Snake, Snake

Snakes are thought to have evolved from either burrowing or aquatic lizards during the Cretaceous period ( c 150 Ma). The diversity of modern snakes appeared during the Paleocene period ( c 66 to 56 Ma).

However 150 million-year-old specimens, readily identifiable as snakes, yet with lizard-like skeletal structures, have been uncovered in South America and Africa. Mehrtens JM. 1987. Living Snakes of the World in Color. New York: Sterling Publishers. 480 pp. ISBN 0-8069-6460-X.

The frontal limbs are non-existent in all snakes and this loss is associated with the evolution of the Hox gene controlling limb morphogenesis. The axial skeleton of the snakes' common ancestor, like most other tetrapods had regional specializations consisting of cervical (neck), thoracic (chest), lumbar (lower back), sacral (pelvic) and caudal (tail) vertebrae. The Hox gene expression in the axial skeleton responsible for the development of the thorax became dominant early in snake evolution and as a result, the vertebrae anterior to the hindlimb buds (when present) all have the same thoracic-like identity (except from the atlas, axis and one to three neck vertebrae), making most of the snake's skeleton being composed of an extremely extended thorax. Ribs are found exclusively on the thoracic vertebrae. The neck, lumbar and pelvic vertebrae are very reduced in number (only two to ten lumbar and pelvic vertebrae are still present), while only a short tail remains of the caudal vertebrae, although the tail is still long enough to be of good use in many species, and is modified in some aquatic and tree dwelling species.thumb|Texas coral snake, tener ]An alternative hypothesis, based on morphology, suggests that the ancestors of snakes were related to mosasaurs — extinct aquatic reptiles from the Cretaceous — which in turn are thought to have derived from varanid lizard.

In the Late Cretaceous, snakes re-colonized land to appear as they are today. Fossil snake remains are known from early Late Cretaceous marine sediments, which is consistent with this hypothesis, particularly as they are older than the terrestrial Najash rionegrina . Similar skull structure; reduced/absent limbs; and other anatomical features found in both mosasaurs and snakes lead to a positive cladistic correlation, although some of these features are shared with varanids. In recent years, genetic studies have indicated that snakes are not as closely related to monitor lizards as it was once believed, and therefore not to mosasaurs, the proposed ancestor in the aquatic scenario of their evolution. However, there is more evidence linking mosasaurs to snakes than to varanids. Fragmentary remains that have been found from the Jurassic and Early Cretaceous indicate deeper fossil records for these groups, which may eventually refute either hypothesis.

Alethinophidia is sometimes split into Henophidia and Caenophidia, with the latter consisting of "Colubroid" snakes (colubrids, vipers, elapids, hydrophiids, and attractaspids) and acrochordids, while the other alethinophidian families comprise Henophidia Pough et al.

In comparison, the largest extant snakes are the reticulated python, which measures about long, and the anaconda, which measures about long CTV: Ancient, gargantuan snakes ate crocs for breakfast and is considered the heaviest snake on Earth. At the other end of the scale, the smallest extant snake is Leptotyphlops carlae with a length of about.

Contrary to the popular notion of snakes being slimy because of possible confusion of snakes with worms, snakeskin has a smooth, dry texture. Most snakes use specialized belly scales to travel, gripping surfaces. The body scales may be smooth, keeled, or granular. The eyelids of a snake are transparent "spectacle" scales which remain permanently closed, also known as brille.thumb|250px|A line diagram from G.A. Boulenger's Fauna of British India illustrating the terminology of shields on the head of a snake The shedding of scales is called ecdysis , or, in normal usage moulting or sloughing . In the case of snakes, the complete outer layer of skin is shed in one layer.

Scales are named mainly according to their positions on the body. In "advanced" (Caenophidian) snakes, the broad belly scales and rows of dorsal scales correspond to the vertebrae, allowing scientists to count the vertebrae without dissection. Snake's eyes are covered by their clear scales rather than movable eyelids; therefore, their eyes are always open.

Renewal of the skin by moulting is supposed to allow growth in some animals such as insects, however this view has been disputed in the case of snakes.

Before a moult, the snake stops eating and often hides or moves to a safe place. Just before shedding, the skin becomes dull and dry looking and the eyes become cloudy or blue-colored. The inner surface of the old outer skin liquefies. This causes the old outer skin to separate from the new inner skin. After a few days, the eyes clear and the snake "crawls" out of its old skin. The old skin breaks near the mouth and the snake wriggles out aided by rubbing against rough surfaces. In many cases the cast skin peels backward over the body from head to tail, in one piece like an old sock. A new, larger, and brighter layer of skin has formed underneath.

Some snakes, such as the Asian vine snake (genus Ahaetulla ), have binocular vision, with both eyes capable of focusing on the same point. Most snakes focus by moving the lens back and forth in relation to the retina, while in the other amniote groups, the lens is stretched.

Contrary to the popular belief that snakes can dislocate their jaws, snakes have a very flexible lower jaw, the two halves of which are not rigidly attached, and numerous other joints in their skull (see snake skull), allowing them to open their mouths wide enough to swallow their prey whole, even if it is larger in diameter than the snake itself, Bebler (1979) p.581 as snakes do not chew. For example, the African Egg-eating Snake has flexible jaws adapted for eating eggs much larger than the diameter of its head.

In species that feed only sporadically, the entire intestine enters a reduced state between meals to conserve energy, and the digestive system is 'up-regulated' to full capacity within 48 hours of prey consumption. Being cold-blooded (ectothermic), the surrounding temperature plays a large role in a snake's digestion.

So much metabolic energy is involved in a snake's digestion that in Crotalus durissus , the Mexican rattlesnake, an increase of body temperature to as much as 1.2 above the surrounding environment has been observed.

The heart is able to move around, however, owing to the lack of a diaphragm. This adjustment protects the heart from potential damage when large ingested prey is passed through the esophagus. The spleen is attached to the gall bladder and pancreas and filters the blood. The thymus gland is located in fatty tissue above the heart and is responsible for the generation of immune cells in the blood. The cardiovascular system of snakes is also unique for the presence of a renal portal system in which the blood from the snake's tail passes through the kidneys before returning to the heart.

Capula (1989), p.118 Cogger (1991), p.182 Recently, it has been confirmed that several species of snake are fully viviparous, such as the boa constrictor and green anaconda, nourishing their young through a placenta as well as a yolk sac, which is highly unusual among reptiles, or anything else outside of placental mammals.

Snake venoms are often prey specific, its role in self-defense is secondary.

Also under this theory, snakes may have evolved from a common lizard ancestor that was venomous, from which venomous lizards like the gila monster and beaded lizard may have also derived, as well as the monitor lizards and now extinct mosasaurs. They share this venom clade with various other saurian species.

With the exception of large constrictors, non-venomous snakes are not a threat to humans. The bite of non-venomous snakes is usually harmless because their teeth are designed for grabbing and holding, rather than tearing or inflicting a deep puncture wound. Although the possibility of an infection and tissue damage is present in the bite of a non-venomous snake, venomous snakes present far greater hazard to humans.

Non-fatal bites from venomous snakes may result in the need for amputation of a limb or part thereof. Of the roughly 725 species of venomous snakes worldwide, only 250 are able to kill a human with one bite.

In the United States for example, all species of venomous snakes are pit vipers, with the exception of the coral snake. To produce antivenin, a mixture of the venoms of the different species of rattlesnakes, copperheads, and cottonmouths is injected into the body of a horse in ever-increasing dosages until the horse is immunized. Blood is then extracted from the immunized horse and freeze-dried. It is reconstituted with sterile water and becomes antivenin. For this reason, people who are allergic to horses cannot be treated using antivenin.

In such a show, the snake charmer carries a basket that contains a snake that he seemingly charms by playing tunes from his flutelike musical instrument, to which the snake responds.

Snake charming as a profession is dying out in India because of competition from modern forms of entertainment and environment laws proscribing the practice.

Snake soup of Cantonese cuisine is consumed by local people in autumn, to warm up their body. Western cultures document the consumption of snakes under extreme circumstances of hunger. Irvine, F. R. 1954. Snakes as food for man. British Journal of Herpetology. 1(10):183189.

In Asian countries such as China, Taiwan, Thailand, Indonesia, Vietnam and Cambodia, drinking the blood of snakes, particularly the cobra, is believed to increase sexual virility.

To supply this demand a captive breeding industry has developed. Snakes bred in captivity tend to make better pets and are considered preferable to wild caught specimens.

Deane (1833). p.61 The cobra is seen on the neck of Shiva and Vishnu is depicted often as sleeping on a seven-headed snake or within the coils of a serpent. Deane (1833). p.6264 There are also several temples in India solely for cobras sometimes called Nagraj (King of Snakes) and it is believed that snakes are symbols of fertility. There is a Hindu festival called Nag Panchami each year on which day snakes are venerated and prayed to. See also Nga.

The Ouroboros or Oroboros is a snake eating its own tail in a clock-wise direction (from the head to the tail) in the shape of a circle, representing manifestation of one's own life and rebirth, leading to immortality.

A festival Nag Panchami is celebrated every year on snakes. Most images of Lord Shiva depict snake around his neck. Puranas have various stories associated with Snakes. In the Puranas, Shesha is said to hold all the planets of the Universe on his hoods and to constantly sing the glories of Vishnu from all his mouths. He is sometimes referred to as "Ananta-Shesha" which means "Endless Shesha". Other notable snakes in Hinduism are Ananta, Vasuki, Taxak, Karkotaka and Pingala. The term Nga is used to refer to entities which take the form of large snakes in Hinduism and Buddhism.

In Judaism, the snake of brass is also a symbol of healing, of one's life being saved from imminent death (Book of Numbers 26:69). In Christianity, Christ's redemptive work is compared to saving one's life through beholding the serpent of brass (Gospel of John 3:14). However, more commonly in Christianity, the serpent was seen as a representative of evil and sly plotting, which can be seen in the description in Genesis chapter 3 of a snake in the Garden of Eden tempting Eve. Saint Patrick is reputed to have expelled all snakes from Ireland while Christianising the country in the 5th century, thus explaining the absence of snakes there.

Source: Wikipedia > Snake