The front side margins of the carapace in many crabs are edged with a row of teeth or with spines, which vary in number and character in different species. In the spider-crabs the whole surface of the carapace is generally studded with tubercles, spines, and stiff hairs of a peculiar character. This armature is for protective purposes, and is often used to secure foreign bodies, such as algæ, hydroids, and polyzoans, which the crabs place upon their backs to disguise themselves. The burrowing crabs are usually smooth. When in motion the crab moves sideways, using the legs [pg274] of one side to pull with and those of the other side to push with. As all the legs do not move at the same time, a continuous and uniform motion is kept up. Some species move with great rapidity, notably the sand-crab, /Ocypoda arenaria/. Often when pursued they will run into the surf instead of to their holes. The common edible crab, /Callinectes sapidus/, of the Atlantic coast, the fiddler-crabs, and the spider-crabs are among the well-known representatives of /Brachyura/. FAMILY ~PORTUNIDÆ~ SWIMMING CRABS GENUS ^Carcinides^ [Illustration: /Carcinides mænas/, the green crab.] ^C. mænas^ (M. J. Rathbun), ^Carcinus mænas^ (Leach), the green crab. This is one of the most common species on the Atlantic coast from Cape Cod to New Jersey, and is very abundant in Vineyard Sound, Buzzard's Bay, and Long Island Sound. It is found between tide-marks, frequently well up on the beach, hiding under loose stones, also in tide-pools and in holes and cavernous places on the shore. It is from one and a half to two inches long, and a little more in breadth. It has five acute teeth on each side of the anterior part of the carapace. Its color is green, spotted with yellow, making it quite conspicuous. The surface of the carapace and limbs is more or less granulated. The posterior feet are flattened to form swimming-paddles. It is a very lively creature and has reckless audacity when brought to bay, which justifies its specific name (which implies frenzy). The French call it /crabe enragé/. [pg275] GENUS ^Callinectes^ [Illustration: /Callinectes sapidus/, the blue crab.]