" The periodic casting of the skin is a needful provision for growth in these animals, as in Insects ; in them it is confined to the caterpillar state, in which alone growth takes place ; but here it prevails, because growth is continued long after the perfect form is attained. The rigidity of the encasing armor forbids the possibility of increase in its capacity. The growth, there fore, is periodic. At certain intervals the hard crust is thrown off in several pieces, a new crust having been prepared beneath, which is at first soft, flexible, and expansible. The body, now freed, instantly enlarges in all directions, and, in a few minutes, has attained the full extent of growth needed. The crust at once hardens, and in a brief space becomes as inflexible as was its predecessor, admitting no further enlargement either of its own surface or of the contained organs. The animal usually un dergoes this process in the most retired situation it can find, in stinctively conscious of its unprotected position when soft, and apparently feeling sick and feeble.
"Another interesting circumstance in the economy of this class, is the power of renewing injured or lost limbs. If one of
the joints of a Crab's legs be wounded, it would bleed to death but for this provision. The animal, however, stiffens the hurt limb, and suddenly throws it off, the separation invariably taking place at the point where the second joint is united with the first. A small gland is placed here, according to Mr. II. Goodsir, which supplies material for future legs as required. ' When the limb is thrown off, the blood-vessels and nerve retract, thus leaving a small cavity in the new-made surface. It is from this cavity that the germ of the future leg springs. A scar forms over the raw surface caused by the separation, which afterwards forms a sheath for the young leg.' As the growth advances, the shape of the new member becomes apparent, and constrictions appear, indi eating the position of the articulation ; but the whole remains unprotected by any hard covering until the next change of shell, after which it appears in a proper case, being, however, still con siderably smaller than the corresponding claw on the opposite side of the body, although equally perfect in all its points."