[Illustration: Fig. 47.--_Amphistomum subclavatum_ of the frog.] [Illustration: Fig. 48.--_Polystomum integerrimum._] Independently of these worms, the vegetable-feeders afford lodging to some which are not their own. We have found in bats two tæniæ, both incompletely developed, and 206 occupying the digestive tube. One has a rostellum without hooks, like the tæniæ of the vegetable-feeders, the other has hooks like those of the carnivora. These cestode parasites are observed to be of two principal forms; the first vesicular, like the finger of a glove partly drawn inwards. They are always lodged in the midst of the flesh, or in a closed organ in the middle of a cyst; under this form the cestode worm is harboured by a host which is to serve as a vehicle to introduce him into his final host. He is a parasite on a journey; he is always agamous, and usually bears the name of cysticercus (Fig. 49). As to the second form, it is like a ribbon; it attains a great length, always occupies the intestine, attains its complete and sexual development, and lays an innumerable quantity of eggs which are disseminated with the evacuations. [Illustration: Fig. 49.--Cysticercus; _a_, upper part of the vesicle; _b_, place where the vesicle is about to separate; _c_, neck of the worm; _d_, the head, showing the suckers and the crown of hooks.] The rabbit harbours a cysticercus which has its final destination in the dog (a xenosite); but independently of this stranger, it gives hospitality to a special tænia in its intestines. This is its own worm, the _Tænia pectinata_, which is a nostosite. All the herbivora are in a similar case; the ox and the sheep possess a peculiar tænia of their own, besides those which they lodge for the sake of the 207 carnivora. The worms of the herbivora have particular characters by which they are easily known; they have no crown of hooks. The tænia of the wolf, which has often been confounded with the _Tænia serrata_, lives in the brain of the sheep, and produces a disease known as the "gid." It was formerly said that every animal has its enemy. We should rather say that each species has its parasites, and each parasite has its vehicle by which it is introduced. These tape-worms are found in all the vertebrate classes. An herbivorous animal usually serves as a vehicle, but it more frequently carries, besides its passengers, species which are peculiar to itself. As the carnivorous animal is not intended to be eaten like the herbivora, it cannot serve as a vehicle, and if by chance its muscles enclose some passenger, he has lost his way and that for ever.