PLATIIELMINTHES 183 



organized than the pporocjst, since they are provided witli 

 a mouth and an intestinal canal, and since the different 

 portions of the body, as well as its organs, appear to he 

 better differentiated (Fig. 89 C). The month is, in fact, 

 surrounded by a kind of suckei', Avhich enables the animal to 

 attach itself to the organs of the host. Moreover, the 

 pharynx e.xecutes swallowing movements, and the intestine 

 appears at some times more, at other times le.^s, filled ; 

 hence it is functional. This new generation has been called 

 by the name of Redicv (Fig. 89 D and E). 



As regards the origin of the Redias, there are two opposing views ; some 

 observers (Leucicaet, Schwaez) trace them directly to the germ cell?, 

 while others (Wagener, Bieheingek) maintain their origin from cells ol 

 the body-wall. Although Schwarz argues strongly for the one and 

 Bieheingek for the other mode of origin, yet this difference does not 

 seem to us to be important, for we have already seen that the parietal 

 cells and the germ cells are embryologically of the same origin. In a 

 portion of the cells of the body-wall even, a differentiation into separate 

 histological elements appears not to have taken place, and for this 

 reason they may continue to develop in the same way as the real germ 

 cells. In harmony with this view is the statement of Tho.mas, who de- 

 rives the EedifB from both the germ cells and the cells of the body-wall ; 

 if the sujiply of the former were exhausted, then the latter might take 

 . their place. 



In regard to the way in which the Ecduc (and later the Cercarice) 

 arise, Schwaez explains this process as corresponding to the cleavage of 

 the egg. The single germ cell divides and produces a 7nonila-like heap 

 of cells, from which the Eedia (or the Cercaria) arises. The germ cell 

 therefore corresponds to the egg, and this would thus be a case of jDar- 

 thenogenetic development (Leuckaet). The entire process of development 

 is therefore to be considered, not as an alternation of generations s. str. 

 (metagenei^is), but as heterocjony, as already suggested by Gkobben 

 {Literature on Cestoda, No. 4). 



When the Redite have attained the proper stage of 

 development, they abandon the sporocyst by rupturing its 

 walls. They migrate from the respii-atory cavity into the 

 other organs of the snail, especially the liver. Here they 

 increase in size, and there can soon be recognized in them 

 in turn .spherical masses of germ cells, from which arise 

 again, if the season be cold, — that is to say, in winter — Redifp 

 of nearly the same form as before (Fig. 89 E). If, on the 



