Because the limbs are placed perpendicular to the mesial line ; and, in the progressive motion, the body is dragged along the ground, as the flexion and extension of the limbs are unable to elevate it above the surface. - Fleming.
Because their walking is performed by so uniform a motion, the feet not moving by pairs, but by divisions, containing from five to twenty, and upwards. The hairs on the rings of caterpillars, it may be here observed, likewise serve as feet, in assisting progressive motion.
Because they bring up the tail towards the head, by bending the body into one or more curves,then resting upon the tail, and extending the body, thus moving forward, at each step, nearly the whole length of the body, or one or more of the curves into which it was formed. In serpents, this motion is well displayed, whence its name ; and, in some cases, it would appear, that they arc assisted in it by means of their ribs, which act as feet. Among the mollusca, (or soft animals) and the annulosa, (or ring-jointed animals) the same kind of motion is performed, by alternate contractions and expansions, laterally and longitudinally, of the whole or parts of the body. The hairs or spines of many of the annulosa, assist their progress; while in others, the body is so soft and pliable, as easily to accommodate itself to the inequalities of the surface over which it glides, and derive assistance from these in its progress.
Because, chiefly, of the cold acting on a frame extremely sensible to its impressions. During the continuance of a high temperature, they remain active and lively; but when the temperature is reduced towards 40°, they become torpid, and in this condition, if placed in a situation where the temperature continues low, will remain torpid for an unknown period of time. Spallanzani kept frogs, salamanders, and snakes, in a torpid state, in an ice-house, where they remained three years and a half, and readily revived when again exposed to the influence of a warm atmosphere.
Because the circulation of the blood is carried on independent of the action of the lungs. Even in a tortoise, kept awake during the winter by a genial temperature, the frequency of respiration was observed to be diminished.
The circulation of the blood is diminished, in proportion to the degrees of cold. Spallanzani counted from 11 to 12 pulsations in a minute in the heart of a snake, at the temperature of 48°, whose pulse in general, in warm weather, gives about 30 beats in the same period. Dr. Reeves observed the number of pulsations in toads and frogs, to be 30 in a minute, whilst they were left to themselves in the atmosphere, of which the temperature was 53°; when placed in a medium, cooled to 40, the number of pulsations was reduced to 12 within the same period ; and when exposed to a freezing mixture at 26°, the action of the heart ceased altogether. The powers of digestion are likewise equally feeble.
Because this condition with them, does not depend on the state of the heart, the lungs or the brain ; for these different organs have been removed by Spallanzani, and still the animal became torpid, and recovered according to circumstances. Even after the blood had been withdrawn from frogs and salamanders, they exhibited the same symptoms of torpidity, as if the body had been entire, and all the organs capable of action.
Because they are so easily acted upon by a cold atmosphere ; while in those countries enjoying a high temperature, they are formed of vast size, of many different kinds, and in great numbers.