mentioned, in his memoir on the habits of the Meloë, a worm found in an egg.
M. Barthelemy has studied a nematode worm (Ascaroides limacis) which inhabits as a parasite the egg of the grey snail; is this not the ordinary worm of the snail which has introduced itself into the eggs?
Many animals establish themselves on their neighbours, not to obtain any advantage from them, except to profit by their fins; they are not themselves sufficiently adapted to rapid motion, so they seize a good courser, mount on his back, and ask from him only a resting-place and no provisions. But it is often very difficult to say where commensalism ends and mutualism begins; the cirrhipedes, for example, establish themselves on a piece of floating wood, or on the bottom of a vessel; on a block of stone, or on one of the piles of a groin; on an immovable animal as well as on a good swimmer.
Some fourteen years ago, Jacobson of Copenhagen wrote an interesting essay, to show that the young bivalves that are found in the branchiæ of anodonts at a certain period of the year are parasitical animals, for which he proposed a new name. But these supposed parasites are only young anodonts, which by the help of a very long cable, which proceeds from their foot like a byssus, attach themselves to their mother, or to a fish which will carry them to a distance.
We see full-grown acephalous molluscs, as mussels and pinnæ, still keep these cables, under the name of byssus, during their whole life. There are also among distomians, worms which though they are hermaphrodite, couple two and two, and have this additional peculiarity,
that while one increases rapidly the other becomes atrophied.
An Egyptian distome, which lives in man, gives an instance of this peculiarity, as well as the D. filicolle, which inhabits a fish (Brama Raii). The caligi which live on the skin of fishes are, when young, fastened by a cord which comes from the anterior edge of their carapace: while quite little, they put themselves under the protection of a kind neighbour, and allow themselves to be led by him.
The new tubularia, which we have dedicated to our learned colleague Dumortier, often fixes itself on the carapace of ordinary crabs, and causes itself to be conveyed like the Echeneis; the tubulary observed by Gwyn Jeffreys, close by the eye of the Rossia papillifera, a cephalopod mollusc, perhaps belongs to the same species.
Every colony of campanulariæ or sertulariæ lodges a crowd of messmates and mutualists; and there are a great number of crustaceans and polyps of all sizes which serve as an abode for infusoria of every kind. Some establish themselves on the carapace or on the swimming appendages, as in a carriage; others on one of the gills, which renders their mode of life more easy, and the danger less great. An amphipod very extensively spread over our sea-coasts, the Gammarus marinus, usually has its appendages covered with Vayinicola crystallina.