The crab has nine of these bundles of gills in each gill chamber; a few of them are shown in fig. 147. Each gill chamber has two openings; the water is admitted by a slit in the base of the claw feet, and ejected by another into the mouth. But the act of breathing is regulated by a plate on the second pair of jaws, so connected with the exterior pair of foot-jaws that, when the crab applies the latter to its mouth, the plate shuts the slit, the water in the gill chamber is ejected by the mouth, and in order to admit a fresh supply, the crab must open the foot-jaws again, so that they are in constant motion. There are plates called whips on all the appendages of the crab, from the last pair of foot-jaws to the fourth pair of walking feet inclusive, which ascend and descend vertically between the bunches of gills to sweep particles of sand or other foreign matter out of them. [Illustration: Fig. 147. Section of a Crab.] The heart of the crab, as in all the Decapods, is placed under the skin of the back next to the throat; and the blood, which is white or bluish, flows from the heart through a complicated system of vessels, and, having nourished the different organs, it is collected in reservoirs at the base of the gills, is aërated while passing through them, and returns to the heart again. The mouth opens through a short gullet into a large globular stomach, from the walls of which calcareous toothed organs meet in the centre. One serves as an anvil, while the others bruise the food on it. Some of the long-tailed crustaceans can evert this apparatus and push it out of their mouth. The bruised food is liquefied by solvent juices from the liver and stomach, and the nutritious part enters the bloodvessels by imbibition. The nervous system is condensed to suit the form of the crab. An oval nervous mass with a hole in its centre surrounds the gullet, from each side of which a nerve extends to a nerve-centre in the head. The organs of sense are as usual supplied with nerves from the latter, and, from the circumference of the massy ring, nerves radiate to every part of the animal, voluntary or reflex, as may be required. [Illustration: Fig. 148. Young of Carcinus mœnas in different stages of development:—A, first stage; B, second stage; C, third stage; D, perfect form.]