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. For it
would be in opposition to what prevails in the whole tribe of
Testacea; for in all the ovum is discontinuous, and in all excepting
the sea-urchins asymmetrical, being placed only on one side of the
body. Owing then to this necessary discontinuity of the ovum, which
belongs to the sea-urchin as a member of the class, and owing to the
spherical shape of its body, which is its individual peculiarity, this
animal cannot possibly have an even number of ova. For were they an
even number, they would have to be arranged exactly opposite to each
other, in pairs, so as to keep the necessary symmetry; one ovum of
each pair being placed at one end, the other ovum at the other end
of a transverse diameter. This again would violate the universal
provision in Testacea. For both in the oysters and in the scallops
we find the ovum only on one side of the circumference. The number
then of the ova must be uneven, three for instance, or five.
Pages:
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189