We shall place in a second section some animals which have been usually classed among parasites, rather because of their living upon their neighbours than on account of their mode of life. If it is necessary in menageries to have keepers to cleanse the animals themselves, it is as requisite to have others to keep the
cages clean, and to remove dung and filth. Many animals perform this office. The rectum of frogs is always literally full of Opalinæ which swarm in this cavity, like ants in their ant-hill, and doubtless live on the contents of the intestine.
These Opalinæ are true infusoria, which do not wait till the fecal matters are decomposed, and till the waters are corrupted by their presence; they prevent accidents which might arise, and interfere in time to purify the water from these excretions. There have been found hitherto in the rectum of frogs, and in the different annelids, the Pachydrili, the Clitelides, the Lumbriculi, and the Enchytrei. We have also seen them in the Planaria and the Nemertians. There is no sight more curious for those who are commencing microscopical studies, than the examination of the contents of the rectum of these Batrachians. Van Leeuwenhoeck knew, two hundred years ago, those animalculæ, to which Bloch at a later period gave the name of Chaos intestinalis. There are also some Rotatoria, the Albertiæ for example, which ought to have a place here, and which Dujardin has described and named. They live in the intestines of the Lumbrici and of snails, and in the larvæ of Ephemerides.
Dujardin first pointed out the Albertia vermiculus; since then Mons. Schultze has made known the Albertia of the Näis littoralis, and Radkewitz has recognized in the small worm of our gardens the Enchytreus vermicularis. Long since, Siebold correctly stated that these animals are not parasites, since they do not live at the expense of their host.
There is a worm in the Philippine Islands, as Professor
Semper has informed me, which lodges in the intestines of a fish, with its head usually projecting outwards, and which watches the crustaceans attracted by the excreta of its host; but although it chooses the intestine of its neighbour as a place of shelter, it is not a parasite.
Fishermen affirm, and the examination of the animal’s stomach confirms their assertion, that the Cyclopterus lumpus feeds on nothing but the excreta of other fishes. Indeed, it is not possible to count the number of intestinal worms known by the name of Scolex, which are found in the contents of the stomach and the intestines. Besides this, we have long known the peculiarities of some insects which cannot live except on the dung of certain animals; and there is an example of one of these insects, found in a fossil state, which anticipated the discovery of the remains of an extinct mammal before unknown in that district. The larvæ of the fly Scatophaga stercoraria live only on excrementary matter.
There are also nematode worms which exist under these conditions, and which develop and propagate their species in the intestines as if in the midst of damp earth. The small eel-like creatures so abundant in cow-dung propagate in it; they are not parasites, and are allied to those of which we speak in this chapter.
Besides those attendants which busy themselves about the cleanliness of other animals, we find some whose duties are less extensive, and whose cares are more limited. Many animals produce a greater number of eggs than they can bring to perfection, and those which are decomposed for want of fecundation, or which
die in the course of evolution, are under the care of an especial attendant, employed to make away from time to time with the addled eggs, or the embryos that have failed to come to maturity.