HIGH SCHOOL ZOOLOGY. 227 



on whichthey feed. Through eating insufficiently-cooked beef, pork or 

 fish, man is liable to several forms of these parasites. 



Fig. 154.— Developmental cycle of Twnia serrata. (A species of tape-worm 

 which occurs ia the don). (After Leuckart). 



1, A young tape-wonn composed of the head, with hooks (a, b), and suckers, and 

 a chain of immature segments ; 2, a mature segment, the branched uterus of which is 

 full of ti-hooked shelled embryos, 3 ; these gain access to the liver of the rabbit, loose 

 their hooks, become encysted, 4, and iuvaginated at one end which gives rise to the head 

 of the future tape- worm, 5 ; G, a fully formed bladder-worm (Cj/sfrVcrcMS ^jfifyon/iis) ; 

 7, section of the head before its eversion ; r, the rostellum which carries the hooks ; s, 

 two of the suckers. 



MOLLUSCA. 



14. From the lowly-organised unsegrnented worms, wliich we 

 have been considering, to a sub-kingdom like the Mollusca, which 

 contains some of the largest and most highly-organised of the 

 Invei'tebrates, seems a very long step, and yet it is to the Vermes, 

 and not to any of the other sub-kingdoms, that we must look 

 for any resemblance to the Molluscan structure. 



