244 



PLATHELMINTHES 



species in a common antrum with) the male sexual opening. The first part of 

 the uterus is called the ootype because here the eggs and yolk cells are formed 

 into eggs (fig. 214) and enclosed in a shell with a lid or cover formed by the 

 secretion of the yolk cells. A second duct Laurcr's canal goes from the 



ootype to the dorsal surface. In the Distomes the 

 canal is rudimentary or lacking. It apparently cor- 

 responds to vagina of the Polystomes, used in copu- 

 lation. On the other hand, it may be homologous 

 with the vitcllo-intestinal canal which connects in- 

 tc-stine and vitelline duct. In the Distomes copula-^ 

 tion occurs in the uterus, leading to self -impregna- 

 tion. 



The Trematodes fall into two great groups, the 

 Polystomeoe, largely ectoparasites, and theDistomeae, 

 exclusively entoparasitic, the distinctions in para- 

 sitism being correlated with differences in structure 

 and development. 



sw 



FIG. 2ig. Polystomum 

 integerrimum (after Zcller). 

 Above two individuals in 

 copulation; below a single 

 animal enlarged, d, diges- 

 tive tract, distended with 

 blood; dg, yolk duct; dst, 

 vitellarium; gp, genital pore; 

 h, testicular vesicles; m, 

 mouth; pit, pharynx, m', 

 ovary; sw, openings of the 

 paired vaginae; u, uterus; 7 r , 

 vaginae; vd, vas deferens; x, 

 vitello-intestinal canal. 



Order I. Polystomeae (Monogenea, Heterocotylea). 



Most Polystomes live on aquatic animals usu- 

 ally fish, where they attach themselves to the gills. 

 Since they are exposed to more dangers, their adhe- 

 sive organs are stronger than in the entoparasites. 

 So while the anterior suckers are weakly developed 

 or absent, the hinder end bears sometimes only a 

 single sucker, but usually a large adhesive disc 

 armed with many suckers and hooks (fig. 219). 

 The transfer of Polystomes from one host to another 

 is easy and the life history is without complications. 

 The stalked eggs are attached near the mother and 

 produce larvae, which soon after hatching have the 

 adult form (hence the name Monogenea). 



Gyrodactylus, parasitic on the gills of the carp, 

 brings forth living young which, even before birth, 

 produce a new generation in their interior, and 

 these may contain a thnd generation. More striking 

 is Diplozoon paradoxum, in which, at the time of 

 sexual maturity, two individuals become fused like 

 Siamese twins (fig. no). The young, called 

 Diporpa, escape from the eggs and only unite later. 

 Each has a ventral sucker and a dorsal papilla. 

 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 integer riir.v.m of 

 the frog (fig. 219) affords a transition to en'oparasit- 

 ism. At first it lives on gills of the tadpole, but at 

 the time of metamorphosis it is forced to leave this 

 place and to pass, by way of the alimentary caral, 

 to the urinary bladder. The monogenetic ASPIDO- 



turtles, fishes 



COTYLID^E are internal parasites m 

 and molluscs. The TEMNOCEPHALID^: of warmer regions are parasitic on 

 Crustacea, molluscs, and turtles. American genera of Polystomeas are Epibdclla, 

 Polystomum, Tristoma, Sphyranura, Microcetyle. 



