It is especially upon fish that these worms are found, and we cannot hesitate to consider the greater part of them as true parasites. We have described a whole series of them which live upon marine fishes, especially on the barbel, the bass or sea-wolf, the halibut, the dab, and different species of gadidæ. A. E. Verril published last year the description of several kinds of American leeches, among which we see two which infest a fish (_Fundulus pisculentus_) of West River, near Newhaven. A large and beautiful species, which is known by the name of _Pontobdella_, is also found upon the Rays. A very skilful naturalist, Mons. Vaillant, has lately made these 113 animals the subject of study. Mr. Baird, in 1869, made known four new Pontobdellæ, one from the coast of Africa, two from the straits of Magellan, and one from Australia, found in one of the Rhinobatidæ. But the most interesting in every point of view are the Branchellions, which inhabit the electrical fishes known under the name of torpedoes, and which do not fear to choose an electric battery as a place of abode. These branchellions always attach themselves, as it appears, to the lower surface of the body, and not to the gills as has been thought; and they are distinguished from all their congeners by tufts of filaments along their sides, which have been compared to lymphatic branchiæ. Many naturalists have considered these curious worms worthy of attention, and have made many interesting observations upon them. One of the finest memoirs on this subject is that of Mons. A. de Quatrefages. We may here mention, in connection with their mode of life, that neither Leydig nor Quatrefages found globules of blood in their digestive cavity. The branchellions live on the mucous products of the secretions of the skin, and instead of being parasites, we may consider them as worms paying liberally for the room which they occupy in their host, by maintaining his skin in good condition. They ought rather to be classed among animals which render service to others; that is, among mutualists. In the fresh waters of Europe, a little leech-like animal, beautiful both in form and colour, fixes itself on carps, tenches, and other Cyprinidæ; this is the _Piscicola geometra_, which also lives on the _Silurus glanis_. They are sometimes found in such great numbers that 114 they form around the gills a kind of living moss, which at last kills the fish.