Animal Parasites and Messmates

by P. J. van Beneden

Some years ago they had no idea of the migration of animals from one body to another. As we have said elsewhere, Abildgard, half a century ago, made experiments on the worms of fishes which he caused ducks to swallow, but these experiments had no result, and formed rather an obstacle to ulterior progress, than an approach to truth. The worms of fishes have been known to live in birds; but these worms were only there as adventitious parasites. Liguli live some days in the goosander, but they do not maintain their position.

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 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 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.

That which adds still more to the difficulty of recognizing them is, that while young they are often enveloped in swaddling clothes which nevertheless permit them to wander freely; then in a simple robe, in keeping with the home which shelters them; and at last in a wedding dress, which hides the eggs and the apparatus which produces them. The nymph in her virgin condition has none of the attributes of future maternity.

It is in this category that we find the Distomes, so common in all the classes of the animal kingdom. This is not all: frequently, among these various forms, these animals when young produce little ones, which in no respects resemble the others, and are not even formed in the same manner. As soon as they quit their swaddling-clothes, they increase by gemmation, and without sexual union, while those which are produced from buds increase sexually. Thus the daughter does not resemble her mother, but her grandmother. This phenomenon has been known by the name of alternate generation; we have called it digenesis.

But all parasites do not resemble those distomes, which change several times both their host and their costume. We find some of them, which the mother deposits with care in the body of a neighbour, and which pass all their early life in the viscera of an alien mother.