It is not the same with a mollusc called _Magilus_, which naturalists considered for a long time to be the calcareous tube of an annelid. All conchologists know the shell of the Magili, so valued by collectors. This gasteropod when young takes up its lodgings in the substance of a madrepore which grows more quickly than he, and in order not to die, stifled in this living wall, he constructs a calcareous tube similar to the shell, of which it appears to be the continuation, and which allows it to procure for itself water, air, 39 and food. The animal, protected by the madrepore, can do without its calcareous mantle, and only shows the end of the tube at the outside. It is this organ which sustains the struggle against the exuberant growth of the polyp, since it is by means of it that the mollusc obtains nourishment. The Magilus is like an oyster which is living in contact with a bank of mussels, with this difference, that the oyster almost always succumbs, while the magilus is always victorious in the struggle. We might also cite as well as the Magili, some _Vermeti_, certain _Crepidulæ_ and _Hipponices_, which struggle with the same success against those which pilot or receive them. As there exist parasites which only depend on others during their youth, so there are messmates which are completely independent when fully grown. Jacobson, of Copenhagen, wrote, in or about 1830, a memoir to show that the young bivalves which are found in the external branchial processes of the Anadontæ are parasites, and he proposed for them the name of _Glochidium_. Blainville and Duméril were charged to make a report on this memoir, which the author had sent to the Académie des Sciences. But his opinion had not many supporters, and it is now thoroughly known that the young anodonts differ considerably in their early and their full-grown state. During their stay in the branchial tubes, each young animal carries a long cable which descends from the middle of the foot, and serves to attach the anodont to the body of a fish, and yet permits it to move to a certain distance.[1] In fact the young anodonts have, not like the other acephala, 40 vibratory wheels in order to move themselves; they are conveyed in this manner by their neighbours. There are also messmate acephala, as the _Modiolaria marmorata_, which lodge on the mantle of ascidians. Professor Semper found attached to the skin of a _Synapta similis_, a mollusc which possesses a peculiarity rare among these animals, that of carrying its shell in the interior and not on the outside.