163 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 

 link between the Trematoda 

 „.„.„, ,^ , and Cestodes. Caryophyllwus 



Fig. 111.— Head of T'. cwnwrus seen from -r. t n i- t 



above, with circle of hoolis ; a-e, hooks ; mutoblllS Kudolphl llVCS m 

 all much enlarged . — After Siehold . .,,. c r-i ''i 



the intestines of Oyprmoid 

 fishes ; the young in a worm, TuMfex 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. simjjlicisfiima 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 

 Tmiia. 



Class I.— PLATYHELMINTHES. 



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

 head in the Gestodes often armed with hooks or suckers. A much-branched 

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

 cavity. {The digestim cavity is entirely wanting in the Gestodes.) Nervous 

 system representedby a cephalic ganglion, which in the Gestodes is absent. A 

 system 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 vitellogeiu ; often par- 

 ihenogenetic, accompanied by strobUation 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. Netthug 

 organs often present. Unisexual, rarely bisexual ; Strobi- 



