The true relation between these molluscs and the holothurians remains to be discovered, and how the entoconchæ become at last simple sexual tubes. At present we must admit that it is the result of a retrogressive development like that of the peltogasters, which, like them, lose all the attributes of their class. They ought, perhaps, to be placed farther on, among parasites.
Some years since, some molluscs were observed which have compromised more or less the dignity of their class. Gräffe cites a species of the genus Cypræa, which one would certainly not expect to find in this category; it lives among the Viti Islands, in the compartments of the Milithæa ochracea. We have referred to it before. Naturalists have given the name of Melithæa to a very beautiful polyp which forms colonies of two or three metres in height. Mons. Steenstrup, with that perspicacity which discerns the most complex phenomena, has also described Purpuræ which live as messmates with the Antipathes and the Madrepores. Quite recently, indeed, Mr. Stimpson has observed in the port of Charleston, a gasteropod mollusc, similar to a Planorbis (Cochliœlepsis parasitus) which lives as a messmate in the body of an annelid (Ocœtes lupina).
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, 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, 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.
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 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.