Such are the Ichneumons, beautiful winged insects, which perfidiously insert their eggs in the body of living caterpillar, whose internal part serves at the same time for a cradle and for food. The young larva devours organ after organ, beginning with the least important, till the last serves for the formation of the last members of the winged insect.

More unfortunate are those which are kept under the bolts and bars of their host from their early youth to mature age; they have no participation in the great banquet of life, except it be in the pleasures of the table and of love. We also find some parasites which occupy different organs in the same animal, and which have different sexual attributes according to the situation which they inhabit. We know some which are hermaphrodite in the rectum or in damp earth, and whose young ones, having the sexes separate, live as parasites in the lungs.

Parasites are not usually reproductive in the animal which they inhabit. They respect the hearth which shelters them, and their progeny are not developed by their side. The eggs are expelled with the feces, and sown at a distance for other hosts.

Parasites may be divided into several categories. We may bring together in the first of these, a certain number of animals, which, without being true parasites, seek for a place of shelter, and, either on account of their wretchedness or their misery, require this protection in order that they may live.

In the second category, we may place those which live at complete liberty, and only require for their sustenance the superfluities of their neighbours; they take

great care of the skin of their host, and use it sparingly. Some also are found which cannot live without assistance, but repay it with some service. Often, indeed,

they associate with their host, and live on a footing of perfect equality with him; and besides these are found associations in which equality is by no means recognized, and where labourers or even slaves perform the work disdained by their masters.

In the last category we shall arrange true parasites, which take both their lodging and their food. And here, again, we shall meet with three distinct subdivisions.

The first includes those which travel from one hotel to another before they arrive at their destination; to-day they lodge in a prawn, to-morrow in a gudgeon, then in some fish which preys upon others, as the perch or the pike. These are nomadic parasites, which do not stop or think of family life until they have found the hotel for which they are destined.

Sometimes the parasite gets into a wrong train, and not being able to retrace his steps, he remains at a station where no other train will take him up. He is condemned to die in a waiting-room.

In the last subdivision, we have parasites that have arrived at their destination, occupying themselves in future only with the joys of a family.

Thus we find some which are really at home, and others which are on their journey, sometimes on the right road, and at others, wandering and lost in an alien "host." The former are autochthonic parasites, the others are foreigners. We may say that each animal species has its proper parasites, which can live only in animals which have at least more or less affinity with

their pecular host.

Thus the Ascaris mystax, the guest of the domestic cat, lives in different species of Felis, while the fox, so nearly resembling in appearance the wolf and the dog, never entertains the Tania serrata, so common in the latter animal.

The same host does not always harbour the same worms in the different regions of the globe which it inhabits. This relates both to the parasites of man, and to those of the domestic animals. Thus the large tape

worm of man, which naturalists call Bothriocephalus, is found only in Russia, Poland, and Switzerland. A small tape-worm, Tania nana, is observed nowhere. except in Abyssinia; the Anchylostoma is known at present only in the south of Europe and the north of Africa; the Filaria of Medina, in the west and the east of Africa; the Bilharzia, that terrible worm, has only been found in Egypt.

There are also parasitic insects dreaded by man, as the Chigoe (Pulex penetrans) which, happily, is only known in certain countries. Some, however, have become cosmopolitan, since man has introduced them wherever he has established himself.

The mammalia which live on vegetable diet have Tænia without any crown of hooks, and man, according to his teeth, ought only to nourish the Tania mediocanellata. We find in a work on the Algerian Tænia, by Dr. Cauvet, that it is the Tania inermis, that is to say, without hooks, which is the species common in Algeria. Among fourteen tænia which he had occasion to examine, there was not a single Tænia solium. I have said long since, that this species ought to be less widely spread than the tænia without hooks. The Tania solium

comes from the cysticercus of the pig, the other from that of the ox; and Dr. Cauvet has ascertained that the latter, in the state of cysticercus, has already lost its crown.

We find extinct fossil genera and species in all the classes of the organic world. Is it the same with worms and animals of other classes which are only known in the condition of parasites? Had the Ichthyosauri and the Plesiosauri worms in their spiral cœcum like plagiostomous fishes, which resemble them so much in the digestive tube? We do not doubt this, and we should have been glad to give some demonstration of it. For this purpose, we have made a collection of the coprolites of these animals, but we have not yet succeeded in getting slices thin enough or sufficiently transparent to discover the eggs or the hooks of their cestode worms.

Not long ago, the partisans of spontaneous generation found in the class of worms their principal argument for their old hypothesis, and it was even after the publication of my treatise on intestinal worms that this question, which seemed forgotten, was taken up again by Pouchet. At present, they appear to have given up parasites, which reproduce their kind like other animals, and to have fallen back upon the infusoria, the last intrenchment which remained to the partisans of spontaneous generation, whence Mons. Pasteur has scientifically dislodged them. It is evident to all those who place facts above hypotheses and prejudices, that spontaneous generation, as well as the transformation of species, does not exist, at least, if we only consider the present epoch. We are leaving the domain of science if we take our arms from anterior epochs. We cannot accept anything as a fact, which is not capable of proof.

CHAPTER VI.

PARASITES FREE DURING THEIR WHOLE LIFE.

THIS first category of parasites includes all those which are not enclosed, and which live at the expense of others, without losing the attributes and advantages of a wandering life; they are as free as the vulture or the falcon which pursues its prey. We shall not, however, include among them the parasitical kite of Daudin, which tears from the hands of the traveller a piece of the flesh which he is preparing in the open air, nor the small Egyptian plover, which keeps the teeth of the crocodile clean. The former is a pirate, a highway robber; the plover, on the contrary, is a kind neighbour, an attendant who performs valuable services.

We are more correct in considering as parasites the Vampires (Phyllostoma), those audacious bats of South . America, which settle on the sleeping traveller or his beasts, and suck their blood by means of the sharp papillæ of their tongue. These animals are winged leeches which bleed their victim and pass on. We place among free parasites the greater part of leeches, some insects, and a certain number of arachnida, crustaceans, and infusoria.

As we have mentioned free messmates, so we have