Of crustaceans,

the female crawfish after copulation conceives and
retains its eggs for about three months, from about the middle of
May to about the middle of August; they then lay the eggs into the
folds underneath the belly, and their eggs grow like grubs. This
same phenomenon is observable in molluscs also, and in such fishes
as are oviparous; for in all these cases the egg continues to grow.

The spawn of the crawfish is of a loose or granular consistency,
and is divided into eight parts; for corresponding to each of the
flaps on the side there is a gristly formation to which the spawn is
attached, and the entire structure resembles a cluster of grapes;
for each gristly formation is split into several parts. This is
obvious enough if you draw the parts asunder; but at first sight the
whole appears to be one and indivisible. And the largest are not those
nearest to the outlet but those in the middle, and the farthest off
are the smallest. The size of the small eggs is that of a small seed
in a fig; and they are not quite close to the outlet, but placed
middleways; for at both ends, tailwards and trunkwards, there are
two intervals devoid of eggs; for it is thus that the flaps also grow.
The side flaps, then, cannot close, but by placing the end flap on
them the animal can close up all, and this end-flap serves them for
a lid. And in the act of laying its eggs it seems to bring them
towards the gristly formations by curving the flap of its tail, and
then, squeezing the eggs towards the said gristly formations and
maintaining a bent posture, it performs the act of laying. The gristly
formations at these seasons increase in size and become receptive of
the eggs; for the animal lays its eggs into these formations, just
as the sepia lays its eggs among twigs and driftwood.