PLATYELMINTHES. 



179 



A. 



FIG. 99 A. TETRARHYNCUS. (From Ge- 

 genbaur ; after Van Beneden. ) 



A. Asexual state. 



B. Sexual stage with ripe proglottides. 



The above development is to 

 be regarded as a case of com- 

 plicated metamorphosis second- 

 arily produced by the necessities 

 of a parasitic condition, to which 

 an alternation of sexual and gem- 

 miparous generations has been 

 added. The alternation of gene- 

 rations only occurs at the last 

 stage of the development, when 

 the so-called head, without gene- 

 rative organs, produces by bud- 

 ding a chain of sexual forms, the 

 embryos of which, after passing 

 through a complicated metamorphosis, again become Cestode heads. 



In the case of Coenurus and Echinococcus two or more asexual 

 generations are interpolated between the sexual ones. It is not quite 

 clear whether the production of the Ta3iiia 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 cellulose?, which becomes the Tannin golium of 

 man. Similarly a Cysticercus infesting the muscles of the ox becomes the 

 Tainia mediocanellata of man. The Cystic&rcus pisciformis of the rabbit 

 becomes the Tcenia serrata of the dog. The Ccenurus cerebralis of the 

 sheep's brain becomes the Tcenin coenurus 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. Redoii 1 lias shewn by experiments 

 on himself that a Cysticercus (cellulose?) taken from a human subject 

 developes into Tcenia soliiim in the intestines of a man. Redoii took 

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

 months passed some proglottides, and subsequently the head of Tcenia 

 solium. 



Some important variations of the typical development are known. 



The so-called head or scolex may be formed without the intervention 

 of a cystic stage. In Archigetes (Leuckart, No. 227), w^liich infests, in 

 the Cysticercus condition, the body-cavity of various invertebrate forms 

 (Tubifex, etc.), the six-hooked embryo becomes elongated and divided into 



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



122 



