The acetabula are set in double line in all the Cephalopoda
excepting in one kind of poulp, where there is but a single row. The
length and the slimness which is part of the nature of this kind of
poulp explain the exception. For a narrow space cannot possibly
admit of more than a single row. This exceptional character, then,
belongs to them, not because it is the most advantageous
arrangement, but because it is the necessary consequence of their
essential specific constitution.
In all these animals there is a fin, encircling the sac. In the
Poulps and the Sepias this fin is unbroken and continuous, as is
also the case in the larger calamaries known as Teuthi. But in the
smaller kind, called Teuthides, the fin is not only broader than in
the Sepias and the Poulps, where it is very narrow, but, moreover,
does not encircle the entire sac, but only begins in the middle of the
side. The use of this fin is to enable the animal to swim, and also to
direct its course. It acts, that is, like the rump-feathers in
birds, or the tail-fin in fishes. In none is it so small or so
indistinct as in the Poulps. For in these the body is of small bulk
and can be steered by the feet sufficiently well without other
assistance.
The Insects, the Crustacea, the Testacea, and the Cephalopoda,
have now been dealt with in turn; and their parts have been described,
whether internal or external.
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