But if
there were only three they would be much too far apart; while, if
there were more than five, they would come to form a continuous
mass. The former arrangement would be disadvantageous to the animal,
the latter an impossibility. There can therefore be neither more nor
less than five. For the same reason the stomach is divided into five
parts, and there is a corresponding number of teeth. For seeing that
the ova represent each of them a kind of body for the animal, their
disposition must conform to that of the stomach, seeing that it is
from this that they derive the material for their growth. 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.
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