Among the many generic divisions, one of the most interesting has received the name of _Trichodectes_; it contains twenty species, one of which lives on the dog, another on the cat, another on the ox; in a word, we discover a distinct species on each of the domestic mammals. It has been said that the _phthiriasis_ of the cat is occasioned by 71 the abundance of ricini. The trichodectes of the dog has lately attracted the especial notice of naturalists, and that from the following circumstances. There is no tape-worm more common in the dog than the _Tænia cucumerina_. But whence comes it? How is it introduced? This had been an enigma for many years, at the time when I dissected some dogs infested with _Tænia serrata_, in the Museum of Natural History at Paris. Together with the _Tænia serrata_, the number and age of which I knew beforehand, since I had myself _planted_ them, there were found in the intestines of one of the dogs some individuals of the _Tænia cucumerina_. My dogs had taken nothing but milk, and _cysticerci pisiformes_. Were there cysticerci of different kinds in the peritoneum of the rabbit? The veil is now withdrawn. We have just said that the dog harbours a tick known under the name of Trichodectes, and in this trichodectes lodges the Scolex, we might even say the larva of the _Tænia cucumerina_. Dogs, especially young ones, lick their hair continually, and it is by this operation that the young tænia is introduced. It is by a similar process that the horse introduces the eggs of the OEstrus which are hatched in its stomach. Many of these ticks live abundantly in birds, and multiply rapidly. The _Liothe pallidum_ lives on the cock, the _Liothe stramineum_ on the turkey, the _Philopterus falciformis_ on the peacock, the _Philopterus claviformis_ on the pigeon. It is to be observed that every bird can nourish many different kinds. Fig. 2 represents the tick which infests the sea-eagle, called Pygarg. [Illustration: Fig. 2.--Ricinus of the Pygarg.] 72 Fishes harbour crustaceans instead of ticks, and their number is not less considerable than on mammals and birds. These crustaceans have perplexed naturalists more than once, because they could only regard them as parasites. They live on the produce of cutaneous secretions, and if they improve, as do the ticks, the cleanliness of their host, they are not less useful in a hygienic point of view, for they prevent the accumulation of cutaneous productions.