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. 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. Now the cartilages of these land animals are without marrow,
that is without any distinctly separate marrow. For the marrow,
which in bones is distinctly separate, is here mixed up with the whole
mass, and gives a soft and mucilaginous consistence to the
cartilage. But in the Selachia the chine, though it is
cartilaginous, yet contains marrow; for here it stands in the stead of
a bone.
Very nearly resembling the bones to the touch are such parts as
nails, hoofs, whether solid or cloven, horns, and the beaks of
birds, all of which are intended to serve as means of defence
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