This shell, as already has been said, may have
one valve, or two valves, or be turbinate. In the latter case it may
be either spiral, as in whelks, or merely globular, as in sea-urchins.
When it has two valves, these may be gaping, as in scallops and
mussels, where the valves are united together on one side only, so
as to open and shut on the other; or they may be united together on
both sides, as in the Solens (razor-fishes). In all cases alike the
Testacea have, like plants, the head downwards. The reason for this
is, that they take in their nourishment from below, just as do
plants with their roots. Thus the under parts come in them to be
above, and the upper parts to be below. The body is enclosed in a
membrane, and through this the animal filters fluid free from salt and
absorbs its nutriment. In all there is a head; but none of the
parts, excepting this recipient of food, has any distinctive name.
8
All the Crustacea can crawl as well as swim, and accordingly they
are provided with numerous feet. There are four main genera, viz.
the Carabi, as they are called, the Astaci, the Carides, and the
Carcini. In each of these genera, again, there are numerous species,
which differ from each other not only as regards shape, but also
very considerably as regards size.
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