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. Now if there
were only one stomach, either the ova would be too far off from it, or
it would be so big as to fill up the whole cavity, and the
sea-urchin would have great difficulty in moving about and finding due
nourishment for its repletion. As then there are five intervals
between the five ova, so are there of necessity five divisions of
the stomach, one for each interval. So also, and on like grounds,
there are five teeth. For nature is thus enabled to allot to each
stomachal compartment and ovum its separate and similar tooth.
These, then, are the reasons why the number of ova in the sea-urchin
is an odd one, and why that odd number is five. In some sea-urchins
the ova are excessively small, in others of considerable size, the
explanation being that the latter are of a warmer constitution, and so
are able to concoct their food more thoroughly; while in the former
concoction is less perfect, so that the stomach is found full of
residual matter, while the ova are small and uneatable. Those of a
warmer constitution are, moreover, in virtue of their warmth more
given to motion, so that they make expeditions in search of food,
instead of remaining stationary like the rest. As evidence of this, it
will be found that they always have something or other sticking to
their spines, as though they moved much about; for they use their
spines as feet.
The Ascidians differ but slightly from plants, and yet have more
of an animal nature than the sponges, which are virtually plants and
nothing more
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