Thus it was considered by Lamarck that parasitical worms are only found in the bodies of animals, and are actually produced there.

Can it be believed that such ideas were put forward by zoologists of the highest merit? and ought we to feel surprised that the theory of spontaneous generation was so long taught in the physiological schools?

A book published in 1859 was entitled, "Heterogenesis, or a Treatise on Spontaneous Generation." The author gives the clue to the origin of his errors in the second line of his preface, in which he says: "When, by meditation, it was evident to me that spontaneous generation was one of the means employed by matter for the reproduction of living beings." . . . According to this philosopher, science is, therefore, not the generalization of facts, but these facts must serve to prop up the theories or hypotheses invented in the silence of the study. This passage of his work shows us that he was no more able to yield to the evidence of experiments made on worms, than to those of Pasteur on the infusoria.

It may be related to the honour of the illustrious Baer, that, from the year 1817, during his stay at Königsberg, he took up arms against this hypothesis, and never ceased to combat it, till evidence succeeded in opening the eyes of the most obstinate.

The worms which present the most remarkable phenomena of transformations, accompanied by metamorphoses, are the Distomians and Cestodes, flat worms, which we will consider in the first place.

Trematode worms include a certain number of large and beautiful parasites which scarcely undergo any change, and are found only on the skin and the gills of

certain fishes; these are the monogenetic trematodes, comprising the Tristomidæ and all the worms of that group, which also stand higher in their organization: we shall speak of them hereafter. The other trematodes, which are called digenetic, live on the most dissimilar animals, under the most varied forms, and, like the greater part of the cestodes, introduce themselves into the individual who is to give them shelter, only by the assistance of a host, acting as a stage-coach which serves them. as a vehicle.

The principal family is that of the Distomidæ, a family par excellence cosmopolitan; as inconstant in their progress as capricious in the choice of their companions. Each distome resembles a small leech which has a sucker in the centre of the belly, and as this sucker was once considered to be perforated, the name of Distoma was given to them.

These parasites are the more interesting to us, from the fact that, though we are not the final resting-place of certain species, we nevertheless find them pass through us on their way. There are two species which occasionally lodge in the liver of man without being peculiar to him, for they properly belong to the sheep. Two other distomes have lately been described by Dr. Bilharz, which are fortunately only known at present in Cairo, and which are interesting, both with respect to their organization and to their manner of life.

The genealogy of the distomidæ is now generally well known; that which remains to be discovered is the itinerary of each particular species; and in several zoological laboratories experiments are daily made with certain species and the hosts which they are supposed

to seek. These investigations have already yielded the best results in the laboratories of Giessen and of Leipzic, under the direction of Leuckart.

The genealogy of the distomidae is as follows: the young distome, when it leaves the egg, is wrapped in a ciliated tunic, and, under the guise of a microscopic infusorial, it abandons itself to all the vagaries of a free and vagabond life; this is the bright period of its life. "It is a youth starting, with all the steam up, without help and without guidance, in the midst of the ocean; if it meets an island on its passage, that is to say, the body of an aquatic larva or a mollusc, it disembarks, brings forth its young, and disappears; its purpose is fulfilled. If it find no island or continent it sinks and perishes, for it carries no provisions with it; it has no organ which permits it to take nourishment on its passage." If life is short, even in the case of a young distome, it is passed in the midst of the water: if fortune is favourable to it, it will at last meet with a living abode, where it will find all that is necessary to the comfort of a parasite.

Abundance always reigns in these living oases; and as these new colonists are really exiles, who will never again see their native country, ciliary oars are useless to them, and their descendants differ entirely from their common mother.

Under the ciliated tunic of the mother appears a daughter under the form of a bag, who is born almost at the same time as herself, and concerning whom we may quote here the words of Réaumur: "Singular and mysterious duality in unity; two beings, living one within the other, which are still only a single individual. Has nature accustomed us to such profusion? Do we

ever see her retrograde thus from a more complicated organization to one more simple ?" That which this great observer did not dare to believe has yet been realized, and in many cases development is clearly recurrent.

Led by a marvellous instinct, and obeying an irrevocable mission, the distomidæ, as well as the monostomidæ, and others besides them, when they claim an asylum from molluscs, introduce into the living body of their new host, not an isolated embryo, but a young animal already impregnated with a rich posterity; if she remain mistress of the situation, this posterity will forcibly invade the various organs, without any consideration whether their host may not give way under the weight of this sudden invasion.

Fig. 41 represents one of these worms which proceeds from a ciliated embryo, and encloses by the side of its digestive tube cercariæ in different degrees of development. In front, we see one provided with eyes and a tail; behind, we see others which are younger; among these ciliated embryos, wandering without guidance and without a compass in the midst of their ocean, but few will reach the land, or, in other words, will find the

Fig. 41.-Monostomum verrucosum, Sporocyst with Cercariæ. In front is the mouth, in the middle the digestive canal, and around the digestive canal are young ones, under the form of Cercariæ in process of development.

port where their progeny may prosper. This first embryonic state is that in which there are the greatest perils. When stripped of their swimming tunic, these young distomes have the form of a bag, which for a long time was called a sporocyst. From these sporocysts we see hundreds and thousands of young ones proceed, resembling in no respect the mother which has brought them into the world. These, in their turn, will resume a free and independent life. They are colonists whom the distome has left on a foreign land. This simple. multiplication is often not sufficient for the preservation of the species; the first sporocyst produces other similar sporocysts, and these bring into the world a rich progeny of tadpoles, which after a certain metamorphosis will become sexual distomes. These tadpoles are often well armed, and devour occasionally even the last scrap of flesh belonging to their host. They have long been known under the name of Cercariæ, which was given to them at a time when their genealogy was unknown. They are not very unlike the tadpoles of the frog (Fig. 45). The mother was only a bag with ciliæ, and sometimes with eyes. The tadpole has a distinct body, with a movable deciduous tail; and after this falls off they have sexual organs.

The cercariæ often abandon their first host in which they have been developed, and live at liberty in the water while waiting for their final host. They are taken sometimes in the open sea. In 1849, J. Müller wrote to me from Marseilles that he had just discovered cercariæ and distomes living at liberty in the Mediterranean. Since then this illustrious naturalist has observed them again at Trieste,. while pursuing his studies on the