180 



PLATHELMINTHES 



excretory pore is at the hinder end of the body and the genital pore is 

 in the ventral surface or on the margin of the body. Special sense 

 organs are with rare exceptions absent. The life history is known of 

 but very few trematodes. The young animal passes out of the host in 

 the egg, on leaving which it is usually a ciliated larva called the 

 miracidium; this larva seeks an intermediate 

 host, often a mollusk, in the body of which it 

 passes its larval life, living actively or encysted 

 in some of the compact tissues. It here 

 passes through one or more metamorphic stages 

 which finally result in the production of young 

 individuals, called cercariae, each of which has 

 usually a locomotive tail (in Bucephalus two) 

 (Fig. 294), which sometimes seek still another 

 intermediate host, and are destined to develop 

 into the adult worms. If, now, the host harbor- 

 ing these larval worms be devoured by the final 

 host, the young worms pass into the intestine or 

 some other hollow organ of the latter and become 

 mature. Tailless cercariae, which are quite numerous, do not leave the 

 first intermediate host, but pass directly with it into the final host. 

 Digenetic trematodes are common parasites, the adult worms being 

 found in almost all vertebrate animals. The larval worms are usually 

 found in snails, small fish, and other small animals. Some of them are 

 dangerous parasites to man and his domestic animals. The order con- 

 tains about 2,000 species, grouped in 2 suborders. 

 Key to the suborders of Digenea: 



tti Mouth median in position 1. Gasterostomata 



Oj Mouth anterior in position 2. Pkostomata 



Suborder 1. GASTEROSTOMATA. 



Mouth in the middle of the ventral surface; intestine sac-like and not 

 bifurcate: 1 family. 



Family BUCEPHALIDAE. 



But one sucker present which is at the front end of the body; mouth 

 in the middle of the ventral surface; intestine sac-like and not bifurcate; 

 genital pore at hinder end: in intestine of fish, in both fresh and salt 

 water; the larval forms in bivalve mollusks; 2 genera. 



Bucephalus von Baer {Gasterostomum von Siebold). Yolk glands 

 in two distinct groups of follicles; male genital pore at end of a papilla: 

 numerous species. 



