Because the former contain much less earthy matter than the latter. In some fishes, the earthy matter is so small, that the cartilage continues, during the whole life of the animal, soft, flexible, and elastic, as the spine of the lamprey; or a little more indurated (harder) as in the bones of the skate or shark. These fishes have been termed cartilaginous. Even in those fishes which are termed osseous, (or bony,) the cartilage bears a much greater proportion to the earthy matter than in quadrupeds. - Fleming.
Because only apart of the blood is aerated, which issues from the heart; and that portion, instead of proceeding directly to the different organs, is again mixed with the circulating fluid. - Fleming.
Because they breathe with lungs; although these are of a much looser texture; and their respiration much more indeterminate, and less regular, than in the two classes of warm-blooded animals. - Blumenbach.
They are capable of living much longer without respiring, or in a vacuum (as, for instance, toads in cavities, within trees, or blocks of stone); they can even endure for a time, an atmosphere of carbonic acid gas; and there are undoubted proofs of newts and frogs having lived in the stomachs of human beings, or that have recovered, after having been frozen perfectly hard.
Because their temperature is greatly influenced by that of surrounding objects.
In this, and warm-blooded animals, as mammalia and birds, whose temperature is high, and not greatly influenced by the changes in the heat of external objects, the temperature is regulated by the vital powers of the animals; and limits are assigned, beyond which it is dangerous to pass.
The range of warm-blooded animals is confined; that of cold-blooded animals extensive.
Because of the force of their nerves, and the comparatively small size of their brain ; as a consequence of which, the nerves are less dependent on the brain; the whole machine has less mobility, presents fewer indications of sympathy, and the whole life is more simple, and more purely vegetative, than in warm-blooded animals; whilst, on the other hand, the separate parts are endowed with a greater share of peculiar and independent vital power ; where as, a stimulus applied to one part, or one system of parts, does not, as in warmblooded animals, excite others by sympathy.
We have thus an explanation of the tenacity of life in animals of this class; (frogs are known to leap about after the heart has been torn out, and tortoises to live for months, after the brain has been removed), and a similar explanation will apply to the long-continued power of motion, in parts of amphibia, when separated from the bodies; as the tails of newts, blindworms, etc.
As an instance of extraordinary reproduction, Blu-menbach tells us of a large water-newt, one of whose eyes had been entirely extirpated; notwithstanding which, within ten months, a perfect new eye was formed, with cornea, pupil, lens, etc. and only differing from the eye on the other side, in being about half its size.