redicc (D, E), the Sporocyst finally disinte- 

 grating, and thus never developing into a fluke. The Redicv which 

 have become free, being developed out of the parthenogenetic eggs of 

 the Sporocysts, reach a higher stage of development than their mother. 

 They have at the front end of their body a sucker-like formation, and 

 also a pharynx, a simple intestinal tube, and a birth aperture behind 

 two blunt processes. Here also we find numerous germ cells between 

 the intestine and the body wall ; these begin early to develop, i.e. to 

 divide. The Redice in fact, like the Sporocysts, do not grow into flukes : 

 they first creep about in the respiratory cavity of their host, Limnccvs 

 truncatulus, and then penetrate into its liver. The germs which develop 

 in them again become He-dice, which pass out by the birth aperture 

 and are parasitic in the liver with their parents. This second genera- 

 tion of liedice (F) again reproduces itself parthenogenetically. From 

 their germs, however, at a warm time of year are developed, not Redid' 

 again, but Iarva3 which are called Cercarice (G). These Cercarice already 

 show the structure of a young Distoma ; they are flat, have oral and 

 ventral suckers, a pharynx and a forked intestine, a double ganglion 

 joined by a transverse commissure in front of and above the pharynx, 

 both the principal branches of the excretory system, and besides these 

 and this is characteristic of the Cercarice a movable caudal 

 appendage. The Cercarice leave the mother body, i.e. the Redicc, by the 

 birth aperture, forsake their host, and reach the water, in which they 

 swim about for a time by means of their tail. They then settle upon 

 grasses or plants growing in water in flooded meadows, lose their tail 

 and become encysted by the help of the secretion contained in two very 

 large glands which lie laterally in the body. In this encysted condition 

 (H) they can remain a long time, and can withstand desiccation. They 

 reach the sheep's intestine if occasion offers in the fodder, and there 

 presumably the cyst is dissolved and the young Distoma enters the 

 liver through the bile ducts. Such a young Distoma, with the first 

 branchings of the intestine, is depicted in Fig. 119, /. 



The life-history of other endoparasitic Irematoda runs, as far as 

 we know, the same course. The free -swimming Cercuria, however, 

 often enters into a second intermediate host, in which it becomes 

 encysted and loses its tail. This second host is generally an inverte- 

 brate animal. The encysted Cercuria enters the body of the final host 

 (generally a vertebrate animal) when the second intermediate host is 

 eaten by the latter. 



Several different generations, therefore, follow each other in a 



