PLATYELMINTHES. 



217 



complicated metamorphosis secondarily produced by the neces- 

 sities of a parasitic condition, to which an alternation of sexual 

 and gemmiparous generations 

 has been added. The alter- A - 

 nation of generations only 

 occurs at the last stage of the 

 development, when the so- 

 called head, without generative 

 organs, produces by budding 

 a chain of sexual forms, the 

 embryos of which, after pass- 

 ing through a complicated 



(From 



FIG. 99 A. TETRARHYNCUS. 

 Gegenbaur ; after Van Beneden.) 



A. Asexual state. 



B. Sexual stage with ripe proglottides. 



metamorphosis, again become 

 Cestode heads. 



In the case of Ccenurus and Echinococcus two or more 

 asexual generations are interpolated between the sexual ones. 

 It is not quite clear whether the production of the Taenia head 

 from the cystic worm may not be regarded as a case of budding. 

 There are some grounds for comparing the scolex to the Cercaria 

 of Trematodes, cf. Archigetes. 



As might be anticipated from the character of the Cestode metamor- 

 phosis, the two hosts required for the development are usually forms so 

 related that the final host feeds upon the intermediate host. As familiar 

 examples of this may be cited the pig, the muscles of which may be 

 infested by Cysticercus celluloses, which becomes the Tcenia solium of man. 

 Similarly a Cysticercus infesting the muscles of the ox becomes the Tcenia 

 mediocandlata of man. The Cysticercus pisciformis of the rabbit becomes 

 the Tcenia serrata of the dog. The Ccenurus cerebralis of the sheep's 

 brain becomes the Tcenia ccenurus of the dog. The Echinococcus of man 

 and the domestic herbivores becomes the Tcenia echinococcus of the dog. 



Cystic worms infest not only Mammalian forms, but lower Vertebrates, 

 various fishes which form the food of other fishes, and Invertebrates liable 

 to be preyed on by vertebrate hosts. So far the Cestodes (except Archi- 

 getes) are only known to attain sexual maturity in the alimentary tracts of 

 Vertebrata. 



The rule that the intermediate host is not the same as the final host does 

 not appear to be without exception. Redon 1 has shewn by experiments 

 on himself that a Cysticercus (cellulosce) taken from a human subject 

 developes into Tcenia solium in the intestines of a man. Redon took 

 four cysts of a Cysticercus from a human subject, and after three months 

 passed some proglottides, and subsequently the head of Tcenia solium. 



1 Annal. d. Scien. Nat., 6th Series, Vol. vi. 1877. 



