//. TREMATODA: DISTOME/E. 



'275 



appear in larval life, and not always then. Since not exposed to 

 danger of being pulled from the host, they possess either the oral 

 sucker alone (Monostomum) or this and a second ventral sucker, 



FIG. 236 -Development of D.stomum hepaticum. (From Korschelt-Heider after Leuck- 

 art ) A young larva; , sporocyst from the lung of Limncea; ( ,, older sporocvst 

 with redue; Z>, redia which has produced rediae internally ; E, redia with cer- 

 cariee; F, cercaria : G. encysted Distomum. ^j, eye spot; Z>, digestive tract; Z>r, 



open- 



and only rarely other attaching apparatus. They are markedly 

 separated from the Polystomes by their life history. The alterna- 

 tion of hosts necessitated by the endoparasitic life is complicated 

 by an alternation of generations (better heterogony, p. 145) with 



