of the Fierasfer, the Holothuria scabra of the Philippine Islands regularly lodges in its interior a couple, and sometimes, though rarely, a greater number of pinnotheres belonging to two distinct species. They choose this domicile at an early period, and must be highly delighted with this obscure abode, since they are seen no more, and when they have once entered never quit this living cavern. This observation is due to Professor Semper, who has made us acquainted with so many curious facts of the China Sea and the Pacific Ocean. In the midst of the slender branches of a coral of the Sandwich Islands, the Pæcilopora cæspitosa of Dana, there lives a little crab (Hopalocarcinus marsupialis, Stimpson), which is at last completely enclosed by the vegetation of the coral. It only keeps up sufficient communication with the exterior to enable it to procure food. The coral, however, furnishes it nothing but a resting-place in the midst of its tissues.
Among the Philippine Islands, also, a brachyurous crustacean lives in the branchial cavity of one of the Haliotidæ, and another on the body of a holothuria. On the coasts of Brazil, F. Müller, during his abode at Desterro, saw some Porcellanæ inhabiting star-fish, not as parasites, as had been supposed, but as true messmates. A crustacean possessed of but little generosity is the Lithoscaptus of Mons. Milne-Edwards. Provided with beak and claws for the purpose of attack, it instals itself, sad to say, in the pantry of a medusa, and instead of making use of its own weapons, takes advantage of the perfidious nematocysts of its acolyte, in order to live quietly at his expense.
Under the name of Asellus medusæ, Sir J. G. Dalyell
has made us acquainted with another messmate of the medusæ which greatly resembles an Idothea.
Another kind of commensalism is that of the Dromiæ. These crabs are of the ordinary size, and lodge, from their earliest youth, under a growing family of polyps, which increases with them. This colony has for its principal foundation a living Alcyonium, which covers the carapace, and as it develops, adapts itself perfectly to all the inequalities of the cephalothorax; one might consider it an integral part of the crab. Sertulariæ, Corynes, Algæ, develop themselves on this Alcyonium, and the Dromia, masked by this living rock which it carries on its shoulders like the fabled Atlas, marches gravely in pursuit of her prey. She has no fear of arousing the attention of her enemies. The greatest vigilance cannot prevent the sudden attack of these dangerous neighbours. There is in the Mediterranean a species which sometimes comes to our coast. They are also known in the Indian Seas and in the Northern Pacific. Rumphius named the dromia Cancer lanosus; it is, said he, a crab which carries grass or moss on its back. It is also mentioned by Renard. Dana has observed a sea-anemone covering a crab in the same manner as the Alcyonium does the dromia, and which is not less dangerous. The mode of life of this anemone has procured for it the name of Cancrisocia expansa. In the north of California, a crab (Cryptolithoides typicus) covers itself in the same manner with a living cloak which hides it from view, and under cover of which it surprises those whom it attacks. It has already cleared the ground of its prey before any alarm has been given to the neighbourhood.
We should perhaps speak here of an association of another kind, the nature of which it is difficult to ascertain; I refer to the little crab, the Turtle Crab of Brown, which is met with in the open sea on the carapace of turtles, and sometimes on sea-weeds. It may be supposed that it takes advantage of the carapace of its neighbour, in order to transport itself at little expense into different latitudes, and it is asserted that the sight of this crustacean gave confidence to Christopher Columbus, eighteen days before the discovery of the New World. Besides this animal, a whole society chooses this movable habitation: in addition to the cirrhipedes we also find the Tanaïs, which is not, however, condemned to live there always.
The macrourous decapods are more rarely found as messmates, but still a Palæmon is sometimes seen on the body of an Actinia, according to Semper, and another in the branchial cavity of a Pagurus. But that which is more generally known, is the presence in the Euplectella aspergillum of the palæmon which lodges in this fairy palace. It is probable that the Euplectella of the Atlantic, recently observed near the Cape Verd Islands by the naturalists on board the Challenger, also conceals this crustacean in its interior. We may also allude here to the Hypoconcha tabulosa, a crab whose carapace is too soft to allow it to venture out undefended, and which covers itself with the shell of a bivalve mollusc.
Among the various associations of this kind, none is more remarkable than that of the soldier-crabs, so abundant on our coasts, and called by the names of Bernard the Hermit and Kakerlot by the Ostend fishermen. It is well known that these crabs are decapod crustaceans,
very like miniature lobsters, which lodge in deserted shells, and change their dwelling-place as they grow larger. The young ones are content with very little habitations.
The shells which give them shelter are such as have been shed, which they find at the bottom of the sea, and in which they conceal their weakness and their misery. These animals have an abdomen too soft to bear the dangers which they meet with in their warfare, and that they may be less exposed to the claws of their numerous enemies, they take shelter in a shell which serves at the same time both as a dwelling and a buckler. Armed cap-à-pie, the soldier-crabs march boldly on the enemy, and know no danger, since they always have a secure retreat.