Animal Parasites and Messmates

by P. J. van Beneden

which lodged twenty-four Filariæ lobatæ in its lungs, sixteen Syngami tracheales in the tracheal artery, besides more than a hundred Spiropteræ alatæ within the membranes of the stomach, several hundreds of the Holostomum excavatum in the smaller intestine, a hundred of the Distoma ferox in the large intestine, twenty-two of the Distoma hians in the œsophagus, and a Distoma echinatum in the small intestine. In spite of this affluence of lodgers the bird did not appear to be in the least inconvenienced.

Krause, of Belgrade, mentions a horse two years old, which contained more than five hundred Ascarides megalocephalæ, one hundred and ninety Oxyures curvulæ, two hundred and fourteen Strongyli armati, several millions of Strongyli tetracanthi, sixty-nine Tæniæ perfoliatæ, two hundred and eighty-seven Filariæ papillosæ, and six Cysticerci. When we consider how many eggs a single worm produces, we can understand how it is that so few animals escape being invaded by them.

Sixty millions of eggs have been counted in a single nematode, and in a single tape-worm, or rather in a colony, even a thousand millions of eggs. Even the very animals which live as parasites, harbour others in their turn. We find parasites on parasites, as we find messmates upon messmates. Almost all writers on this subject give examples of these; some in the larvæ of ichneumons, others in the lernæans, and we have more than once met with nematodes in different crustacea still attached to their host.

In order to understand thoroughly the living furniture of an animal, especially of a fish, it is necessary to examine it while young; the feces are the Kitchen-middings

of the stomach; it is from them that we can appreciate the bill of fare of each. This study of the food will one day excite much interest, not only in a scientific point of view, but also with reference to fishing as an occupation.

There are some animals which are infested at every period of their life, and at every season; others in far greater number only during their youth, and they gather in at the commencement of their life the harvest for the rest of their days. The greater part of parasites, especially of fish, are introduced with the first nourishment. As soon as they issue from the egg, young rays, like young turbots, are already stuffed with worms which afterward obstruct the digestive organs. The stomach of each of these fishes is like a filter which allows every thing which is food to pass, but detains on its passage and without any change all that is living. When we examine the stomach and observe the food in its different degrees of digestion, we see distinctly the worms coming out of their holes, wallowing in that which physiologists call chyle, and choosing afterwards at their convenience the place where they may completely develop themselves. At the end of a few days, the fish may have swallowed an innumerable quantity of small animals, and if each of them introduces some worms, we can easily understand in how short a time the intestine becomes literally filled.

There is no organ which is sheltered from the invasion of parasites: neither the brain, the ear, the eye, the heart, the blood, the lungs, the spinal marrow, the nerves, the muscles, or even the bones. Cysticerci have been found in the interior of the lobes of the brain, in the eye-ball, in the heart, and in the substance of the

bones, as well as in the spinal marrow. Each kind of worm has also its favourite place, and if it has not the chance of getting there, in order to undergo its changes, it will perish rather than emigrate to a situation which is not peculiar to it. One kind of worm inhabits the digestive passages, some at the entrance, others at the place of exit; another occupies the fossæ of the nose; a third the liver, or the kidneys.

We may even divide parasites into two great categories, according to the organs which they choose: those which inhabit a temporary host, almost always instal themselves in a closed organ—in the muscles, the heart, or the lobes of the brain; those, on the contrary, which have arrived at their destination, and which, unlike the preceding, have a family, occupy the stomach with its dependencies, the digestive passages, the lungs, the nasal fossæ, the kidneys, in a word, all the organs which are in direct communication with the exterior, in order to leave a place of issue for their progeny. The young ones are never enclosed. Even the blood is not free from these animals, but there are few which lodge there, except during the act of migration.

In Egypt, Dr. Bilharz observed a distome in the blood of a man (Distoma hœmatobium); the Strongylus of the horse has been long known, which causes serious injuries in its vessels (Strongylus armatus); as also the strongylus of the dolphin and of the porpoise (Strongylus inflexus), and the filaria of the dog (Filaria papillosa); and some are also found in the blood of many birds, of reptiles, batrachians, and fishes; so that there is no class of vertebrates which escapes.