It appears from Sir John’s observations that crustaceans begin to throw off their shells at a very early period of their life, even in that embryo state in which they first appear after having left the egg, and before they have yet assumed the real form of their mature state.  During every successive exuviation in this embryo state, they assume more and more of their perfect and established form.  While the crab is young and rapidly growing, frequent exuviations take place at short intervals, from three to five times in the course of one year.  Previous to the change, the animal almost ceases to feed, and becomes rather inactive; the proper time having at length arrived, exuviation is effected in the course of a few hours, body and limbs being alike relieved from their hard covering.  Until the new shell acquires firmness and strength, the creature is very shy, and in the state of nature, retires into cavities below rocks or heaps of protecting sea-weed.  Sir John had kept for some time one of our smaller species of shore-crabs (Carcinus monas), of medium size, of a brown colour, with one white limb.  One summer evening it was put outside the window in a capacious glass-vessel of sea-water.  In the morning a form exactly resembling its own, only somewhat larger, lay in the vessel.  This was the same animal, which had performed exuviation, and extricated itself from the old shell during the night.  The resemblance between both forms was complete—­everything was the same, even the white limb was seen in both.  Another specimen kept was of smaller size, the opposite extremities of the limbs being only thirteen lines asunder; its colour was green, with three white patches on the back.  In the course of little more than a year five exuviations took place at irregular intervals, the new shell and animal becoming larger each time.  The third shell came on uniformly green, the white spots being entirely obliterated.  On the fourth exuviation, the limbs expanded two inches and a half.  From the long slender form of the limbs of crustacea, they are very liable to mutilation.  Crabs are also a very pugnacious family, and in their battles limbs are often snapped off.  These mutilations, however, are readily repaired; although, contrary to what was the common belief, the restoration takes place only at the next regular period of exuviation.

The full-grown common crab (Cancer pagurus) is of a reddish-brown colour, the claws tipped black; but some of the young are naturally of the purest white, which remains long unsullied.  This does not arise from confinement, which, according to Sir John, has no influence on colour.  ’A young white specimen of the common crab was subjected to observation on 29th September.  The body might have been circumscribed in a circle three-quarters of an inch in diameter, and the extended limbs by one-and-a-half inch in diameter.  Its first exuviation ensued on 8th November, the second on the 30th of April following, and the shell then produced subsisted till 12th September, when another exuviation took place, introducing a new shell of such transparent white that the interior almost shone through it.  All the shells were white, and increased somewhat in size successively.  This last shell of 12th September subsisted until 29th March, being 197 days, when it was thrown off during another exuviation.’