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The Oxystomata ([Plate X].), which form the second tribe of the Brachyura, are distinguished by the form of the mouth-frame, which is narrowed in front so as to be triangular instead of square in outline. The passages through which the water passes out from the gills, which in other Crabs open at the front corners of the mouth-frame, are carried forwards to the front of the head. The Oxystomata are most abundant in tropical seas, but are represented on the British coasts by species of Ebalia, small and compact Crabs which are not unlike pebbles of the gravel among which they live.

The remaining Crabs form the tribe Brachygnatha, in which the mouth-frame and the maxillipeds that close it are more or less quadrilateral in shape. The tribe is divided into two subtribes, which may be recognized by the general shape of the carapace. In the Brachyrhyncha this is generally rounded or square-cut in front, without a projecting rostrum. In this subtribe are included the great majority of Crabs. The Edible Crab and the Shore Crab ([Plate IX].) are familiar examples. In the Oxyrhyncha, on the other hand, the carapace is generally narrowed in front, with a projecting rostrum, either simple or forked, and is often armed with spines. In this subtribe are included the long-legged Spider Crabs, several species of which are common on our coasts. The Giant Spider Crab of Japan ([Plate XI].) is the largest of living Crustacea.

PLATE XI

THE GIANT JAPANESE CRAB, Macrocheira kæmpferi, MALE. THE SCALE OF THE FIGURE IS GIVEN BY A TWO-FOOT RULE PLACED BELOW THE SPECIMEN

(From Brit. Mus. Guide)

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The last division of the Eumalacostraca, the Hoplocarida ([Plate XII].), is one of very small extent, comprising only a single order (Stomatopoda) of very remarkable Crustacea which are common in tropical seas, and of which at least one species, Squilla desmarestii, is occasionally captured on the south coast of England. The Stomatopoda are prawn-like Crustaceans, usually with a flattened body, and are easily recognized by the form of the large claws (the second pair of thoracic limbs), in which the last segment shuts down, like the blade of a pocket-knife, on the preceding segment, and forms a very efficient weapon, so that the larger species are not to be handled without caution. The resemblance of these claws to those of the mantis-insect of Southern Europe led to a common Mediterranean species receiving long ago the name Squilla mantis ([Plate XII].).