The Thoracostraca are subdivided into the small group of the Stomatopoda, whose branchiæ are external and the feet prehensile or formed for swimming, and the far more numerous and important Decapods, which are either long-tailed like the scyllarus or short-tailed like the crab. In these the branchiæ no longer float in the water, but are enclosed in two chambers, situated one at each side of the under surface of the broad shelly plate which covers the back of the animal. Each of these chambers is provided with two apertures, one in the front near the jaws, the other behind.
Scyllarus equinoxialis.
The disposition of the anterior or efferent orifice varies but little; but in the long-tailed species the afferent or posterior orifice is a wide slit at the basis of the feet, while in the short-tailed kinds it forms a small transverse aperture generally placed almost immediately in front of the first pair of ambulatory extremities. By means of this formation, the short-tailed decapods or crabs, like those fishes that are provided with a narrow opening to their gill covers, are enabled to exist much longer out of the water than the long-tailed lobsters. Some of them even spend most of their time on land; and, still better to adapt them for a terrestrial life, the internal surfaces of the branchial caverns are lined with a spongy texture, and the gill branches separated from each other by hard partitions, so as to prevent them from collapsing after a long penury of water and thus completely stopping the circulation. While in fishes the water that serves for respiration flows from the front backwards, so as not to impede their motions, we find in the interior of the branchial cavity of the decapods a large valve attached to the second pair of maxillary feet, which, continually falling and rising, occasions a rapid current from behind forwards in the water with which the cavity is filled, a structure perfectly harmonising with their retrograde or sidelong movements.
The digestive apparatus of the decapods is of a very complicated structure. The mouth is here furnished with at least eight pieces or pairs of jaws, which pass the food through an extremely short gullet into a stomach of considerable size. This stomach is rendered curious by having within certain cartilaginous appendages, to which strong grinding-teeth are attached. These are placed at the outlet of the stomach, so that the aliment, after being subjected to the action of the jaws, is again more perfectly comminuted by the stomach-teeth before entering the digestive tube. The different pieces composing the masticatory apparatus of the stomach vary considerably in the different genera, and even in the several species of the same genus; but in every case they are always singularly in harmony with the kind of food taken and the general habits of the animal.
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.
Dromia Vulgaris.
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.