This solitary worm is introduced by means of beef, and the cysticercus, during its abode in the cow, manifests already the peculiar characteristics which enable us to recognize the species, that is to say, no crown of hooks, but four suckers, and in the middle of them, some blotches of pigment. Leuckart fed a calf with eggs of this tænia, and at the end of seventeen days, the animal died of acute miliary tuberculosis, produced by the great abundance of cysticerci. This second species, which had been always confounded with the preceding, and which is nevertheless the more common, has therefore a different origin from the _Tænia solium_. Observations made quite recently in the north of Africa demonstrate this. Great difficulty had sometimes been felt in explaining the presence of the tænia in persons who had not eaten pork. This embarrassment arose from the confusion of the two species, and this confusion is the more easy as the head of the colony must necessarily be found in order to distinguish them. Scharlau, at Stettin, found tæniæ in seven children who had been fed, on account of anæmia, with raw meat. The tæniæ were those of this species. We have ourselves found them in children to whom the use of raw meat had been prescribed. We do not think it necessary to speak here of a third species of tænia (_T. nana_), which also lives at our expense, but which has been 220 hitherto found only in Egypt. We know perfectly well the itinerary of the _Tænia serrata_ of the dog, which is so abundant, that there are few of these animals that do not enclose some and even many of them. There are few except lapdogs which do not harbour them. We can easily assign the reason. Every tænia, like every animal, has its eggs; each plant has its seeds. These eggs are laid by the mother in the most favourable condition for the development of her progeny. The dog deposits its dung on the grass rather than in any other spot, because the eggs of its tænia, which are destined to the rabbits or hares, will have greater chance of arriving at their destination than if they were exposed on the bare earth, or in the water. Their prodigious number is calculated according to the chances of their arriving safely. The egg, when introduced into the stomach of the rabbit, is rapidly hatched in this organ under the action of the gastric juice, and the embryo which is produced from it seeks its hiding-place in the midst of the tissues which surround it; it bores into them, and establishes itself in the folds of the peritoneum. Then, once in its resting-place, it barricades itself, and waits patiently for an opportunity of introducing itself into the stomach of the dog.