v PHYLUM PLATYHELMINTHES 277 



primitive endodeim — among which a cavity appears. But in 

 some Rhabdocceles no definite intestinal epithelium is developed, 

 and the syncytial mass which represents it is only to be dis- 

 tinguished from the surrounding mesodermal syncytium by its 

 enclosing the remains of the yolk. No metamorphosis is known 

 to occur. 



An account has already been given (p. 244, Fig. 190) of the 

 development and metamorphosis of the Liver-Fluke (Fasciola 

 hepcUiea) which may be looked upon as typical of the Digenetic 

 Tiviiiatodi\s in general. There is thus to be recognised in the 

 Digenetic Trematodes an alternation of generations comparable to 

 that which has been described as so general in the Ccelenterata. 

 In the Trematoda, however, it is to be observed, it is an alterna- 

 tion of a sexual, not with an asexual, but with a parthenogenetic 

 generation (the sporocyst), the ova of which develop into a second 

 parthenogenetic generation (the rediae) ; and these finally produce 

 larvas (the cercariae) capable of developing into the sexually 

 mature form. The term heterogeny is applied to a life-history 

 of this kind, in which several distinct generations succeed one 

 another in a regular series. 



In some of the Distomidae the eggs, instead of becoming free as 

 in the case of the Liver-Fluke, are taken directly into the digestive 

 canal of the intermediate host, and there hatched out. The 

 sporocyst stage may take the form of a branching tube in the 

 interior of which cercariae are developed — the redia stage being 

 omitted. Sometimes the sporocyst becomes directly developed 

 into a redia instead of giving rise to a generation of the latter by 

 such a process of internal development as that described in the 

 case of the Liver-Fluke. The cercariae in most Digenetic Trema- 

 todes only develop further if they succeed in establishing themselves 

 in a second intermediate host instead of merely becoming encysted 

 on the surface of herbage, as in the case of the Liver-Fluke. The 

 cercariae of different Trematodes differ greatly, particularly with 

 regard to the nature of the tail. In some forms the cercaria is 

 tail-less : such cercariae do not become free, but are taken directly 

 — with the intermediate host in which they have been developed — 

 into the digestive canal of the final host. 



Among the Heterocotylea, Gyrodactylus (Fig. 200, A) is vivi- 

 parous, and the remarkable phenomenon is observed that the 

 embryo (A'), while still within the body of the parent worm, 

 develops another embryo (h 2 ) in its interior, and this again 

 develops a third. The rest of the Heterocotylea deposit eggs 

 each of which, within a chitinous shell, contains an oosperm and 

 a number of yolk-cells. Usually there is a stalk and often an 

 operculum. In general the development appears to be direct ; 

 but Polystomum passes through a larval stage with five rows of 

 cilia, and in Diplozoon paradoxitm, a parasite on the gills of certain 



