//. TEEM AT OD A: POLTSTOME^E. 



273 



enclosed in a shell with a lid or cover. A second duct Laurer's 

 canal goes from the oviduct to the dorsal surface. In many 

 Polystomeae the canal is double (fig. 234, sw) and is connected 

 with copulation, but in the Distomes it is rudimentary or lacking 

 and the opportunities are present for 

 self-impregnation. Many Trematoda 

 have a third canal the vitello-intes- 

 tmal duct leading to the digestive 

 tract. 



The Trematodes fall into two great 

 groups, the Polystomeae, largely ecto- 

 parasites, and the Distomeae, exclu- 

 sively entoparasitic, the distinctions in 

 parasitism being correlated with differ- 

 ences in structure. 



Order I. Polystomese 

 (Monogenea, Heterocotylea). 



Most Polystomes live on aquatic 

 animals usually fish, rarely Crustacea, 

 where they attach themselves especially 

 to the thin-skinned and richly vascular 

 gills. Since as ectoparasites they are 

 exposed to more dangers, their adhesive 

 organs are stronger than in the ento- 

 parasites. So while the anterior suckers 

 are weakly developed, in some cases 

 absent, the hinder end bears sometimes 

 only a single sucker, but usually a large 

 adhesive disc armed with many suckers 

 and hooks. The transfer of Polystomes 

 from one host to another presents few 

 difficulties and hence the life history is 

 without complications. The stalked 

 eggs are attached near the mother and 

 produce larvae, which soon after hatch- 

 ing have the adult form (hence the 

 name Monogenea). 



The American species have been scarcely touched, hence most of our 

 knowledge is of European species. Gyrodactylus, parasitic on the gills of 

 the carp, is interesting, since it brings forth living young which, even 

 before birth, produce a new generation in their interior. More striking is 

 Diplozoon paradoxum (gills of Cyprinoids), which owes its name to the 



234. Polystonnim 

 rimum. (After Zeller.) Above 

 two individuals in copulation ; 

 below a single animal enlarged, 

 d, digestive tract, distended 

 with blood; rfgr, yolk duct; dst, 

 vitellarium; gp, genital pore: h, 

 testicular vesicles ; m, mouth; 

 p/j, pharynx ; ov, ovary ; s?, 

 openings of the paired vaginae ; 

 M, uterus; v, vaginae; rd, vas 

 deferens; x, vitello-intestinal 

 canal. 



