We often find among the eggs of the ordinary crab of our coasts 82 (_Cancer moenas_) a nemertian which probably performs the same office. He is lodged while young in a kind of firm sheath attached to the abdominal processes. We have been able easily to study the first phases of its evolution. We have given it the name of _Polia involuta_. This nemertian had been observed at Messina, and described before by Kölliker under the name of _Nemertes carcinophilus_, and it has just been described and figured anew by Mr. M'Intosh, in a monograph of British annelids published by the Ray Society. The sturgeon seems to give lodging in its eggs to a polyp which plays the same part. In fact, Mons. Owsjannikoff, at the congress of Russian naturalists at Kiew, described an animal, _Accipenser ruthenus_, which lives in the eggs of the sterlet. Some eggs placed in water for a few hours at first show tentacles on the outside, then a whole colony, and each part consists of four individuals, which have a common digestive cavity, resembling somewhat a hydra divided longitudinally in four. Each has six tentacles, two of which are terminated by transparent corpuscles, perhaps nematocysts; the digestive cavity extends into the arms, as in the hydra; the mouth is not between the tentacles, but at the opposite pole. They are not all lodged within the eggs; some are found outside, according to the observations of Mons. Koch. Does not this animal fulfil in the egg of the sterlet, the same office as the histriobdella in the egg of the lobster? The eggs of some insects are attacked by very little ichneumons, the _Proctotrupidæ_; they empty them, and then instal themselves in the shell. Mons. Fabre has mentioned, in his memoir on the habits of 83 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.