SNAKES, an order (Ophidia) in the class of REPTILES (q.v.). They are elongate animals without limbs, or with but claw-like rudiments of the hinder pair; without eyelids or external ears, with pointed re-curved teeth fused to the supporting bones, with a forked slender tongue which can be withdrawn into a sheath at its base, and, as a rule, with the two halves of the lower jaw not fused but joined by an elastic ligament. Like the lizards, with which they have many affinities, they are one of the dominant groups of reptiles of the present day and at least 2,000 different species are known. Their distribution is cosmopolitan with the exception of New Zealand, Ireland and some of the more recent, completely isolated, oceanic islands such as the Azores; as in other groups of terrestrial, cold-blooded animals the area of per manently frozen subsoil limits their northern and southern range and the greatest profusion of species and individuals is to be found in the tropics.
In the loss of limbs and the absence of well-differentiated neck, body and tail they may be regarded as degenerate and, by analogy with certain groups of limbless lizards, it seems probable that this type of bodily form arose in correlation with a habitat amongst dense vegetation; under these conditions limbs do not appear to be such efficient organs of locomotion as the lateral un dulations which form the basis of serpentiform locomotion. Con trary to the idea expressed in the conventional representations of snakes, the body is not undulated vertically but laterally, and loco motion is effected by the passage of a series of "waves" from be fore backwards, each wave in its progress pressing against the sur rounding medium and forcing the animal forwards. Such a system, is efficacious only when the sur rounding medium is sufficiently dense to offer an appreciable resis tance to the passage of the waves and the majority of terrestrial snakes possess an additional mechanism. The scales of the lower surface are enlarged to form transverse, overlapping plates, whose free edge is directed backwards, and to each of these plates is attached a pair of movable ribs. When the ribs are moved f or
wards they carry the scute with them, and as this is smooth and has its leading edge protected by the one in front of it, it slips easily over any irregularities of the surface. But, when the scute is moved backwards its free, hinder edge catches on the slightest projection and so enables the snake to push itself forward. As there may be as many as 300 of these ventral shields, each of which can utilise any slight irregularity, progress is possible over almost any surface which is not absolutely smooth.
Probably also correlated with the ancestral habitat is the devel opment of a hard transparent covering over the eye to protect the delicate cornea; in many lizards a transparent covering is developed from the lower eyelid, but in snakes the analogous covering is probably the modified nictitating membrane. In most snakes the outer horny covering of the scales is shed in one piece; sloughing commences at the lips and by vigorous rubbing move ments the slough is turned back on itself and the animal works its way out, the old skin being turned inside out in the process. Though the sense of sight is well-developed, hearing must be of a different nature from that of most vertebrates, as there is no external ear or ear-drum ; the columella auris, which normally transmits vibrations from the ear-drum to the inner ear, in snakes rests with its outer end on the quadrate, the bone which supports the lower jaw; possibly the ear is sensitive, not to air-borne vibrations, but to vibrations transmitted through the substratum on which the animal rests. The sense of smell is well developed but a snake's most important sensory organ is its tongue; exactly what sense is centred here is not known, but the constant play of this organ makes it evident that the sensations which it registers are of paramount importance in determining the animal's reactions to external conditions.