To enable the decapods to seize their victims or to defend themselves against their enemies, their anterior thoracic extremities generally assume the form of "chelæ," claws, or pincers of considerable strength, armed with teeth or sharp hooks, which give them increased powers of prehension. This form results mainly from the state of extreme development in which the penultimate articulation frequently occurs, and its assumption of the shape of a finger by the prolongation of one of its inferior angles. Against the finger-like process thus produced, which is of great strength, and quite immovable, the last articulation can be brought to bear with immense force, as it is put into motion by a muscular mass of great size, and in relation with the extraordinary development of the penultimate articulation. In most cases only the first pair of legs is converted into these formidable weapons, but in the Dromiæ, which are very common in the warmer seas, we find the two posterior pairs of legs, which are of a much smaller size, and raised above the plane of the others, similarly armed. These posterior claws, however, are not intended for active warfare, but merely for strategical purposes, as they serve to hold fast the pieces of sponges, shells, medusæ, and other marine productions, under whose cover the wily robber approaches and entraps his prey.
While the lower crustaceans abound in the polar seas, the crabs are completely wanting in those desolate regions; their number increases with the warmer temperature of the waters, and attains its maximum in the tropical zone. Here we find the most remarkable and various forms, here they attain a size unknown in our seas; and here they do not, as with us, inhabit solely the salt waters, but also people the brooks and rivers, or even constantly sojourn on land,--as, for instance, the _Thelphusæ_ and _Gecarcini_. There are even some species of land-crabs that suffocate when dipped into water. They breathe indeed through branchiæ, but the small quantity of oxygen dissolved in water does not suffice for the wants of their active respiration. They generally live in the shades of the damp forests, often at a great distance from the sea, concealing themselves in holes. At breeding time they generally seek the shore for the purpose of washing off their spawn, and depositing it in the sand, and no obstruction will then make them deviate from the straight path. They feed on vegetable substances, and are reckoned very excellent food. When taken, they will seize the person's finger with their claw, and endeavour to escape, leaving the claw behind, which for some time after it has been separated from the body, continues to give the finger a friendly squeeze. In the dusk of the evening they quit their holes, and may then be seen running about with great swiftness.