DEVELOPMENT OF DISTOilEJE. 



129 



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, b) 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 Rediae provided with an 

 alimentary canal (fig. 115, d). 



These stages in the development of Distomum which are apparently 



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

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

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

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

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

 Th 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 Cercariae (fig. 115, e), 

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

 host, and, after the loss 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. 



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