120 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 

 I link between the Trematoda 



Fig. 81._Head of T. cvnuru, seen from 

 above, with circle of hooks; a-|, hooks; mutaUUs Rlldolphi lives in 

 all much enlarged. After Siebold. . /-,- j 



the intestines of Cyprmoid 

 fishes ; the young in a worm, Tubifex rivulorum. 



Tetraryhnchus 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 Eud. 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 L PLATYHELMINTHES. 



More or less flattened worms, with the body usually unsegmented ; th 

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

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

 cavity. (The digestive cavity is entirely wanting in the Cesfodes.) Nercous 

 system represented by a double cephalic ganglion, with two or more nervous 

 cords. A system of vessels corresponding to the water-vascular system of 

 Echinoderms, but supposed to be mainly excretory in function. Monte- 

 cious, rarely bi-sexual. Ovaries differentiated into a germigene and vitel- 

 logene; often parthenogenetic, accompanifd by strobUation in the tape- 

 worms. When alternation of generations occurs by budding, the sexual ani- 

 mals 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. Bisexual, rarely unisexual; strohi 



