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.
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.
The eggs of the Brachyura are attached by gluten to the false ciliated feet of the tail of the female, which being bent up under the body forms a temporary protection till they are hatched. On leaving the egg the young have not the smallest resemblance to the parent; it is only after the fourth moult that they even acquire the crab form. When the young of our common shore crab, the Carcinus mœnas, leaves the egg, it is scarcely half a line in length. The body is ovoid, the dorsal shield large and swelled ([fig. 148], A). On the middle of its upper edge there is a long, hollow spine bending backwards, in which the white blood may be seen to circulate with a sufficient microscopic power. In front there is a pair of large sessile eyes, and the circumference of the pupils is marked by radiating lines: behind, there is a long, six-jointed tail, the last segment of which is forked and spined. On each side of the shield there is a pair of swimming feet attached to its waved margin. Fixed also to the margin, but in advance of these, there are three pairs of jointed feet ending in slender hairs. Immediately in front, between the eyes, there is a very long compressed appendage, which is bent backwards between the claws when the animal moves. Under each eye there is another appendage, shorter and rather more compressed. There are three pairs of claws, each composed of three joints, and ending in four long slender hairs: the claws stand at right angles to the body. The young, when it escapes from the egg, is quite soft, but it rapidly hardens by the deposition of calcareous matter on its surface. The progress of the consolidation is shown by the circulation of the white blood in the hollow dorsal spine. When the creature is yet soft, the blood globules may be seen ascending to its apex; but, as the consolidation advances, the circulation becomes more and more limited till at length it is confined to the base. This creature, whose shield is sap green and the rest transparent, swims with great activity, beating the water with his claws and tail. Such is the first stage in the life of the common shore crab. At this period the young of the Decapods bear a strong resemblance to one another, whether they are afterwards to become long or short tailed crustaceans.
After a time this creature loses its activity, moults, and is no longer to be recognised as the same, so great is the change ([fig. 148], B). The dorsal spine has vanished, the shield has become flatter, its anterior part pointed, the eyes raised on stalks, and certain rudimentary organs that were below the eyes now form long antennæ. The first pair of feet have got hands, the others are jointed and simple, except the last pair, which are still natatory: with these and with the tail, which is now much smaller, these creatures swim and congregate round sea-weeds and floating objects. After the third moult they have the form of a crab, though neither that of the genus nor species of the parent ([fig. 148], C). The tail is folded under a square carapace, the four pairs of walking feet spread widely and laterally, while the great hand-feet attached to the anterior sides of the carapace stretch straightforwards, the antennæ are short, and the eye-stalks bent to the right and left. It requires several moults to bring this creature to its final size and form.[[38]]
Crabs sometimes die while moulting, and occasionally are unable to extricate a limb from its shell, and consequently lose it. But if a limb be fractured they can cast it off at the second joint, and soon after a diminutive limb is formed, which attains its full size at the next moult; but if the crab has not strength enough to cast it off, it bleeds to death.