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.
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.