274 



PL A THELMINTHE8. 



fact that, at the time of sexual maturity, two individuals become fused 

 like Siamese twins (fig. 109). The young, described under the name 

 Diporpa, escape from the eggs and only unite later. Each has a ventral 

 sucker and a dorsal papilla. When they unite each of the pair seizes the 

 papilla of the other with the sucker, and then the two grow together so 

 that the male opening of one comes opposite the female opening of the 

 other. Polystomum integerrimum of the frog (fig. 234) affords a transition 



FIG. 235. A, Polystomum hassalli* (after Goto), from bladder of mud-turtle. 

 B, Acnnthocotyle verrilli* (after Goto), from skate. C, Gyrodactylus elegans (after 

 von Nordmann). 



to entoparasitism. At first it lives on gills of the tadpole, but at the time 

 of metamorphosis it is forced to leave this place and p<iss, by way of 

 the alimentary canal, to the urinary bladder. The TEMNOCEPHALID.E of 

 warmer regions are partially ciliated, and have from four to twelve 

 anterior tentacles and a posterior sucker. They are parasitic on Crustacea, 

 molluscs, and turtles, and are regarded by some as a distinct order. 

 American genera of PolystomeaB are Epibdella, Polystomum, Tristoma, 

 Sphyranura, Microcotyle. 



Order II. Distomeae (Digenea). 



The entoparasitic Trematodes occur largely in the digestive 

 tract and its appendages ; more rarely in blood-vessels, urogenital 

 organs, and ccelom of vertebrates and other animals. As inhabitants 

 of the dark they have, with few exceptions, lost the eyes, which 



