244 



PLATHELMIXTHES 



species in a common antrum witli) the male sexual opening. Tlie first part of 

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

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



secretion of the volk eel 





.d^t 





Fig, 2iq. — Polystomum 

 intcgerrimum (after Zeller). 

 Above two individuals in 

 copulation; below a single 

 animal enlarged, d, diges- 

 tive tract, distended with 

 blood; (/.q-, yolk duct; dsl, 

 vitellarium;/^/), genital pore; 

 /;, testicular "\-esicles; ;;;, 

 mouth; ph. pliarynx; cn\ 

 o\'ar}'; yze, openings of the 

 paired vagina'; )(, uterus; -', 

 vaginae; vd, vas deferens; .v, 

 vilello-intcslinal canal. 



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- 

 resfionds to \agina of the Polystomcs, used in copu- 

 lation. On the other hand, it may be homologous 

 with the vitello-intestinal cana! which connects in- 

 testine and \ itelline duct. In the Distomes copula- 

 tion occurs in the uterus, leading to sclf-impiregna- 

 lion. 



The Trematodes fall into two great grou]is, the 

 Polystomea?, largely ectoparasites, and tlieDistoniea?, 

 exclusively enloparasitic, the distinctions in para- 

 sitism being correlated with diil'crcnces in structure 

 and development. 



Order I. Polystomeae (Monogenea, Heterocotylea). 



Most Polystomcs 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 cntO]iarasiles. 

 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. 210). 

 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 

 jiroduce larvx, which soon after hatching have the 

 adult form (hence the name INIonogenea). 



Cyrfldactyhis, 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 thhd generation. ]\Iore striking 

 is Diplozooi! parajflxuiii, in which, at the time of 

 sexual maturity, two individuals become fused like 

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

 Difiorpa. esca]ie 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. PoJvslommii iiitcgcrrimiiin of 

 the frog (fig. 2ig) affords a transition to en'oparasil- 

 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 canal, 

 to the urinary bladder. The nionotrcnetic AsriDO- 



C0TYLID.1'. arc internal parasites in turtles, fishes 

 and molluscs. The Temnocephalid.e of warmer regions arc parasitic on 

 Crustacea, molluscs, and turtles. American genera of Polystomea; arc Epihdclla, 

 Polysloiniiin, Tristoiim, Spliyraiiura, Miirmolylc. 



