Here is an essay on snakes.
Snakes are a very specialised group of Reptiles under order Ophidia. Nearly 3,000 species of snakes are now found in the tropical and sub-tropical parts of the world. Out of these about 300 are Poisonous and others are Non-Poisonous. WHO has estimated that thirty to forty thousand people die annually of snake bite. Snakes are not found in New Zealand and Ireland.
Snakes are slender and elongated reptiles without limbs, eyelids and external ears. The whole body is covered with epidermal scales and shields. The scales and small and usually overlap each other but the shields are large and join each other by their margins. Shields are prominent on the head. The occipital condyle is single and tripartite.
The lower jaw is attached to the skull with the help of quadrate which articulates in such a movable fashion that the jaw can move laterally and downwards. The two halves, of the lower jaw are loosely articulated. All these are special adaptation for- its peculiar feeding habit.
The teeth are conical, recurved backward and fixed on jaw bones. In poisonous snakes two maxillary teeth (one on either side) become enlarged and pointed to form the Fangs. There are two types of fangs—open type and closed type according to the nature of the groove.
In poisonous snakes the labial glands have become modified into poison glands and these do not help them in digestion. The tongue is bifid at the apex. Left lung is reduced. Urinary bladder is absent. Most snakes are terrestrial. Few are arboreal and aquatic.
Some of the poisonous snakes are:
1. Cobra:
There are about 10 species of cobras of which two are found in India. They are Ophiopkagus hanah and Naja bungarus. Cobra is a very poisonous snake. It is usually found in forests. Usual length is 2-3 metres. The colour is black or buff.
The hood is well developed. It is supported by long ribs located just behind the head. The hood bears marks like spectacles. In some cobras there are two spectacles marks while in others there is a single spectacles mark. Under the neck there are found 2 or 3 dark belly plates. The fang is open type.
2. Krait:
Kraits (Naja bungarus) is smaller in size compared to the cobras and measures 1-1-25 metre. The colour of the body is glistening black with linear arches. They are found in gardens, on the roof of bung- lows. The poison fangs are small. The shields in the belly are broad. The tail is round at the tip. Central rows of scales down the back are large and hexagonal. The plates under the tail are entire.
3. Vipers:
These are rather large and stout snakes. The triangular head is broad, flat and covered with small scales. The neck is narrow. Eyes are with vertically elliptical pupil. The plates in the belly are broad and extend across (Fig. 142A).
The effects are observed within half an hour after bite. The venom is neurotoxic type. Symptoms are giddiness, high pulse rate, salivation, partial paralysis of tongue and larynx, vomiting and contraction of pupil.
The effects are observed within quarter of an hour. The venom affects the circulatory system. Symptoms are swelling of the affected region, discolouration, acute burning pain, pupil dilation, profuse vomiting and watery discharge from the rectum.
Poison Apparatus:
The poison apparatus consists of a pair of poison glands and a pair of fangs. The glands are situated one on either side of the upper jaw. These are probably modified superior labial glands. Each gland is sac like with a narrow duct emerging from its anterior end. The duct passes forward along the side of the upper jaw and opens either at the base of the fang or at the base of the tunnel on the fang.
The glands are held in position by ligaments. An anterior ligament attaches the anterior end of the gland to the maxilla. A posterior ligament runs between the gland and quadrate. Many fan-shaped ligaments are found between the side walls of the gland and squamoso quadrate junction.
The fangs are pointed and enlarged maxillary teeth which regenerate when lost. There are two types of fangs. In open type as in cobras the poison groove on the fang is open and in the closed type as in vipers the poison groove forms a tunnel having two Openings, (Fig. 142B) one at the base (intake aperture) and the other near the apex (discharge aperture).
The poison apparatus is associated with specialised muscles like digastric, sphenopterygoid, anterior and posterior temporalis etc. These muscles help in squeezing the gland and opening and closing of the mouth.
