In the case of Cœnurus 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 Cœnurus cerebralis of the sheep’s brain becomes the Tænia cœnurus 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.
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.
The embryo of Tænia elliptica similarly gives rise to a Cysticercus infesting the dog-louse (Trichodectes canis), without passing through a vesicular condition; but the caudal vesicle disappears, so that it forms simply a scolex. These cases may, it appears to me, be probably regarded as more primitive than the ordinary ones, where the cystic condition has become exaggerated as an effect of a parasitic life.
In some cases the larva of a Tænia has a free existence in the scolex condition. Such a form, the larva of Phyllobothrium, has been observed by Claparède[97]. It was not ciliated, and was without a caudal vesicle; and was no doubt actively migrating from an intermediate host to its permanent host.
Scolex forms, without a caudal vesicle, are found in the mantle cavity of Cephalopoda, and appear to be occupying an intermediate host in their passage from the host of the cystic worm to that of the sexual form.
Archigetes, already mentioned, has been shewn by Leuckart (No. [227]) to become sexually mature in the Cysticercus state, and thus affords an interesting example of pædogenesis. It is not known for certain whether under normal circumstances it reaches the mature state in another host.