Body Types
The snake’s spine consists of the precaudal and the caudal vertebrae, which needs to be highly flexible to allow locomotion and various acts of contortion. Each precaudal consists of a centrum which is cylindrical in shape, with a convex and a concave end. The corresponding ends of each centrum and the outward jutting processes or spines, articulate with one another. Above the centrum is the neural arch, through which the spinal cord runs.
The caudal vertebrae in the tail are more simple and do not have ribs attached. In some species of the genera Coluber and Scaphiodontophis the tail can be truncated as a defence mechanism. Although no species have pectoral girdles, some primitive families retain vestigial pelvic girdles such as the Anillidae, Boidae, Cylindrophiidae, Leptotyphlopidae, Typhlopidae and some of the members of the Uropeltidae. The Boidae retain vestigial limbs and the spurs are visible on either side of the anal scale. The male snakes use these spurs to stimulate females during courtship.
The Scull The unique skull consists of bones that are loosely joined. The jaws are loosely connected rather than being fused to the cranium or brain case, enabling them to move outwards as well as backwards and forwards, independently of the rest of the scull or each other. The two halves of the lower jaw are joined by an elastic ligament, allowing them to move away from one another, thus increasing the capacity of the mouth.To avoid damage to the brain when large prey is forced through the mouth, the bones in the roof of the mouth are extended and strengthened. The jaws of primitive species have limited movement and as a result these species feed on soft-bodied invertebrates.The coronoid bone is a small bone located in the lower jaw. This bone is quite large in primitive snakes and it becomes progressively smaller, or absent altogether in the more advanced families.Their teeth are of pleurodont type and are found along the ridge of the mandible (the lower jaw), the maxilla (the outer portion of the upper jaw), the palatine bones (long bones that run along the roof of the mouth inside the maxillary bones) and the pterygoid bones (paired bones that are fixed to the palate at the front of the quadrate bones, at the back of the mouth). The replacement teeth develop at the base of the existing ones, ready to be swung upwards and sideways when the old teeth are shed.
Members of the family Boidae have many long recurved teeth which enable them to hold onto their prey while constriction takes place.
Elapids have short, fixed, hollow fangs which are positioned at the front of the maxilla In the spitting cobras the fang outlet is situated in the front of the fang, somewhat above the tip.
Snakes are defined by having vertebrae, ribs and the absence of limbs, eyelids and external ear openings. The snake body is supple, muscular and may be of varying shapes and lengths depending on the species habits.
Burrowing species such as the African Asps, Mole vipers, Purple-glossed snakes and the Centipede-eating snakes in Atractaspididae, the Pipe snakes in Aniliidae, Anomochilidae and Cylindropheidae, the African burrowing snake of the Boidae, the Coral snakes in Elapinae, the Mexican burrowing snake of Loxocemidae, the Shield-tailed snakes of Uropeltidae, the primitive burrowing species of the infraorder Scolecophidia, several genera of the Colubridae and several species of the terrestrial Hydrophinae, are cylindrical in cross section. This body shape enables them to burrow into loose soil with ease. These species generally have shortened tails, their heads are not distinct from their bodies and their scales are smooth and shiny.
Heavy bodied species which are more muscular and also cylindrical in cross section include some of the boas and pythons of the Boidae, this body shape gives them a considerable advantage when constricting their prey.
Arboreal and aquatic species which are often laterally flattened, include some of the boids, Boidae, some colubrid snakes, Colubridae and the aquatic Hydrophinae, this enables them to move with ease through vegetation or be more efficient swimmers. The arboreal snakes often use their elongated, prehensile tails to anchor themselves onto vegetation, where the aquatic species which have paddle shaped tails are more proficient in propelling themselves through the water.
Large heavy bodied, terrestrial snakes which are dorsally flattened, include a few colubrid snakes, some Australian elapids such as the Death adders Acanthophis, the Copper heads in Austrelaps, the Tiger snakes in Notechis and most of the larger vipers from the family Viperidae. This body shape allows them to quickly soak up heat from the sun and substrate, while their ability to flatten their bodies greatly enhances their camouflage. These species generally have much shorter tails.
Some snakes are triangular in cross section and these species include the African file snakes in Mehelya and Asian kraits of the genus Bungarus, the advantage to this body shape is unknown.
The Skeleton
Each precaudal vertebra bears a rib, the ends are connected to each other and to the ventral and dorsal scales by muscles. This network from the bones, spinal cord, nerves, muscle and finally the scales, are key factors which enable the fluid and graceful movements of snakes.
The downward pointing hypapophyses on the precaudals may be present in primitive species or absent in the more advanced families. However, hypapophyses are absent in the more primitive Leptotyphlopidae, Typhlopidae and some of the burrowing species in other families and invariably prevalent in the aquatic species. The Egg-eating snakes of the genera Dasypeltis from Africa and Elachistodon from India, have several elongated hypapophyses in the neck region which are used to puncture the shells of eggs.
The caudal vertebrae in the tail are more simple and do not have ribs attached. In some species of the genera Coluber and Scaphiodontophis the tail can be truncated as a defence mechanism. Although no species have pectoral girdles, some primitive families retain vestigial pelvic girdles such as the Anillidae, Boidae, Cylindrophiidae, Leptotyphlopidae, Typhlopidae and some of the members of the Uropeltidae. The Boidae retain vestigial limbs and the spurs are visible on either side of the anal scale. The male snakes use these spurs to stimulate females during courtship.
The Scull The unique skull consists of bones that are loosely joined. The jaws are loosely connected rather than being fused to the cranium or brain case, enabling them to move outwards as well as backwards and forwards, independently of the rest of the scull or each other. The two halves of the lower jaw are joined by an elastic ligament, allowing them to move away from one another, thus increasing the capacity of the mouth.To avoid damage to the brain when large prey is forced through the mouth, the bones in the roof of the mouth are extended and strengthened. The jaws of primitive species have limited movement and as a result these species feed on soft-bodied invertebrates.The coronoid bone is a small bone located in the lower jaw. This bone is quite large in primitive snakes and it becomes progressively smaller, or absent altogether in the more advanced families.Their teeth are of pleurodont type and are found along the ridge of the mandible (the lower jaw), the maxilla (the outer portion of the upper jaw), the palatine bones (long bones that run along the roof of the mouth inside the maxillary bones) and the pterygoid bones (paired bones that are fixed to the palate at the front of the quadrate bones, at the back of the mouth). The replacement teeth develop at the base of the existing ones, ready to be swung upwards and sideways when the old teeth are shed.
Members of the family Boidae have many long recurved teeth which enable them to hold onto their prey while constriction takes place.
Elapids have short, fixed, hollow fangs which are positioned at the front of the maxilla In the spitting cobras the fang outlet is situated in the front of the fang, somewhat above the tip.
Rattlesnake scull The family Viperidae have enlarged, hollow, fangs on a pair of jointed maxilla which fold up into the roof of the mouth. Both these fangs are sheathed in protective membranes and when the mouth is opened the fangs swing forwards and the membranes are pulled back. The vipers may have several fangs in reserve, positioned behind the current fangs and ready to swing into position if one should be broken or shed.
Rear-fanged colubrid snake Some of the colubrid snakes have enlarged, grooved, rear teeth positioned behind the eye. In some of the species these rear teeth are positioned horizontally when the mouth is closed. But when the mouth is opened wide, the upper jaw swings into a vertical position and these rear teeth are brought into play. Other species in this family, although nonvenomous, also have enlarged teeth. Members of the family Atractispididae have very similar fang structure to the venomous colubrid snakes and elapids, although several members of the genus Atractaspis have large, hollow, erectile fangs at the front of the toothless maxilla.Internal Organs
The internal anatomy of snakes is similar to that of other vertebrates, the only exception being that the organs are elongated and paired organs such as the lungs and testis are staggered, rather than being situated side by side. In aquatic or primitive species the tracheal lung is much larger than those of advanced snakes, note the dissection of Daboia russelii above.
In snakes the oral glands, located in and under the tongue and the especially large labial glands in the lips, contain strong digestive juices. The glottis or wind pipe is highly muscular and can be thrust forward and held open when swallowing prey. The throat and oesophagus which have to accommodate large prey are greatly distendable and highly muscular enabling the snake to force food towards its stomach. The oesophagus becomes gradually wider until it opens into a muscular stomach where additional secretions break down the food. The gall bladder and pancreas are positioned towards the far end of the stomach. As snakes are carnivorous the intestines are not as long as in other animals and the excretory system lacks a bladder. Therefore snakes excrete uric acid, which is a semi-solid, white material containing the minimum amount of fluid to carry the excrement out of the body. The intestines merge into the rectum and in turn the cloaca and finally the vent. In the majority of species the vestigial or left lung is either very small or absent altogether. Only the Boidae and the Xenopeltidae have a sizable left lung. The right lung usually consists of several parts, including the bronchial or tracheal lung and the saccular lung. In the marine snakes and highly aquatic species, the saccular lung is greatly enlarged and the walls are thick and muscular. This enables them to store extra air for increased buoyancy and when needed, the air can be forced into the bronchial or functional part of the lung, where the oxygen can be extracted. The heart has three chambers, not four as in crocodilians and the large artery or aorta leaving the heart is modified and may vary from one family to another. The plumonary system which feeds blood to the lungs is more important among snakes, since most do not absorb oxygen through their skins as amphibians are able to. Although some aquatic species can absorb up to one fifth of their oxygen requirements through their skins. The nervous system is similar to that of most animals, although less complex. The spinal cord runs the entire length of the spine and in some of the primitive species there are networks of nerves which are the remnants of hind limbs, supporting theories that their earlier ancestors had legs. The differences are those connecting to the Jacobson’s organs, heat sensitive pits and scale tubercles.
The male hemiphines are paired and inverted, extending from the cloacae for eight to sixteen subscaduals down the base of the tail. They are covered with spine-like bristles to ensure the snakes are not accidentally separated during mating. The paired testes are staggered and long, coiled, seminiferous tubes run from the testes to the ureter where the sperm is stored in a bladder-like structure. This organ opens into the cloaca, where another organ, the papilla urogenitalis is situated and the sperm is transported along this groove to the base of the hemiphines.In female snakes the cavity is smaller, extending up to three subscaduals and they may be either oviparous, egg laying, or ovoviviparous, birthing live young. Females have a pair of staggered ovaries and in some species the left oviduct is absent. The eggs mature simultaneously and once the yolk is formed, they break through the walls of the ovary into the body cavity and then move into the oviduct. It is here that fertilization takes place and some species are able to store sperm in the seminal receptacle, which is situated at the posterior end of the oviduct. Some snake species are viviparous to an extent, as in the Red-bellied black snake, P. porphyriacus, found in Australia, which passes electrolytes and amino acids to the developing embryos through osmosis.
(For information concerning Reproduction, see the appropriate page)
External morphology
Scales The advanced snakes have a single row of wide scales on the underside of their bodies called the ventral’s, with the anal plate and the subscaduals on the tail being either singular or divided. In the primitive species the ventral’s are only slightly larger than the dorsal scales.
Fig 1 Fig 2The dorsal’s may be smooth in most of the boids, colubrids and elapids (Fig. 1) or keeled in the majority of viperids, some colubrids and the sea snakes (Fig. 2). The dorsal scales are overlapping or tile-like, except for the small granular scales of the aquatic file snakes.Their heads may be covered with large plate-like scales in the elapids, colubrids and some of the viperids (Fig. 3) and (Fig. 5), or small, keeled head scales in most of the viperid species (Fig. 4). The boids tend to have either small granular or large plate-like head scales, although a few species do have keeled scales.
The linual fossa is an opening between the rostral and mental scales situated on the snout and lower lip, which allows snakes to extend their tongues without opening their mouths (Fig. 3) and (Fig. 5).
Sensory organs The snake’s eyes, which are covered by single transparent scales called the brille, differ greatly from one species to another. The pupils may be vertically elliptical or horizontally elliptical as in some crepuscular, or the nocturnal species (Fig. 4), rounded in diurnal or burrowing species (Fig. 5), and key-hole shaped in some arboreal species which require binocular vision (Fig. 6). Primitive species such as blind snakes have rudimentary eyes which consist of a few light receptive cells.
The sensory organs in snakes are highly tuned, enabling them to track and capture their prey. The nervous system connects to the Jacobson’s organ in the roof of the mouth (Fig. 7). This organ registers scent particles which are first trapped and then deposited by the forked tongue.
Additional nerve endings connect to heat sensitive pits, (Fig. 10) in the pit vipers, boas and pythons (Fig. 8 & Fig. 9) and below scale pits and tubercles, which also register heat. The stapes are small bones reminiscent of the inner ear which are very sensitive and register vibrations. These connect to the quadrate bones which conduct the vibrations when the jaw touches the ground.
Fig 9 Fig 10 The Aquatic file snakes sense vibrations through microscopic hair-like bristles found on the skin. Visual adaptations may also be prevalent such as the ability to see in ultraviolet light, which may enable the snake to see the urine trail of its prey.
Caudal ornamentationThere are a wide variety of tail shapes among snakes which serve various purposes. These include the sea snakes with paddle-shaped tails to aid in swimming, or the shield tailed snakes of the family Uropeltidae which have rough shields or plates armed with spines. The most elaborate caudal ornaments would be the rattle on a rattlesnakes tail, which consists of the successive remains of hourglass-shaped scales covering the tip, used as a warning, and Pseudocerastes urarachnoides which has a number of long, bristle-like scales before the tip which terminates in a bulb-like structure, This spider-like lure is used to catch birds.
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