When we consider the numerous encysted and agamous nematodes, which are found in the different orders of mammalia, birds, reptiles, batrachians, and fishes, there is little doubt that all these beings are only migratory parasites, which pass together with their hosts into the animal to which they are destined. They are found, like ascarides, in animals of all classes. Some are to be met with in all the organs--the brain, the eye, the muscles, the heart, the lungs, the tracheal artery, the frontal sinus, the digestive tube, the skin, and even in the blood. Sometimes the two sexes live under the same conditions; sometimes the male is dependent on its female, or else one generation is parasitical, and the next is independent. There is a great diversity with respect to development. Some nematodes, like trichinæ, are developed so rapidly, that the embryos are already perfect in the egg before it has quitted its mother. Others, like the ascarides lumbricoides, lay eggs, in which the embryos do not appear till several weeks or many months after they have been laid. Between these two extremes we find all the intermediate degrees. Diezing, who has done more for systematic helminthology than any 234 other naturalist, brought together, under the name of _Agamonema_, all the migratory agamous nematodes which wait for the opportunity of entering their final host. Diezing had kept himself quite independent of the discussion by fixing his attention exclusively on form, without taking account of migration and digenesis. One of these agamonemata, lodged in the midst of a pediculated cyst on the vagina of a bat (the little horse-shoe), was probably a worm that has lost its way; if not, we must admit that these mammals become the prey of some carnivorous animal. But what carnivore can habitually feed on the cheiroptera? There are but few fishes, either in fresh or salt water, which do not enclose in the folds of their peritoneum, especially round the liver, cysts full of these agamonemata. We see in some of the nematodes examples of migration which are quite peculiar to them. Some of these worms are always free, others free at one part of their life only, others migrate from one animal to another; others again from one organ to another. The _Ascaris nigro-venosa_ of the frog lives sometimes in the lungs, at others in the rectum or quite out of the body in damp earth. The _Filaria attenuata_ lives in the rook (_Corvus frugilegus_), and it is said that it becomes sexual in the intestines of the same bird. These worms are usually very tenacious of life; many of them can, it is said, be dried for weeks, months, or years together, and return to life as soon as their organs are moistened. Their eggs resist even the action of alcohol and the most active chemical agents, and eggs that had been prepared for the microscope, and had served for many years 235 the purposes of study, have been known to produce young ones as if they had been just laid.