\ KK.MES 



I I ! 



there they develop into asexual individuals which eventually 

 produce, asexually, a second type of larva, the cercaria ( Fig. 101 ), 

 and this is the larva which develops into the adult worm. It 

 makes its way out of the snail and swims about in the water, 

 either becoming attached to blades of grass, upon which it cnc\ sts 

 itself, or entering the body of some aquatic animal as an inter- 



m"P 



en.E 



fprx. 





FlG. ioi. Distomum hepaticutn. Development. A, cili- 

 ated larva; B, sporoevst, containing redire in various 

 stages of development; C, redia, containing a daughter- 

 redia and cercaria? ; D, fully developed cercaria. b.op, 

 birth opening; ent, enteron of redia; eye, eye-spots; 

 gust, gastrula stage of redia ; germ, germ-cell to develop 

 into cercaria; mor, morula stage of cercaria; ass, <rsn- 

 phagus; or.su, oral sucker; pap, head-lobe of ciliated 

 embryo; ph, pharynx; proc, processes of redia ; vent.su, 

 ventral (posterior) sucker. (After Thomas, tmm Parker 

 and Haswell's Manual.) 



mediate host, becoming encysted there. Its final host, usually a 

 vertebrate, receives it either from the grass, or by eating the 

 aquatic animal within which it lies, and then the encysted cer- 

 caria escapes from its cyst and develops into a sexually mature 

 trematode. Thus there is here a marked alternation of genera- 

 tions, the sexual with the non-sexual, and three hosts are often 

 necessary for the complete cycle of development. 



