Adaptations shown by Snakes.

The Body of Snakes

The body of a snake is long and cylindrical without a well-marked neck. No snake has even vestiges of fore legs, but some pythons possess small claw-like processes which represent vestigial hind legs. The shape and great flexibility of the body of a snake enable it to move easily and rapidly through the narrow cracks and passages present in thick vegetation and in stony ground. In such places rapid movement would be hindered by the possession of legs. The body and tail of a snake are covered by a protective layer of overlapping horny scales which on the under side of the body are large and transversely elongated.

A snake moves by throwing its body into a series of lateral waves which pass backward to the tail. As each wave presses against any irregularity in the ground, the body of the snake moves forward. Movement is also helped by the action of the ribs, which are attached to the vertebra? at their dorsal ends, and fixed ventrally to the transversely elongated scales on the under side of the body. When the ribs are moved forward by the contraction of certain muscles, the scales are also moved fonvard, but when the ribs are moved backward, the free hind edges of the scale catches on any projection in the ground, and so enable the snake to lever itself forward.

Colour of Snakes

In many snakes the colour of the body matches that of the surroundings, as in fishes, so that the animal tends to be invisible to its enemies and to its prey. The grass snake, which is our largest British species, is coloured grey, dark green or brown, and has two rows of black spots along the back and another row along the sides. It can be recognized by the two yellowish patches at the back of the head, immediately behind which are two black patches. A full-grown female snake may reach a length of 4 ft., while the male is generally about a foot shorter.

An average adder is slightly over 2 ft. long, and has a ground colour similar to that of the grass snake, with a dark zigzag wavy line running down the middle of the back. It can be recognized from the grass snake by its head, which broadens behind the eyes and is separated from the body by a distinct neck. Behind the head of the adder there are two dark lines in the shape of a V or X.

Feeding Habits of Snakes

Most snakes are carnivorous and swallow their prey whole without mastication. The adder kills its prey by means of its poison fangs, A, Head of Adder. but the grass snake swallows its victim alive, B. Head of Grass Snake. and the captured animal may sometimes be seen struggling inside the snake ’s body. The pythons and boa constrictors crush their prey to death before swallowing it. The skull of a snake shows various adaptive features which enable the jaws to be opened very widely, so that a snake can swallow an animal which may be several times larger than its head. The lower jaw is hinged behind the skull to a bone known as the quadrate, which itself is movably attached to the skull. When the jaws are opened the base of the quadrate bone swings forward, so depressing the lower jaw and giving the mouth a very wide gape. The upper and lower jaws are provided with sharp backwardly curved teeth, which are replaced when broken and which serve to prevent the prey from escaping. The two halves of the lower jaw are not firmly fixed together, but are joined by elastic ligament which allows them to be separated. When a snake swallows an animal the teeth are hooked into it and the two halves of the lower jaw are alternately moved forward a short distance, so that the snake slowly works its way over the prey.

In the adder the poison glands represent modified salivary glands, and the poison ducts lead to the two long hollow teeth at the front of the mouth. The teeth are fixed to bones known as maxilla?, which are hinged to the skull and which are joined by rod-like bones to the quadrate. Ordinarily the fangs lie back against the roof of the mouth, but when the mouth is opened the forward swing of the quadrate moves the rod-like bones running to the maxilla:, and so raises the fangs into the striking position.

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