The above development is to be regarded as a case of complicated metamorphosis secondarily produced by the necessities of a parasitic condition, to which an alternation of sexual and gemmiparous generations has been added. The alternation 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 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 Tænia 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 metamorphosis, 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 cellulosæ_, which becomes the _Tænia solium_ of man. Similarly a Cysticercus infesting the muscles of the ox becomes the _Tænia mediocanellata_ of man. The _Cysticercus pisciformis_ of the rabbit becomes the _Tænia serrata_ of the dog. The _Coenurus cerebralis_ of the sheep's brain becomes the _Tænia coenurus_ of the dog. The Echinococcus of man and the domestic herbivores becomes the _Tænia 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 Archigetes) 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[96] has shewn by experiments on himself that a _Cysticercus_ (_cellulosæ_) taken from a human subject develops into _Tænia 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 _Tænia solium_. [96] _Annal. d. Scien. Nat._, 6th Series, Vol. VI. 1877. 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), which infests, in the Cysticercus condition, the body cavity of various invertebrate forms (Tubifex, etc.), the six-hooked embryo becomes elongated and divided into two sections, one forming the head, while the other, with the six embryonic hooks, forms an appendage, homologous with the caudal vesicle of other Cysticerci.