The tænia, as well as the other tape-worms, is generally looked upon as an imprisoned parasite during the whole of its existence. This is a mistake; the last stage of the life of cestodes is a phase of liberty. The cucumerina, or, as we have proposed to call it, the proglottis, that is to say, the complete and sexual animal, is evacuated with the feces; and when we notice a dog leaving his dung upon the grass, it is not uncommon to see there worms which move like leeches, and whose white colour is in strong contrast with the mass which contains them. The duration of this last stage is very short, it is true; but it is, nevertheless, during this period of her life that the mother scatters the eggs which are to disseminate the species. We repeat that each animal has its parasites, and these in their turn are not always exempt from them. We have already cited some examples of this. Man has the dental system of a vegetable feeder; but, thanks to fire, which he alone knows how to produce and maintain, he eats flesh. It is by these means that he nourishes the solitary worm, which, by its crown of hooks, is a cestode belonging to the carnivora, and the _Tænia mediocanellata_ with the _Botriocephalus_, which are cestodes 213 peculiar to vegetable-feeders. As a feeder on vegetable diet he also harbours vesicular agamous cestodes, which are only found in him as passengers. The _Tænia serrata_ of the dog lives at first as a passenger in the peritoneum of the hare and the rabbit; and every one knows how greedily the dogs eat the viscera of these animals. The cat entertains another kind of tænia, and, as we may easily suppose, in its young state it lives as a passenger in the mouse or the rat. Who then has traced out for it this itinerary, and pointed out the way, the only one by which the parasite can hope to take possession of its proper abode? Evidently it is neither the tape-worm nor the cat. The plan for all these various species is marked out beforehand, and each animal as soon as it is born knows it without being taught. A Danish naturalist, Mons. H. Krabbe, has just finished a special work on cestode worms of the genus _Tænia_, and he remarks that there is no class in which these worms are so abundant as in that of birds. It is among the rapacious and carnivorous birds of this class that they are less abundant. Among mammals, the carnivora possess the greater number. This fact, as M. Krabbe remarks very rightly, seems to indicate that the cestodes of birds especially employ the inferior aquatic animals as their vehicles when in their incomplete state.