274 



PL A TIIELMINTHES. 



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. Eaclj has a ventral 

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

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

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

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



Fig. 2.3.5.— .4, Polystonnon hassaUi* (after Woto), from bladder of mud-turtle. 

 B, Aanithocoti/le verrilh* (after Goto), from skate. C, Gyrodactylus elegans (after 

 von Nordmann). 



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

 of metamorphosis it is forced to leave this place and pass, by way of 

 the alimentary canal, to tlie 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 crustaeea, 

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

 American genera of Polystomea' are Ej>iliclena, PoIi/stoiDitin, Tn'stoina, 

 Sphijranura, Microcotyle. 



Order II. DistomeEe (Digenea). 



The entoiiarasitic Trciuatoiles occur largely in the digestive 

 tract and its apiicndages; mure rarely in hlood-vessels. urogenital 

 orga-ns. and Cddom of vertebrates and otlier aninuils. As inhabitants 

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



