PHYLUIVI PLATYHELMINTHES 



287 



i.r., that different families may ]ia\'e become devol()ped from 

 different families of Turbellaria altogether independently, some 

 of them appearing to be nearer the Rhabdoccelcs, others nearer 

 the Polyclads, and others, again, nearer the 'LViclads, in the majority 

 of their characters. 



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

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

 nation of generations — -the altei-nation of a scxt'al with a pado- 

 geneiic and 'parthcnof/enetic generation (heterogeny). The sporocyst 

 and redia are to be regarded as intercalated stages — as cercariiB 

 which exhibit psedogenesis. The cercaria is the characteristic 

 larval stage of the Trematodes, and corresponds to the cysticereus 

 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. Garyopliyllani,s (Fig. 206) is a Cestode 



Monogene+ica 



Nemertinea 



Polycladida 



iTemnocephalea 

 Tricladida 



Rhabdocoelida 



Polyzoa 



Lower Coelenterata 



Fxa. 227. — Diagi-am of the relationships of the Platyhelminthes (together with the Neincrtinca). 



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 Gyrorotylc 

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



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

 Cestodes which have failed to reach tlie mature stage, and liave luidei-gono 

 a precocious development of the sexual apparatus. 



