In a few Entomostraca (some Phyllopoda and Ostracoda) the chitinous lining of the fore-gut develops spines and hairs which help to triturate and strain the food, and among the Ostracods there is occasionally (_Bairdia_) a more elaborate armature of toothed plates moved by muscles. It is among the Malacostraca, however, and especially in the Decapoda, that the "gastric mill" reaches its greatest perfection. In most Decapods the "stomach" or dilated portion of the fore-gut is divided into two chambers, a large anterior "cardiac" and a smaller posterior "pyloric." In the narrow opening between these, three teeth (fig. 10) are set, one dorsally and one on each side. These teeth are connected with a framework of movably articulated ossicles developed as thickened and calcified portions of the lining cuticle of the stomach and moved by special muscles in such a way as to bring the three teeth together in the middle line. The walls of the pyloric chamber bear a series of pads and ridges beset with hairs and so disposed as to form a straining apparatus.
The mid-gut is essentially the digestive and absorptive region of the alimentary canal, and its surface is, in most cases, increased by pouch-like or tubular outgrowths which not only serve as glands for the secretion of the digestive juices, but may also become filled by the more fluid portion of the partially digested food and facilitate its absorption. These outgrowths vary much in their arrangement in the different groups. Most commonly there is a pair of lateral caeca, which may be more or less ramified and may form a massive "hepato-pancreas" or "liver."
[Illustration: FIG. 10.--Gastric Teeth of Crab and Lobster.
1a, Stomach of common crab, _Cancer pagurus_, laid open, showing b, b, b, some of the calcareous plates inserted in its muscular coat; g, g, the lateral teeth, which when in use are brought in contact with the sides of the median tooth m; c, c, the muscular coat. 1b´ and 1b´´, The gastric teeth enlarged to show their grinding surfaces. 2, Gastric teeth of common lobster, _Homarus vulgaris_. 3a and 3b, Two crustacean teeth (of _Dithyrocaris_) from the Carboniferous series of Renfrewshire (these, however, may be the toothed edges of the mandibles).]