CRUSTACEA are the Crustaces of the French, and the Krustentheire of the Germans. The common crab, the lobster, and crayfish, the com mon shrimp and the water-fleai may be taken as types of different sections of tins family.
Cancer is a genus of short-tailed crustacea, and is the type of the family Canceridte of Linnmus ; it includes a large number of species of the genus Cancer ; and the term Crab, which is a translation of it, is in common parlance applied' te the great bulk of the brachyurous crustaceanA. Dr. Leach restricted the genus Cancer to the form of Cancer pagurus, Linn., the large eatable crab of British coasts, which was, when he defined the genus, the only species known. For the Blood-spotted Crab of the Asiatic seas (Cancer maculatus, etc.), and the CoraEine Crab (Cancer corallinus, Fain.), Dr. Leach instituted the genus Carpilius, characterized by the existence of a single tooth on the border of the carapace, and by the tri clentated front ; and for the ` Eleven-toothed Crab ' (Cancer undecimdentatus, Fabr.), Egeria is a genus of brachyurous decapod crustaceans estab lished by Dr. Leach. E. Indica, in size, general form of the body, and length of the feet, bears a great reseniblance to Inachus scorpio ; but, besides generic differences, the arms are rather short and slender. It inhabits the Indian seas. The Hermit Cmbs are very common ; and the nimble little Calling Crabs, Gelasimus tetragonon, Edw., G. annulipes, Edw., G. Dussumieri? Edw., scamper over the moist sands, carrying aloft their enor mous hand, sometimes larger than the rest of the body. They are of the E. ancl 1Y. Indies. They bore holes for themselves in the black soil of the coasts. • On some coasts of the East Indies the sands at ebb-tido swarm with them. They are the food of the inshore sea fishes, and some of them are the best bait that can bo used ; and ono species of Gelasimus is common in the cassava fields of Brazil.
Several small crabs aro parasites, or take shelter within other animals. Cymothoe, a genus of the Indian Ocean, is too imperfectly organized to catch its own food, and tho species take up their home in the mouth cavity of fishes of the genus Stromatea, where they snap up all that comes within their reach ; Cymothoo stromatei is found inside tho mouth of Stromatca nigra on the Coro mandel coasts; another species has been found in the mouth of a chetodon, and inside that of a cyprinus of the Amur, and the Ceratothoa exoecti has been found within. the mouth of the flying fish.
Ostracotheres tridaenfe, Ruppell, is a little crab which lives within the great tridatna mollusc, whose immense shell serves in European churches as a vessel for holy water. The crab takes shelter
in the branchial chamber. Conchodytes tridacme inhabits the Tridacna squamosa ; the Concho dytes meleagrina) liTeS in tho shell of the pearl mussel ; the Epichtys giganteus lives on a fish of the Indian Archipelago ; the Ichthyoxenus Jellinghausii lodges in a fresh-water fish of the island of Java. Pinnetheres is a genus of small crabs which live within mussels, amongst others the Avicula margaritifcra or pearl mussel, and in the holothurians of the Philippine Islands, and Pin. Fischerii is of New Caledonia ; one species lives mithin the Chama.
Again, species of Pagurus, about 30 in number, all lodge in deserted shells, and change their dwelling-places n.s they grow older. They are known as the Hermit Crabs. Darwin thinks (p. 544) that certain species always use certain kinds of shells. Their abdomen is too soft to be exposed. In the Keeling islands, the large claws of some of the hermit crabs are beautifully adapted, when drawn back, to form an operculum to the shell.
Crustacea, occupying deep waters in places to which light is inaccessible, are found without eyes. Three species of Amphipoda and one Isopoda, from Kaiapai, NOW Canterbury, New Zealand, were found in this state.
The Birgus latro of the Keeling islands is famed for tho skilfal manner in which it tears off the husk and opens the cocoanut, in order to extract the medullary matter of the interior. It is a kind of intermediate link between the short and long tailed crabs, and bears a great resemblance to the Paguri. Darwin observed that they live on the cocoanuts that fall from the trees. The story of their climbing these palms and detachin,,e. the heavy nuts is mere fable. Its front pair of legs are terminated by very strong, heavy pincers, the last pair by others narrow and weak. To extract the nourishment, it tears off the husk, fibre by fibre, from that end in which the three eyes are situated, and then hammers upon one of them with its heavy claws until an openin,g is effected. It then, by its posterior pincers, extracts the white albuminous substance. It inhabits deep burrows, where it accumulates surprising quan tities of picked fibre of cocoanut husks, on which it rests as on a, bed. Its habits are diurnal, but every night it is said to pay a visit to the sea, perhaps to moisten its branchire. It is very good to eat, and the great mass of fat accumulated under the tail of the larger ones, sometimes yields, when inched, as much as a quart of limpid oil. They aro esteemed great delicacies, and aro fattened for the table.