The Ascidians are sometimes free, sometimes united to others in a manner more or less intimate. Hence their division into the three groups of _simple_, _social_, and _composite_ Ascidians. [Illustration: Fig. 124. Ascidia microcosmus (Cuvier).] Simple Ascidians attach themselves, each individual singly, to rocks and other submarine bodies, and generally at a fixed depth. _Ascidia microcosmus_ a Mediterranean species, represented in Fig. 124, may be quoted as a type of this division of Ascidians. The name of Microcosmus, or the little world, is probably given from its being inhabited by quite an animated colony of algæ and corals, which dwell upon its surface, and give it a very peculiar, but not very attractive, appearance. The flavour of these molluscoids is very strong, which does not, however, hinder the poorer dwellers on the sea shore from eating them. The genus _Phallusia_ is another type of the group. _Phallusia grossularia_ is of a reddish colour, and about the size of a currant-berry: it usually lodges itself in the oysters of certain localities. At Ostend another species, _Phallusia ampulloïdes_, is found in prodigious quantities in the oyster parks, and is parasitic on living lobsters. Social Ascidians comprehend living _Tunicata_, connected together on a common prolongation by the roots, but free and unconnected in all other respects. _Ascidia pedunculata_ (Fig. 125) may be quoted as an example. The Composite Ascidians are still more intimately associated together; a great number of these little beings live together in a single mass. Such are the Botryllus and the Pyrosoma. The Botryllus is a genera the most interesting of all the groups under consideration. Only imagine from ten to twenty individuals, oval in form, more or less flattened, adhering by their dorsal surface to some submarine body, and holding on by their sides, so as to form a sort of wheel. "When we excite one of the branches," says Frédol, "a single mollusc contracts itself; when we touch the centre, they all seem to contract themselves (Cuvier). The buccal orifice is at the outer extremity of the radius; but the intestinal terminations abut on the common cavity, which occupies the centre of the wheel. Here we behold certain animals which eat separately, but which fulfil together as a community very singular functions--a kind of union and communism of which the moral world presents no prototype. With our molluscs, in place of two individuals united, we have a score. We may consider the entire star as one single animal with many mouths. But then, we have with it a luxury of organs for the function of intelligence which seeks and chooses, and parsimony of the organ of stupidity, which neither seeks nor chooses." [Illustration: Fig. 125. Ascidia pedunculata (Milne Edwards).]