We find from official reports that the French soldiers often suffered, during the campaigns in Egypt and Algeria, from an aquatic leech (Hœmopis vorax), which attacked the mouth and the nostrils, and did not respect man any more than horses, camels, and oxen. The leech discovered by Dr. Guyon under the eyelids and in the nasal fossæ of the crab-eating heron of Martinique, is probably a monostomum, and not one of the hirudinidæ. Leeches have also been found on turtles under the name of Eubranchella Branchiata. Say saw one on a chelonian, and others on tritons and frogs.
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 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
they form around the gills a kind of living moss, which at last kills the fish.
There are different leeches which inhabit invertebrate animals. Rang mentions a little creature of this kind in Senegal, living as a parasite upon the respiratory apparatus of an anodont. Gay discovered in Chili one of the Hirudinidæ in the pulmonary sac of an Auricula, and another on the branchiæ of a crab (Branchiobdella Chilensis). Mons. Blanchard has noticed a malacobdella in the branchiæ of the Venus exoleta; and it was known in the last century that the Mya truncata of our coast also lodges a malacobdella which lies always under the foot of the animal. This is the hirudinean of which we have spoken above, which is allied transitionally to the trematoda.
Together with the Hirudinidæ, we find very small worms, transparent, bristling with daggers and spikes of every form, which are found everywhere in fresh water. They are known by the name of Naïs. They are so completely transparent that we can see the action of all their organs through the substance of the skin. They have been the subject of several remarkable works.
They live freely among the leaves of Lemna and other aquatic plants; but there is one species much more restricted in their habitat than the others; these seek assistance from the Lemneæ, and live at their expense. It is because of this kind, of which the genus Chœtogaster has been formed, that we mention them here. Their long bristles are veritable halberds, which they employ with astonishing skill, both in attack and defence.
Among free parasites are found many very important