16 COMPARATIVE ANATOMY 



product of any single egg arriving at the condition of a mature 

 fluke are enormous, but where so many eggs are produced, and 

 where the larvae which survive multiply themselves so rapidly in 

 the body of the snail, it is nearly certain that some few cercariae 

 will make their way back again into their final host the sheep. 

 It may be difficult to imagine how such a complicated life- 

 history, involving residence of different forms in an intermediate 

 and a final host, can have been established in the course of 

 evolution, but the difficulty is lessened byan extended study of the 

 Trematoda, which shows that there is every gradation between 

 direct development without alternation of generations, such 

 as occurs in the monogenetic Trematodes, and the elaborate 

 alternation of generations in such a typical digenetic form as 

 the liver-fluke. 



Lastly, the anatomy of the redia throws light upon a question 

 of purely morphological interest. It has been shown that the 

 adult fluke has no body-cavity. In the sporocyst there is a 

 large body-cavity, but no gut, in the redia there are both body- 

 cavity and gut, and the relations of one to the other can be 

 satisfactorily studied. Now, what is this body-cavity ? It is a 

 large space which does not communicate with the excretory 

 system or with any other organs, but is lined by an epithelium, 

 which we have recognised as a germinal epithelium, because it 

 gives rise to the germ cells from which daughter rediae or 

 cercariae are developed. It is, in fact, a greatly developed 

 generative pouch, which has its own proper opening, the birth- 

 opening, to the exterior. The relations of this pouch to the 

 gut and other organs are just those of the ccelom in other 

 animals ; and we shall see, in studying the earthworm and other 

 types, that there is good reason to believe that the extensive 

 spaces which We call coelom are to be considered as having 

 arisen from the enlargement and extension of primitive genera- 

 tive sacs. It will be remembered in this connection that the 

 germ cells of the frog are developed from a part of the epithelium 

 lining the pleuro-peritoneal cavity or ccelom (vol. i. p. 1 16). We 

 may, therefore, call the body cavity of the redia, and also that 

 of the sporocyst, a coelom. In the sexual fluke we must regard 

 the cavities of the generative organs, the ovaries, testis, and 

 their accessory glands as the representatives of the coelom, 

 which, in this instance, retain their primitive characteristics. 



