The _Phronima sedentaria_ which lodges with the salpa seems to be scattered over the warm seas of both hemispheres. For the honour of the species, the females alone seek the assistance of their neighbours, without at the same time abandoning their characteristic robe. The sexes differ little from each other except in size, in the abdomen, and in the antennæ. Maury has described certain amphipod crustaceans which also inhabit the Salpæ. Another phronima described by Professor Claus, the _Phronima elongata_, lives in the same manner; but instead of occupying a living house, it generally seeks an empty lodging, in which it establishes itself like a pagurus. The "Bernard the Hermit" of the Marseillaise fishermen, the _Pyades_, becomes the messmate of an anemone which Dugès has called _Actinia parasitica_. According to the observations of the learned professor at Montpelier, the mouth of this anemone is always situated opposite to that of the crustacean, to take advantage of the morsels which escape from his pincers. Both of them profit by this association; and the opening of the shell is prolonged by a horny expansion furnished by the foot of the actinia. On the coast of England lives another soldier-crab (_Pagurus Prideauxii_), which has as its principal messmate a sea anemone called _Adamsia_, which Mons. Greeff found at the island of Madeira. This pagurus is especially remarkable for the good understanding which exists between himself and his acolyte--he is a model Amphitryon. Lieut.-Col. Stuart Wortley has watched it in its private life, and thus relates the result of his observations: this 27 animal after he has fished, never fails to offer the best morsels to his neighbour, and often during the day, ascertains if it is not hungry. But more especially when he is about to change his dwelling, does he redouble his care and his attention. He manoeuvres with all the delicacy of which he is capable, to make the anemone change its shell; he assists it in detaching itself, and if by chance the new dwelling is not to its taste, it seeks another until the _Adamsia_ is perfectly satisfied. This association is not confined to the union of a decapod with a nereid and an actinia; a curious cirrhipede often establishes itself on the body of the pagurus, and on the outside of the shell we generally find a colony of polyps, of a rose or yellow colour, which extend like a living carpet round this habitation. Thirty-six years ago we have given the name of _Hydractinia_ to these polyps, which were till then entirely unknown to naturalists, and which form habitually a double overcoat for the paguri, if I may employ the expression of my learned colleague, Mons. Ch. Desmoulins.