There is no organ which is sheltered from the invasion of parasites: neither the brain, the ear, the eye, the heart, the blood, the lungs, the spinal marrow, the nerves, the muscles, or even the bones. Cysticerci have been found in the interior of the lobes of the brain, in the eye-ball, in the heart, and in the substance of the bones, as 93 well as in the spinal marrow. Each kind of worm has also its favourite place, and if it has not the chance of getting there, in order to undergo its changes, it will perish rather than emigrate to a situation which is not peculiar to it. One kind of worm inhabits the digestive passages, some at the entrance, others at the place of exit; another occupies the fossæ of the nose; a third the liver, or the kidneys. We may even divide parasites into two great categories, according to the organs which they choose: those which inhabit a temporary host, almost always instal themselves in a closed organ--in the muscles, the heart, or the lobes of the brain; those, on the contrary, which have arrived at their destination, and which, unlike the preceding, have a family, occupy the stomach with its dependencies, the digestive passages, the lungs, the nasal fossæ, the kidneys, in a word, all the organs which are in direct communication with the exterior, in order to leave a place of issue for their progeny. The young ones are never enclosed. Even the blood is not free from these animals, but there are few which lodge there, except during the act of migration. In Egypt, Dr. Bilharz observed a distome in the blood of a man (_Distoma hoematobium_); the _Strongylus_ of the horse has been long known, which causes serious injuries in its vessels (_Strongylus armatus_); as also the strongylus of the dolphin and of the porpoise (_Strongylus inflexus_), and the filaria of the dog (_Filaria papillosa_); and some are also found in the blood of many birds, of reptiles, batrachians, and fishes; so that there is no class of vertebrates which escapes. There are some which, like leeches, seek assistance from their 94 neighbours, but are content to snatch their food as they pass, and only attach themselves for a short time to the host which they despoil; they retain their fishing or hunting tackle, as well as their organs of locomotion. These parasites, which never take up their lodging on the host which nourishes them, have no sooner sucked his blood, or devoured his flesh, than they resume their independent life. They do not disfigure themselves, nor put on any special costume, like those which seek a permanent abode. Gluttony is not with them the only moving principle of existence; they do not forget what they owe to the world, and keep up an appearance which allows them at all times to present themselves afresh.