The internal anatomy (Fig. 6) presents many points of interest which can only be briefly touched on here. The food-canal consists of a short gullet leading into a capacious stomach, from which the straight intestine runs to the vent on the under-side of the telson. The stomach has a most remarkable and complicated structure. It consists of two chambers, a larger in front and a smaller behind, which are lined by a continuation of the chitinous outside covering of the body. This chitinous lining is thickened in places to form a system of plates and levers connected with three strong teeth set in the narrow opening between the two chambers. By the action of muscles attached to certain of these plates the teeth work together so as to divide up the food more finely than had been done by the mandibles and other jaws. The whole apparatus, in fact, serves as a kind of gizzard, and is known as the _gastric mill_. A small part of the intestine at the hinder end is lined, like the stomach, by a continuation of the chitinous covering, which is turned in at the vent. This lining and that of the stomach, with the plates and teeth of the gastric mill, are cast and renewed when the shell is moulted. On each side of the food-canal in the thorax lies a large mass of soft tissue, yellowish-green in colour. This is the _digestive gland_, or "liver," which secretes the digestive juice, discharging it into the food-canal by a short duct on each side just behind the stomach. The _heart_ lies in the middle of the back, just under the hinder part of the carapace, and gives off, in front and behind, a number of arteries which carry the blood to the various organs of the body. From the smaller branches of these arteries the blood passes, not, as in vertebrate animals, into capillaries, but into the spaces lying between the organs of the body, and it finds its way back to the heart, not in definite veins, but by ill-defined venous channels which open into the pericardium, or space surrounding the heart. From the pericardium the blood enters the heart by six openings in its walls, each guarded by a pair of valves which close when the heart contracts, and prevent the blood from returning to the pericardium. [Illustration: FIG. 7--GILLS OF THE LOBSTER, EXPOSED BY CUTTING AWAY THE SIDE-FLAP OF THE CARAPACE (BRANCHIOSTEGITE)]