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. Therefore it is that
they are found in better condition in summer than at any other season;
and this all over the world excepting in the Pyrrhean tidal strait.
There the sea-urchins flourish as well in winter as in summer. But the
reason for this is that they have a greater abundance of food in the
winter, because the fish desert the strait at that season.
The number of the ova is the same in all sea-urchins, and is an
odd one. For there are five ova, just as there are also five teeth and
five stomachs; and the explanation of this is to be found in the
fact that the so-called ova are not really ova, but merely, as was
said before, the result of the animal's well-fed condition. Oysters
also have a so-called ovum, corresponding in character to that of
the sea-urchins, but existing only on one side of their body. Now
inasmuch as the sea-urchin is of a spherical form, and not merely a
single disk like the oyster, and in virtue of its spherical shape is
the same from whatever side it be examined, its ovum must
necessarily be of a corresponding symmetry. For the spherical shape
has not the asymmetry of the disk-shaped body of the oysters. For in
all these animals the head is central, but in the sea-urchin the
so-called ovum is above [and symmetrical, while in the oyster it is
only one side]. Now the necessary symmetry would be observed were
the ovum to form a continuous ring. But this may not be
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