PHYLUM PLATYHELMINTHES 



287 



i.e., that different families may have become developed from 

 different families of Turbellaria altogether independently, some 

 of them appearing to be nearer the Rhabdocoeles, others nearer 

 the Polyclads, and others, again, nearer the Triclads, in the majority 

 of their characters. 



The remarkable life-history of the Digenetic Trematodes is, as 

 already pointed out, to be looked upon as a special form of alter- 

 nation of generations the alternation of a sexual with a pcedo- 

 (jenetic and partheno genetic generation (heterogeny). The sporocyst 

 and redia are to be regarded as intercalated stages as cercarise 

 which exhibit psedogenesis. The cercaria is the characteristic 

 larval stage of the Trematodes, and corresponds to the cysticercus 

 or cysticercoid of the Cestode. The most important difference 

 between these is in the presence in the former of an enteric cavity, 

 and its absence in the latter. There seems to be something to be 

 said in favour of the view that the enteric cavity of the cercaria is 

 represented by the frontal sucker of some scolices, and by the 

 rostellum of the majority. 



Between the adult Cestodes and the Trematodes an intimate 

 relationship is traceable. Caryvphyllceus (Fig. 206) is a Cestode 



Monogenetica 



Nemertinea 



Polycladida 



iTemnocephalea 

 Tricladida 



Rhabdocoelida 



Lower Coelenterata 

 FIG. 227. Diagram of the relationships of the Platyhelminthes (together with the Nemertinea). 



which, but for the absence of an enteric cavity and the want 

 of organs of adhesion at the posterior end, is not far distant 

 from the Trematodes ; and the same might be said of Gyrocotyle 

 (Fig. 207), Amphilina, and Archigetcs (Fig. 208). 1 The most 



1 It is possible, however, that in the last two forms we have to do with larval 

 Cestodes which have failed to reach the mature stage, and have undergone 

 a precocious development of the sexual apparatus. 



