With oviparous quadrupeds, the skeleton of the
larger ones is more or less osseous; of the smaller ones, more or less
spinous. But all sanguineous animals have a backbone of either one
kind or other: that is, composed either of bone or of spine.

The other portions of the skeleton are found in some animals and
not found in others, but the presence or the absence of this and
that part carries with it, as a matter of course, the presence or
the absence of the bones or the spines corresponding to this or that
part. For animals that are destitute of arms and legs cannot be
furnished with limb-bones: and in like manner with animals that have
the same parts, but yet have them unlike in form; for in these animals
the corresponding bones differ from one another in the way of relative
excess or relative defect, or in the way of analogy taking the place
of identity. So much for the osseous or spinous systems in animals.