Our great initiator into the world of parasites, Mons. Siebold, arrived also at a conclusion which could not be maintained. Having observed, with his habitual sagacity, that the cysticercus of the mouse is the same worm which lives in the cat, he published his opinion that the eggs of this tænia had lost their way in the mouse, that the young worms had become sick there, and that in the cat alone, they could be healthily and completely developed. It was like a plant lost on a soil where it could not live, and still less flourish. May I be permitted to state by what means we have arrived at the knowledge of the transmigration of worms? I had commenced the study of encysted Tetrarhynchi in the peritoneum 101 of the Gadidæ in 1837. Ten years afterwards, shortly after a visit from my learned friend, Mons. Kölliker, I discovered that this world of parasites did not live such a monotonous life as was supposed. I ascertained by my dissections of fishes, that the tetrarhynchi also, which were supposed to be disinherited by Nature, knew how to vary their pleasures; that instead of spending their whole life in a prison cell, they change their home at a certain age, and pass the latter part of their existence in more spacious habitations. I had seen the _Tetrarhynchus agamus_ inhabiting a cyst in the peritoneum of the gadidæ, and I had met with the same tetrarhynchus completely developed and sexual in the spiral intestine of the voracious fishes known under the name of squalidæ, or sharks. This caused me to write to the Academy of Brussels, at the meeting on January the 13th, 1849, that the order of vesicular worms, admitted by all helminthologists, ought to be suppressed. These worms began to be understood when these cysticerci ceased to be regarded as sick creatures. Siebold had mistaken the _crèche_ for the hospital, and instead of seeing in the cysticercus a young animal full of life and of the future, he looked upon it as a gouty individual, ready to breathe its last sigh. These fish had directed me in the right road; I had closely followed up certain very characteristic worms, which lived under a very simple form in certain fishes, and which, passing with their host into the stomach of another, finished in the latter their toilet and their evolution. I had been a witness of all their changes of form from 102 the cradle to the tomb, by following them from fish to fish, or rather from stomach to stomach. In fact these parasites are perpetually on their journey, and constantly changing their host, and at the same time their dress and mode of locomotion, so that frequently, at the end of their voyage, they preserve only shapeless rags to cover their eggs or their offspring.