either establishing themselves voluntarily in his organs, or quitting him after each meal, like the leech or the flea.

But when the larva of an ichneumon devours, organ after organ, the caterpillar which serves him as a nurse, and at last eats her entirely, can we call him a parasite? According to Lepelletier de Saint-Fargeau, who has so successfully treated these questions, the parasite is he who lives at the expense of another, eating that which belongs to him, but not devouring his nurse herself. Nor is the ichneumon a carnivorous animal, for the true beast of prey cares nothing at any period of his existence for the life of his victim.

True parasites are very commonly found in nature, and we should be wrong were we to consider that they all live a sad and monotonous life. Some among them are so active and vigilant that they sustain themselves during the greater part of their life, and only seek for assistance at certain determinate periods. They are not, as has been supposed, exceptional and strange beings, without any other organs than those of self-preservation. There is not, as was formerly supposed, a class of parasites, but all the classes of the animal kingdom include some among their inferior ranks.

We may divide them into different categories.

In the first of these we will place together all those which are free at the commencement of their life, which swim and take their sport without seeking assistance from others, until the infirmities of age compel them to retire into a place of refuge. They live at first like true Bohemians, and are certain of getting invalided at last in some well-arranged asylum. Sometimes both the male and female require this

assistance at a certain age; with others it is the female only, as the male continues his wandering life. In some cases, the female carries her partner with her, and supports him entirely during his captivity; her host nourishes her, and she in her turn feeds her husband. We find few female gill-suckers which have not with them their Lilliputian males, which, like a shadow, never quit them. But we also find males, living as parasites of their females, among those curious crustaceans known by the name of cirrhipeds. All the parasitical crustaceans are placed in this first category.

We find others, the ichneumons for example, which are perfectly at liberty in their old age, but require protection while young. There are many of these, which as soon as they escape from the egg, are literally put out to nurse; but from the day when they cast off their larval robe, they are no longer under restraint, but, armed cap-à-pie, they rush eagerly in quest of adventure, and die like others on the high road. In this category are generally found parasitical hymenopterous and dipterous insects.

Other kinds are lodgers all their lives, though they change their hosts, not to say their establishment, accordingly to their age and constitution. As soon as they quit the egg, they seek for the favours of others, and all their itinerary is rigorously traced out for them beforehand. Fortunately we are at present acquainted with the halting-places and magazines of a great number of those which belong to the order of cestode and trematode worms. These flat and soft worms begin life usually as vagabonds, aided by a ciliary robe which serves

as an apparatus for locomotion; but scarcely have they tried to use their delicate oars, before they demand assistance, lodge themselves in the body of the first host that they meet, whom they abandon for another living lair, and then condemn themselves to perpetual seclusion.

That which adds to the interest inspired by these feeble and timid beings is, that at each change of abode, they change also their costume; and that when they have reached the limit of their peregrinations, they assume the virile toga—we had almost said, the wedding robe. The sexes appear only under this later envelope; up to this period they have had no thoughts of the cares of a family. It has always been somewhat difficult to establish the identity of those persons who frequent the public saloons one day, and are found on the next in the most obscure haunts, dressed as mendicants. Most of the worms which have the form of a leaf or a tape give themselves up to these peregrinations, and those which do not arrive at their last stage, die usually without posterity.

It is interesting to remark that these parasitical worms do not inhabit the various organs of their neighbours indiscriminately, but all begin their life modestly in an almost inaccessible attic, and end it in large and spacious apartments. At their first appearance they think only of themselves, and are contented to lodge, as scolices or vesicular worms, in the connective tissue of the muscles, of the heart, of the lobes of the brain, or even in the ball of the eye; at a later stage, they think of the cares of a family, and occupy large vessels like the digestive or respiratory passages, always

in free communication with the exterior; they have a horror of being enclosed, and the propagation of their species requires access to the outer air.

In the last category are found those which need assistance all their lives; as soon as they have penetrated into the body of their host, they never remove again, and the lodging which they have chosen serves them both as a cradle and a tomb.

Some years since, no one suspected that a parasite could live in any other animal than that in which it was discovered. All helminthologists, with few exceptions, looked upon worms in the interior of the body as formed without parents in the same organs which they occupy. Worms which are parasites of fish, had been seen a long time before this in the intestines of various birds: experiments had even been made to satisfy observers of the possibility of these creatures passing from one body to another; but all these experiments had only given a negative result, and the idea of inevitable transmigration was so completely unknown that Bremser, the first helminthologist of his age, raised the cry of heresy, when Rudolphi spoke of the ligulæ of fishes which could continue to live in birds.

At a period nearer to our own times, our learned friend, Von Siebold, deservedly called the prince of helminthologists, was entirely of this opinion, and compared the cysticercus of the mouse with the tape-worm of the cat, considering this young worm as a wandering, sick, and dropsical being.

In his opinion, the worm had lost its way in the mouse, as the tænia of the cat could live only in the cat. Flourens considered it a romance when I myself announced to the “Institut de France,” that

cestode worms must necessarily pass from one animal to another in order to complete the phases of their evolution.

At the present time, experiments respecting these transmigrations are repeated every day in the laboratories of zoology with the same success; and Mons. R. Leuckart, who directs with so much talent the Institute of Leipzig, has discovered, in concert with his pupil Mecznikow, transmigrations of worms accompanied by changes of sex; that is to say, they have seen nematodes, the parasites of the lungs of the frog, always female or hermaphrodite, produce individuals of the two sexes which do not resemble their mother, and whose habitual abode is not in the lungs of the frog but in damp earth. In other words, let us imagine a mother, born a widow, who cannot exist without the assistance of others, producing boys and girls able to provide for themselves. The mother is parasitical and viviparous, her daughters are, during their whole