The antero-lateral margins of the carapace in many of the crabs of our own and of foreign coasts are beset with a row of teeth or spines, which vary in character and number in different species and genera. In the Oxyrhyncha (Spider-crabs) the whole surface of the carapace is generally studded with spines and stiff hairs of a peculiar character, but there is no general restriction of these processes of the carapace to the antero-lateral margins of the body. These crabs, moreover, do not adopt burrowing habits. Their armature of spines, tubercles, and hairs is employed, as is well known, for protective purposes: in some cases possibly as an actual defence against attack, in others (i.e., Eurynome aspera) as a means of protective resemblance to their surroundings; but in the great majority as mere pegs and hooks for the fixation of foreign bodies, such as algae, hydroids, polyzoa, and ascidians, for purposes of concealment and disguise.

In the Catometopa (Land-crabs, etc.) the carapace is usually smooth over its whole surface. These animals often burrow in sand, but for the most part their burrows are permanent subterranean tunnels and chambers.

In the Cyclometopa, however—the group which includes most of our commoner British crabs—the back of the carapace is generally smooth, while the antero-lateral margins are in most forms conspicuously serrated. Most of these animals inhabit sandy or gravelly areas, and show a marked propensity towards burrowing habits. Their burrows are never permanent channels or tunnels in the sand, but are mere temporary excavations, the sand, mud, or gravel being in actual contact with their bodies when imbedded.