In many of these, recurrent development (I was about to say degradation) sometimes proceeds so far, that their animal nature becomes doubtful, and more than one of them, having no longer any mouth by which to feed, are reduced to a mere case which shelters their progeny. The messmate very nearly takes its rank among parasites. There are also cirrhipedes which live on different genera of their own family; and some species which are always found in society with other species. Some also live as messmates with each other; some of the Sabelliphili have one of the sexes parasitical on the other sex. Crustaceans are usually dioecious; but because of their manner of life, the cirrhipedes sometimes unite the two sexes and thus render the preservation of the species more certain. The whole family of the _Abdominalia_, a name proposed by Darwin, if I am not mistaken, have the sexes separate; and the males, comparatively very small, 56 are attached to the body of each female. It is a case of polyandria which we see realized in the _Scalpellum_. Darwin made known the existence of supplementary males, so small and so little developed, that they are with difficulty discovered, and so badly are they provided with organs that they have neither those of motion nor a stomach to digest. We have not exhausted the strange peculiarities of this particular group; there are some which live without shells and claws in the inside of other cirrhipedes, and atrophied males which only exist at the expense of their own females. It is almost useless to make the remark that more especially here there exist almost insensible gradations of difference between parasites, messmates, and free animals, and we shall find more than one example of this in the crustaceans to which we now allude. The most interesting fixed messmates are evidently those cirrhipedes, which, under the name of _Tubicinella_, _Diadema_, or _Coronula_, cover the skins of whales. They are, like all the rest, free in their infancy, but soon they take shelter on the back or on the head of one of these huge cetaceans, which they never quit when they have once chosen their abode. That which gives them great importance is, that each whale lodges a particular species; so that the crustacean messmate is a true flag which indicates in some respect the nationality, and it would not be without interest for voyagers who are naturalists to study these living flags.