228 



UNSEGMENTED WORMS 



many millions to one against the embryo becoming an 

 adult. 



The above history is true, mutatis mutandis, for many other tape- 

 worms. The embryo grows into a proscolex or bladder, which buds off 

 a scolex or head, which, in another host, buds off the chain of proglottides. 



HEAP 



Fig. 124. — Diagram of Ufe-history of TcBnia solium. 



First chapter : Tape-worm in man ; H., head ; PR., proglottides. Second 

 chapter : Free proglottis and egg-cases ; ut., uterus ; g.a., genital 

 aperture ; embryo within the egg-case. Third chapter : Within the 

 intermediate host, the pig ; H., hexacanth embryo ; p.sc, proscolex or 

 bladder-worm ; m., muscle of pig ; sc, scolex or head, everted in final 

 host. 



As it is virtually the same animal throughout, the life-history does not 

 include an " alternation of generations." It is doubtful, however, what 

 term should be applied to those cases in which the bladder-worm 

 (Ccenurus and Echinococcus) forms not one head only but many, each 

 of which is capable of becoming an adult tape-worm. The only known 

 eTtception to the fact that sexual tape-worms are parasites of Vertebrates 

 is Archigetes sieboldii, a simple cestode which is sexual within the small 

 fresh-water oligochaete Tubifex rivulorum. 



