For of
all the sense-organs that of touch is the only one that has
corporeal substance, or at any rate it is more corporeal than any
other, and its medium must be corporeal like itself.
It is obvious also to sense that it is for the sake of the flesh
that all the other parts exist. By the other parts I mean the bones,
the skin, the sinews, and the blood-vessels, and, again, the hair
and the various kinds of nails, and anything else there may be of a
like character. Thus the bones are a contrivance to give security to
the soft parts, to which purpose they are adapted by their hardness;
and in animals that have no bones the same office is fulfilled by some
analogous substance, as by fishspine in some fishes, and by
cartilage in others.
Now in some animals this supporting substance is situated within the
body, while in some of the bloodless species it is placed on the
outside. The latter is the case in all the Crustacea, as the Carcini
(Crabs) and the Carabi (Prickly Lobsters); it is the case also in
the Testacea, as for instance in the several species known by the
general name of oysters. For in all these animals the fleshy substance
is within, and the earthy matter, which holds the soft parts
together and keeps them from injury, is on the outside. For the
shell not only enables the soft parts to hold together, but also, as
the animal is bloodless and so has but little natural warmth,
surrounds it, as a chaufferette does the embers, and keeps in the
smouldering heat.
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