474 
THE OCEAN WORLD. 
and near the shore. Some few of them frequent the deep waters, 
others hide themselves in the sand or under stones, while the common 
crab ( Carcinus mamas, Leach) loves the shore almost as much as the 
salt water, and establishes itself accordingly under some moist clifl 
overhanging the sea, where it can enjoy both. 
One of the necessary consequences of the condition of these animals, 
enclosed in a hard shell, is their power of throwing it off. The solidity 
of their calcareous carapace would effectually prevent their growth, 
but at certain determinate periods Nature despoils the warrior of his 
cuirass ; the creature moults, and the calcareous crust falls off, and 
leaves it with a thin, pale, and delicate tunic. In this state the Crus- 
tacean is no longer worthy of its name — its skin has become vulner- 
able as that of the softest mollusc ; but it has the instinct of weakness 
—it retires into lonely places, and hides its shame in some obscure 
crevice, until another vestment, more suitable for resistance, and 
adapted to its increased size, has been restored, and, along with it, h s 
coat-armour and crustacean dignity. 
The Crustacean has not, like the more advanced vertebrata, a 
vertebral column. In its nature it is altogether different from the 
internal skeleton ; still its functions are the same. The tegumentary 
skeleton of the Crustacean consists of a great number of distinct pieces, 
connected together by means of portions of the epidermal covering 
which have not yet become hardened, in the same way as the bones 
in the internal skeleton of the vertebrata are connected by cartilage®- 
the ossification of which only takes place in old age. The skeleton, 
or framework, consists of a series of rings varying in number, the 
normal number of the body-segments being twenty-one. Each ring 
is divisible into two arcs — one upper, or dorsal, the other lower, ° r 
ventral ; and each arc may present four elementary pieces, two of which 
are united in the mesial line from the tergum, or back ; the lower 
arc is a counterpart of this, while the others form the two side, ° r 
epimeral, pieces. Them skeleton, in short, is the stony envelop 6 • 
the bone is not condensed within the body, but is accumulated on th e 
circumference. The skin, therefore, performs the functions of *h e 
skeleton, so that the Crustaceans, as was said by Geoffroy Saint Hilah’ e > 
like the molluscs, live inside and not outside the bony column. Thei' e 
exists, then, among the vertebrata, animals with an interior — a i' rlie 
skeleton ; and others with a dermal, or tegumentary skeleton. 
