[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.]
Dr. Carpenter has proved, by microscopic observations, that the shell of the Decapod, in its most complete form, consists of three strata: the first is a horny structureless layer covering the exterior; the second, a cellular stratum; and the third is a laminated tubular substance. In the large, thick-walled crabs, as the Cancer pagurus, the three strata are most distinctly marked. The tubuli of the lowest layer rise up through the pigment stratum in little papillary elevations, which give the coloured parts of the shell a minutely speckled appearance. There are various deviations from this general plan. In many of the small crabs belonging to the genus Portunus, the whole substance of the shell below the structureless horny investment is made up of hexagonal, thick-walled cells; and in the prawns there are large stellate coloured cells.