II. TREMATODA: DISTOME.^. 



275 



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

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

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



^^'■^x^^^^^. 



Fig. :W1.— Development of Distonuim liepaticum. (From Korsfhelt-Heider after Leuck- 

 iirt.) .4, young Itirva; B, sporocyst from the lunfc of Liwim-a; a, older sporocyst 

 witli redia'.; D, redia whinli has produced redise internally ; E, redia with cer- 

 curiie; J',- ceroaria ; G, encysted JJistomum. .4, eye spot ; ZJ, digestive tract; I)i\ 

 glands; Ei\ ciliated lobules and main trunks of excretory system ; G, birth open- 

 ing ; Kz, germ cells; N^ nervous system. 



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 



