162 ZOOLOGY. 



in the young, and there are no suckers or hooks ; while 

 there is but a single set of male and female reproductive 

 organs situated in the posterior end of the body, which can 



be detached from the ante- 

 rior part of the body, form- 

 ing a proglottis. In fact, 

 this form is a connecting 

 * t link between the Trematoda 



Ffe. lll.-nearl of T. cvnvrm seen from and CestodeS. CaryOpliyttCBUS 

 above, with circle of hooks ; a e, hooks ; miltnTiil 1 !* "Rnrlftlnhi livpsj in 

 all much enlarged. -After Siebold. 



the intestines of Cyprinoid 

 fishes ; the young in a worm, Tubifex rivulorum. 



Tetraryhnclius is provided with four very long slender 

 extensile spiny cephalic processes or beaks. The young live 

 encysted in bony fishes, the adults occurring in the intestines 

 of sharks and rays. 



In Ligula the body is ribbon -shaped, not jointed, with a 

 series of sexual organs, and there are no suckers, and some- 

 times no hooks. L. simplicissima Rud. lives in fishes and 

 amphibians, and attain maturity in the intestines of water- 

 birds, which feed on the former animals. This genus con- 

 nects the simpler tape-worms with Bothriocephalus and 

 Tcenia. 



CLASS I PLATYHELMINTHES. 



More or less fattened worms, with the body usually unsegmented ; the 

 7tcftd in the Cestodes often armed with hooks or suckers. Simple or branched 

 (Turbellaria) or forked (Trematoda) digestive tract, but no general body - 

 cavity. (The digestive cavity is entirely wanting in the Cestodes.) Nervous 

 system represented by a cephalic ganglion, which in the Cestodes is absent. A 

 si/xtem of vessels corresponding to the water-vascular system of Echinoderms, 

 but supposed to be mainly excretory in function. Monoecious, rarely bi- 

 sexual. Ovaries differentiated into a germigene and vitellogene ; often par- 

 thenogenetic, accompanied by strobilation in the tape-worms. When alter- 

 nation of generations occurs by budding, the sexual animals are united with 

 their nurse or a sexual form into a polymorphic colony. 

 Order 1. Turbellaria. Flattened ovate worms, with a nervous gan- 

 glion in the head ; usually eye-specks ; body externally cili- 

 ated, with a much-branched digestive canal. Nettling 

 organs often present. Unisexual, rarely bisexual ; strobi- 



