The characteristic contour of a snake's body is too familiar to need elaborate description; its leading features are, in fact, so nearly approximated by certain of the legless lizards, previously described, that the distinctions between the two can with difficulty be defined. Many of the snake-like lizards, including the Common Blind-worm, are altogether devoid of external limbs. In some snakes, on the other hand, and notably the large terrestrial Pythons, a spur-like development on each side of the base of the tail represents rudimentary hind legs. The Snakes agree essentially with the Lizards in the character of their scaly covering, the scales, however, being larger on their under-surface and specially adapted, as in the legless lizards, for creeping locomotion. The essential distinctions between the two groups have to be sought in the structure of the head. The most notable of these, as it obtains in the Snakes, is the very loose manner in which all the bones connected with the jaws are held together, thus providing for the greatest possible distension in the act of their swallowing their prey whole, as is the custom of all ordinary snakes. To achieve this end, the two halves of the lower jaw are not united together at their extremity or chin, as in lizards, but are merely connected with one another by an elastic ligament. In most snakes the bones of the upper jaw and palate are also attached to one another in a similar way.
The eyes of a snake differ in a very marked manner from those of ordinary lizards. No snake possesses movable eyelids. The eye, in compensation, is protected by a transparent horny disk, continuous with the general epidermis, and is shed with it when the snake casts its skin. This feature imparts to snakes that fixed, stony expression of the eyes which undoubtedly contributes very materially towards increasing the feeling of repulsion with which snakes are commonly regarded. A few exceptional lizards, such as the Geckos, have a similar eye-construction, but it is not met with in any of the limbless or snake-like forms. No snakes, again, show any trace of external ear-openings, such structures, on the other hand, being distinctly developed in almost all lizards. The head itself of the snake is never compressed or elevated, as in most lizards, but flattened down and usually wider than the body, to which, however, it is united without a distinct neck. The tongue of the snake is slender, and terminates in two long, thread-like points; basally it is inserted into a hollow sheath, into or out of which the entire organ can be retracted or exserted at will. The somewhat uncanny, flickering action with which a snake, while moving, displays and as it were feels its way with its long, forked tongue represents another element which adds to the disfavour with which these reptiles are commonly regarded. Among the uneducated even at the present day it is not unusual to hear the tongue, with reference to its peculiar shape and vibrating action, pronounced to be the seat and instrument of the animal's poisonous properties. The swift, silent, stealthy, gliding motions with which, apart from any visible organs of locomotion, a snake slides, as it were, along the ground and over all obstacles fill to the brink the measure for its condemnation in the estimation of all but the snake-devotee or the naturalist.
The locomotion of the snake is, as a matter of fact, one of the most remarkable and beautifully contrived phenomena in animal mechanics. The peculiarly jointed and abnormally mobile ribs constitute the mystic deus ex machinâ by which the reptile accomplishes its migration. These ribs articulate in pairs by a single mobile head with their respective segment of the vertebral column. At their opposite extremity they impinge on and are in muscular connection with the broad, slightly overlapping, shield-like scales which clothe the under surface of the body. The rib-muscles, contracting in rhythmical succession, raise the free overlapping edges of the shield-like scales, which, striking against the ground in the same regular order, push the body forward. Adopting an easily comprehensible simile, the snake's body is carried along the ground on the same principle as a paddle-wheel steamer is pushed along the surface of the water, the paddle-boards in the case of the snake being affixed to a long, narrow plane instead of a circular wheel.
Photo by H. G. F. Spurrell, Esq.] [Eastbourne.
DARK GREEN SNAKE.
Closely allied to the rat-snake of India, and preys in a similar manner on rats, mice, and birds.
The poison-fangs of snakes are highly specialised structures, and their presence or otherwise was formerly considered sufficiently distinctive for the separation of these reptiles into two sharply defined natural series. More recent investigations have, however, shown that such a system of classification is entirely artificial, both venomous and harmless species occurring among groups which are related to one another by essential structural characters. The teeth in the ordinary or harmless snakes are usually represented by two rows of slender, recurved, sharply pointed teeth in the upper jaw, and a single row of a similar character in the lower one. This recurved character of the dentition effectively assists the snake in gorging its quarry whole, nothing once seized by the hook-like teeth having a chance of retreating, the snake itself being unable to eject the prey upon which the teeth have fastened. In the most poisonous series, such as a rattle-snake, there is but a single row of recurved teeth in the upper jaw, and these are the equivalents of the inner set of the harmless species. Among the most venomous snakes the poison-fangs are tubular in character, the poison being received from the venom-glands at their open base, and discharged at the apex. In other forms the fangs have grooved channels only for the passage of the virus, while in other species there may be an intermediate condition. In all cases the poison-secreting glands are modifications of the ordinary salivary glands of other vertebrate animals. They are situated, one on each side, immediately below and behind the eyes, and are in some instances so abnormally developed as to extend backwards along the sides of the body. Special muscles envelop these glands, and force the poison into the hollow base of the fangs when the mouth is opened to strike.
Photos by Fredk. Downer & Sons] [Watford.