a large species of Modiola (_Modiola Papuana_) which is especially found in deep and almost inaccessible parts, and which always encloses a couple of pinnotheres about the size of a hazel-nut. We have opened hundreds of these modiolæ, and we have never met with any without their crabs. We have long since deposited some specimens of these 17 pinnotheres in the galleries of the Natural History Museum at Paris. The large mussel, which furnishes fine pearls (_Avicula margaritifera_), lodges also pinnotheres of a particular species by the side of another messmate more allied to a lobster than a crab. It is not even impossible that these crustaceans, with other messmates or parasites, contribute to the formation of pearls, since these gems, so highly prized in the fashionable world, are only the result of vitiated secretions, and are usually the result of wounds. We also meet with a little crab (_Ostracotheres tridacnæ_, Ruppel) in the acephalous mollusc, whose immense shell sometimes serves as a vessel for holy water; and it lives doubtless in many other bivalves which have not yet been examined. Dr. Léon Vaillant has written a very interesting memoir on the Tridacnæ, and informs us that the crab takes shelter in their branchial chamber. Therefore, since the molluscs live only on vegetable substances, while the Ostracotheres feed entirely on animal matter, Mons. Vaillant supposes that the latter take their choice of the food as it enters, and seize on its passage that which suits them best. Mr. Peters, during his abode on the coast of Mozambique, studied a great many of these acephala and pearl-mussels, and found their interior inhabited by three crustacean decapods, a pinnothere, and two macrouræ allied to the _Pontonia_, to which he has given the name of _Conchodytes_; the _Conchodytes tridacnæ_ inhabits the _Tridacna squamosa_; the _Conchodytes meleagrinæ_, as its specific name indicates, lives in the shell of the pearl-mussel. Professor Semper has recently observed pinnotheres in holothurians at 18 the Philippine Isles, and Mons. Alphonse M. Edwards has described some from New Caledonia (_P. Fischerii_); so that these little crabs, the friends of the molluscs, are known in both hemispheres. Do not these conditions seem to authorize the conclusion that the same thought has presided over the appearance of all living creatures; that they have all come into existence, not according to the chance arrangement of surrounding media, but according to the laws established from the very origin of all things?