Then come the Oviparous fishes, where there is no bone,
but merely fish-spine. In the Serpents too the bones have the
character of fish-spine, excepting in the very large species, where
the solid foundation of the body requires to be stronger, in order
that the animal itself may be strong, the same reason prevailing as in
the case of the Vivipara. Lastly, in the Selachia, as they are called,
the fish-spines are replaced by cartilage. For it is necessary that
the movements of these animals shall be of an undulating character;
and this again requires the framework that supports the body to be
made of a pliable and not of a brittle substance. Moreover, in these
Selachia nature has used all the earthy matter on the skin; and she is
unable to allot to many different parts one and the same superfluity
of material. Even in viviparous animals many of the bones are
cartilaginous. This happens in those parts where it is to the
advantage of the surrounding flesh that its solid base shall be soft
and mucilaginous. Such, for instance, is the case with the ears and
nostrils; for in projecting parts, such as these, brittle substances
would soon get broken. Cartilage and bone are indeed fundamentally the
same thing, the differences between them being merely matters of
degree. Thus neither cartilage nor bone, when once cut off, grows
again.
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