There are few animals so infested with parasites as the Ascidians in general. Not only does their surface sometimes become a _microcosm_, as the name of one Mediterranean species indicates, but even in the substance of their testa lodge _Crenellæ_ and other molluscs and polyps, which choose by preference to place their dwelling there. There are also Annelids which hollow out galleries in their interior, Lernæans which establish themselves in their respiratory cavity, Nematodes, Pycnogonidæ, Ophiuræ, and many others besides. Mons. Alfred Giard has described several Amphipods and Isopods which establish themselves on Tunicates. One cannot say that there is always such a complete agreement between animals of such different kinds, for Mons. Alfred Giard gives examples of grave disagreements which he has seen break out, and which have caused the death of several among them. Another association is that of a gasteropod with one of the acephala. In the environs of Caracas lives an Ampullaria (_Crocostoma_) which lodges in the umbilicus of its shell another mollusc, the only fluviatile species of those countries, called the _Sphaerium modioliforme_. We have every reason to suppose that the Sphaerium lives on good terms with the Ampullaria, since they are usually found associated. The Bryozoaria, the animal mosses, establish themselves on all solid 41 bodies at the bottom of the sea, like true mosses on stones or on trees. One species, a _Membranipora_, is usually found on the common mussel. These animals are of small size, group themselves in colonies on the surface of shells and of polyparies, or even on crustaceans, and form by their union a fine kind of lace, the dazzling whiteness of which often comes out sharply on the varying and glittering colour of the shell. This is because each animal lodges in a cell which is not larger than the head of a pin, and all the cells of a colony are grouped together with the symmetrical regularity of the façade of a Gothic building. Many Bryozoaria live in such a manner that it is impossible to say whether they are messmates, or have installed themselves by chance in a hiding-place for which they have no predilection. A charming bryozoon is developed in abundance on the carapace and the claws of the _Arcturus Baffini_, on the coast of Greenland, and propagates itself with extreme rapidity. On a single Arcturus we have found, scattered over its claws by the side of each other, Balani, Spirorbes, Sertulariæ, and vast colonies of Membranipora. One can see, merely by this example, the great zoological riches of the polar seas.