We may here mention the Pycnogonons, the Saphyrinæ, the Peltidiæ, and the Hersiliæ; these crustaceans often crawl over the skins of their congeners, but without ever renouncing their independence; and they are all more or less occupied with the toilet of their neighbours. 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 78 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 79 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.