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.