They are encased in armour of shell, and this has given
to them and their relations the name of Crustaceans, or Crusty creatures,
because what bones they have are outside, not hidden beneath the flesh. But
unlike the snail's house, which grows with the growth of its inmate, and
unlike _our_ skeleton which grows as _we_ grow, this close-fitting armour
does not increase in size, nor is it elastic enough to expand, but every
year one coat of mail is cast off, in a way not unlike the sloughing of the
serpent, to make room for a fresh soft suit. This new suit soon hardens,
and the creatures embrace the opportunity to make a little progress in
growing, which they do by fits and starts, not continuously; for the shell,
when once hardened, gives them no room to increase in size--they have to
wait till next year! The long pointed claws of the crab and lobster are
easily broken, and sometimes lost altogether, so that the power which they
have of growing new ones is a wonderful provision for their life among the
rough rocks and tangled sea-weeds.
One of the crusty creatures you know well enough, and you can find it
without going to the seaside, I mean the wood-louse, which I used to hear
called a "carpenter" when I was a child. In damp places, you can hardly
turn over a mossy stone, or pick off a bit of bark from a fallen tree,
without disturbing a whole colony of these slate-coloured creatures, with
their mailed coats, made of ten rings, or plates of armour.
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