226 



HIGH SCHOOL ZOOLOGY. 



generally have two suckers (genus Distomum), one round the mouth, 

 the other on the ventral surface. The eggs are very numerous 

 for only a very small number of them can meet with the com- 

 plicated conditions favourable to their complete life. They do not give 

 rise at once to the Distome form, but, by internal budding, to inter- 

 mediate broods, which differ from the adult in form, and are found in 

 some diflferent animal (Fig. 153). 



Fig. 153.— Developmental cycle of Distomumhepaticum,— The liver-fluke of sheep. 



(After Leuckart). 



a, the adult showing the position of the suckers, and the branched intestine ; 6, 

 an egg with oijerculum, and contained ciliated embryo, with some unconsumed food- 

 yolk.— This embryo gives rise to a " spoiocyst " in which " rediie," Mke c, are formed 

 by internal budding. The redia gives rise, also by internal budding, to larval distomes— 

 " cercaricB " rf,— which loose the tail and after encystment attain the adult form, a. 



13. Apart from the absence of the intestine, the Cestodes differ from the 

 Trematodes in the formation of chains by budding, each segment in the 

 chain or proglottis, resembling its neighbour, but differing from the 

 original or head-segment, by the absence of organs of lixatiou. The seg- 

 ments have sometimes more, sometimes less capacity for independent 

 life. The eggs formed witliin them do not at once develop into original 

 head-segments, but into larval bladder-like forms (Cysticerci) — which are 

 found in. some different animal from the host of the adult — and these 

 may, by budding, give rise to more than one head-segment. The adult 

 chains are found in the intestines of all the classes of Vertebrates ; the 

 cystic stages in the flesh, liver or brain of some animal, which serves as 

 food for the host of the adult chains (Fig. 154). Thus the tape-worms of 

 the carnivorous sharks pass through their cystic stages in the Teleosts, 



