DEVELOPMENT OP DISTOME^. 



IL'D 



Further it cannot be doubted that the development of the 

 Distomese, which has hitherto been regarded as a case of alternation 

 of generations really represents a form of heterogamy allied to 

 psedogenesis. After the completion of the segmentation and em- 

 bryonic development, the ciliated embryos (fig 115, a, 6) pass from 

 the egg into the water, where they swim about, and eventually make 

 their way into the body of a Snail, in which they give rise to sac-like 

 or branched Sporocysts (fig 115, c) or to Redire provided with an 

 alimentary canal (fig. 115, d). 



These stages in the development of Distomum which are apparently 



d 



a. 



fh 



FIG. 115. Developmental history of Distomum (in part after R. Leuckart). a, Free- 

 swimming ciliated embryo of the liver fluke. 6, the same contracted, with rudiment of 

 alimentary canal D ; and aggregations of cells ; On, rudiment of genital gland ; Ex, 

 ciliated apparatus of the excretory system. c, Sporocyst, which has proceeded from 

 Distomum embryo, filled with Cercarife C ; B, spine of a Cercaria. d, Redia with pharynx 

 PA ; alimentary canal, D ; Ex, excretory organs ; C, contained Cercariae. e, free Cercaria ; 

 S, sucker ; D, gut. 



comparable to larvae, produce by means of the so-called germ granules 

 or spores a generation of offspring known as Cercarise (fig. 115, e), 

 which become free, and then make their way into the body of a new 

 host, and, after the bss of the oral spine and caudal appendage, encyst 

 (fig 115 /}. Hence they are carried into the body of the permanent 

 host to develop into the sexual adult form. 



