for many long years found the delight of his leisure hours
in watching the habits of the animals, and in discovering and
describing many singular circumstances in the constitution of their
bodies, and the peculiar adaptations of their structure and instincts
to their modes of existence. One of his last communications to the
public, imparted with all the modesty and simplicity of true genius,
at the last meeting of the British Association in Edinburgh, was on
this subject of the exuviation of the crustacea.[2]

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