PHYLUM PLATYHELMINTHES 



273 



en 



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 

 cercariee of different Trematodes differ greatly, particularly with 

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

 tailless : such cercarise 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. 203, A) is vivi- 

 parous, and the remarkable phenomenon is observed that the 

 embryo (AJ, 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, con- 

 tains an oosperm and 

 a number of yolk- 

 cells. Usually there 

 is a stalk, and often 

 an operculum. In 

 general the develop- 

 ment appears to be 

 direct ; but Polysto- 

 mum passes through 

 a larval stage with 

 five rows of cilia, and in Diplozoon paradoxum, a parasite on the 

 gills of certain fresh-water fishes, in which there is also a ciliated 

 larval stage, the young animals do not become sexually mature 

 until two of them have permanently united with one another. 



Temnocephala produces relatively large eggs, stalked or sessile, 

 with a thick chitinous shell, enclosing a single oosperm and a mass 

 of yolk-cells, which later become fused into a continuous mass. 

 Segmentation results in the formation of an irregular, massive 

 blastoderm composed of cells of several sizes. In this collects 

 a rounded group of larger cells, in the middle of which a space 

 appears (Figs. 223, 224). The space (endoccele) increases greatly 

 in size, the boundary cells becoming spread out to form a thin 

 layer, and approaches the periphery of the egg. A part of the 



VOL. i. T 



FIG. 223. Section through the blastoderm of Temno- 

 cephala, showing early stage of endococle (en). 



