Since they have neither feathers nor
scales such as either reptiles or other fishes have, all which are
signs rather of a dry and earthy nature, the egg they produce is soft;
for the earthy matter does not come to the surface in their eggs any
more than in themselves. This is why they lay eggs in themselves,
for if the egg were laid externally it would be destroyed, having no
protection.
Animals that are cold and rather dry than moist also lay eggs, but
the egg is imperfect; at the same time, because they are of an
earthy nature and the egg they produce is imperfect, therefore it
has a hard integument that it may be preserved by the protection of
the shell-like covering. Hence fishes, because they are scaly, and
crustacea, because they are of an earthy nature, lay eggs with a
hard integument.
The cephalopods, having themselves bodies of a sticky nature,
preserve in the same way the imperfect eggs they lay, for they deposit
a quantity of sticky material about the embryo. All insects produce
a scolex. Now all the insects are bloodless, wherefore all creatures
that produce a scolex from themselves are so. But we cannot say simply
that all bloodless animals produce a scolex, for the classes overlap
one another, (1) the insects, (2) the animals that produce a scolex,
(3) those that lay their egg imperfect, as the scaly fishes, the
crustacea, and the cephalopoda. I say that these form a gradation, for
the eggs of these latter resemble a scolex, in that they increase
after oviposition, and the scolex of insects again as it develops
resembles an egg; how so we shall explain later.
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