CESTODA. 



183 



from the same joints ; the proglottis becomes distended with 

 developing embryos. These ripe joints are liberated, the 

 embryos are set free by rupture, and the vicious circle may 

 recommence. Happily, however, the chances are 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 oft" 

 a scolex or head, which, in another host, buds off" the chain otproglottides. 

 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 

 (Ca-nnnis and Echinococcus) forms not one head only but many, each 

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

 exception 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 worm Tubifex rivulontni. 



Life Histories. 



ADULT, SEXUAL, OR TAPE-WORM 

 STAGE. 



NON-SEXUAL, PROSCOLEX, OR BLADDER- 

 WORM STAGE. 



1. Tcenia soliuui, in man, with four 

 suckers and many hooks. 



2. Tcenia saginata or tnediocanellata, 

 in man, with four suckers, but no hooks. 



3. Bothriocephalus latus, in man, with 

 two lateral suckers, but no hooks, with 

 less distinct separation of the proglottide.s 

 than in Tcenia. It may be u yards in 

 length. 



4. Tcenia echinococcus, in dog. 



5. Tcenia. ccemtrus, in dog. 



6. Tcenia. serrata, in dog. 



7. Tcenia cucuierina, in cat. 



8. Tcenia, elliptica, in dog. 



1. Cysticercus cellulosce, in muscles of 

 the pig. 



2. Bladder-worm in cattle. 



3. The ciliated, free-swimming embryo 

 becomes a parasite in the pike or burbot, 

 but without a distinct bladder-like stage. 



4. Eckinococcus veterinormn, in do- 

 mestic animals, and sometimes in man, 

 producing brood capsules, which give 

 rise to many " heads." 



5. Ccenuruscerebralis, causing sturdie 

 or staggers in sheep, producing numerous 

 "heads." 



6. Cysticercus pisifonnis, in rabbit. 



7. Cysticercus fa.sciola.ris, in mouse. 



8. Cysticercus, in dog-louse or perhaps 

 in flea. 



Zoologically the cestodes are interesting, on account of their life 

 histories, the degeneration associated with their parasitism, the pre- 

 valence of self-impregnation, and the complexity of the reproductive 

 organs. Practically they are of importance as parasites of man and 

 domestic animals. The medical student should consult Leuckart's 

 great work, "The Parasites of Man," part of which has been translated 

 by W. E. Hoyle (Edin., 1886). 



