Let us look at the
builders of these pretty homes.
The shell-builders have soft, juicy bodies, and they are put in one big
division of the animal kingdom--the _mollusca_, which only means
_soft-bodied_. Some of these molluscs do not build shells. But most of
them build a shelly house for themselves; they do this to defend their
soft bodies from the attacks of a host of enemies. Some build two
shells--the Oyster and Mussel do, as you know. These are called
_bi-valves_; that is, two valves or shells; and others, like the Garden
Snail, the Limpet, and Periwinkle, have one shell only, and so are
called _uni-valves_.
The crab, and other _crustaceans_, also have a hard covering to their
soft bodies; but it is not at all like the shell of a Snail, or other
_mollusc_. The Snail's shell is like the little boy's suit which is
altered and made bigger as the boy grows. The crab's covering is a suit
which cannot be altered. It must be thrown away, and replaced by a
larger one.
The body of the shell-builder is wrapped in a soft covering, a kind of
outer coat, which is called the _mantle_.
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