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 92 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.