In this article we will discuss about the structure of snakes.
Due to fossorial adaptation, snakes show many peculiar features (Fig. 8.20).
External Features:
(i) The body of snakes is extremely elongated and cylindrical.
(ii) The limbs are absent, except in certain genera where traces of posterior limbs are present at the base of the tail and bear conical claws (Fig.8.20C).
(iii) Distinct neck is absent.
(iv) The body is covered with scales and shields.
(v) The osteoderms are absent.
(vi) The scales are usually small and overlap each other, but the shields are usually large and join each other by their margins.
(vii) The head is covered mostly by the head shields (Fig. 8.20A).
(viii) The head shields are named according to their placement (Fig. 8.21).
(ix) The shields situated in the mental groove on the ventral side between the rami of the lower jaws are called mental shields. The mental shields are considered to be peculiar features.
(x) The other shields are frontal shield, super-ciliary shields, rostral shield, upper labial shields, nasal shield, pre-orbital shields, loreal shield, median labial shield, jugular shields, ventral shields, etc.
(xi) The scales on the ventral side of the body are broad and cover the body like transverse bands. These ventral scales are called gastrosteges (Fig. 8.20B).
(xii) The whole of the scaly epidermis is cast off several times in a year. This phenomenon is called the ecdysis or moulting.
(xiii) The head is more or less conical.
(xiv) The nostrils are fused over the eyes.
(xv) The tympanic membrane, tympanic cavity and Eustachian tube are absent.
(xvi) The cloacal aperture is a transverse slit.
(xvii) Males are provided with paired copulatory organs.
Internal Structures:
A. Soft Parts:
The internal peculiar structures of the snakes are seen according to their peculiar mode of living in different habitats.
Digestive System [Fig. 8.22]:
(i) The mouth cavity is bounded by the upper and lower jaws.
(ii) The tongue is provided with a muscular sheath into which the tongue may be retracted and this part is highly sensitive, and used as a tactile organ.
(iii) The teeth are conical, backwardly curved and are situated in the maxillae, palatines and pterygoid bones in the jaw. Dentition is pleurodont type.
(iv) The labial glands are modified into poison glands in case of poisonous snakes and do not help in digestion and the large backwardly directed maxillary teeth become into fangs in venomous snakes.
(v) The buccal cavity leads into shorter pharynx.
(vi) The oesophagus is thin walled and opens into a muscular dilated and elongated part, called stomach.
(vii) The distal part of the stomach becomes narrower and is marked by the pyloric valve. After the pyloric valve, the intestine begins.
(viii) The intestine is divided into well- defined small intestine and large intestine that ends in a cloaca.
(ix) Liver is elongated and is formed by two unequal lobes. The right lobe is longer than the left.
Respiratory System [Fig. 8.23]:
(i) The respiratory system consists of a long trachea and lungs. The wall of the trachea is supported by numerous cartilaginous rings.
(ii) The anterior part of the trachea is dilated and form, the larynx which is supported by cricoid and arytenoid cartilages.
(iii) The lungs in the snakes exhibit an asymmetrical feature and the left lung is often small and sometimes absent.
The right lung is always very long and in marine species, it may extend up to the cloaca and it performs the hydrostatic function. In the more primitive groups both lungs lobes are present (e.g., Python, Boa). The elongated trachea opens at the anterior portion of buccal cavity and be protruded between the two halves of the lower jaw.
This is a modification for respiration during swallowing of prey. In some forms, a part of the trachea is specialized and acts as a tracheal lung. This is due to the incomplete tracheal rings and this part expands as a tracheal membrane. This membrane develops as an alveoli and extends up to the right lung proper.
Circulatory System:
(i) The heart consists of two auricles and one incompletely divided ventricle. The ventricular septum is well developed which indicates the division of the ventricle.
(ii) The arterial arches are asymmetrical—mainly the carotid and jugular.
(iii) Ductus caroticus is absent.
Due to imperfections in the respiratory and circulatory systems, the reptiles, specially the snakes, are compelled to supply enough oxygen to their tissues for food combustion. So their body temperatures can be raised a few degrees above the surrounding temperature.
Nervous System and Organs of Special Sense:
(i) The brain is narrow, elongated and projected between the eyes and is more highly organised than the amphibians.
(ii) There are ten pairs of cranial nerves.
(iii) The vomeronasal or Jacobson’s organ (Fig. 8.24) is well developed and is used for taste and small analysis of the dust particles (Fig. 8.25).
(iv) The eyes of snakes have many peculiarities (a) There are no scleral ossicles or cartilages and accommodation is brought about by the lens displacement (b) The visual cells include peculiar types of cones, developed from rods. From the above peculiarities of the eyes, Walls (1942) concluded that snakes originated as burrowing creatures and later developed as the animals of the above surface of the ground.
(v) In the ear portion, the middle ear cavity—the tympanic membranes, tympanic cavity and eustachian tube— is reduced or absent and the columella auris occurs and attached externally to the quadrate. The general belief is that the snakes cannot perceive the air-borne sound.
But according to Marshall and Williams (1964), “the snakes are highly sensitive to earth- borne vibrations due to the attachment of columella to quadrate”, and according to Young (1981), “they can hear the air-borne sounds only of low frequency”.
Urinogenital System:
(i) The kidneys are asymmetrical and elongated. In the land snakes the number of glomeruli is highly reduced.
(ii) The urinary bladder is absent.
(iii) The gonads are asymmetrical. The males are provided with a pair of eversible copulatory organs, the apex of which bears spiny projections.
(iv) Anal glands are present at the base of the tail and discharge into cloaca. There is some evidence that the secretion is odorous in nature and attracts the opposite mates during the breeding season.
(v) The sexes are distinguished by a number of features—
(a) the size of the body—the males are smaller than the females.
(b) the sizes of the tail between the male and female sexes. The males have a longer swollen base tail and this swelling is due to the paired hemipenes, the male copulatory organs, at the base of the tail. The tail in the female is generally tapered and ends in a fine point.
(vi) Snakes are mostly oviparous, some are ovoviviparous or viviparous (laying young ones directly), Uropeltis ocellatus, various vipers (vipera sp.) and sea snakes (except amphibious forms) are ovoviviparous or viviparous in nature.
Oviparous and ovoviviparous can be distinguished in such way as, if laying precedes hatching the animal is called oviparous but the individuals of the same species may hatch eggs in the “oviducts”, called ovoviviparous.
Development:
(i) The eggs are moderately telolecithal and the egg shell is leathery.
(ii) Segmentation is meroblastic.
(iii) Some snakes have developed yolk sac or chorioallantoic placentation.
(iv) The embryos of snakes possess a small “egg tooth” that helps to puncture the shell and shell-membrane.
B. Hard Part:
Skull (Fig. 8.26 A,B):
(i) The skull is streptostylic (i.e., the quadrate is highly movable) in nature and occipital condyle is distinctly tripartite.
(ii) The fossae and inter-parietal foramen are absent.
(iii) The parietals are fused to form a single piece (Fig. 8.26A).
(iv) The frontals are provided with lateral projections which unite at the floor of the skull.
(v) The inter-orbital septum is absent.
(vi) The palatines are slender and widely separated. They are connected by the pterygoids behind. The transverse or epipterygoid bones are connected with pterygoid and maxillary bones at the rear and front ends.
(vii) The posterior border of the orbit is formed by the post-frontal.
(viii) The premaxillae are united into a single piece and are usually toothless. The maxillae, palatines and pterygoids are freely movable.
(ix) The quadrate and squamosal of the skull (Fig. 8.26B) are loosely articulated and squamosal is horizontal in position and extends beyond the posterior limit of skull.
(x) The rami of the lower jaw are connected by elastic ligamentous tissue so that the gape of the mouth can be increased several times to engulf the large prey during swallowing.
Vertebral skeleton:
(i) The vertebrae are strongly procoelous and are divided into precaudal and caudal regions.
(ii) Besides pre- and post-zygapophyses, extra-articulating surfaces, called zygosphenes, at the anterior face and zygantra at the posterior end are present (Fig. 8.26C, D). The zygosphene is a wedge-like process, situated at the base of the neural arch of the anterior surface.
(iii) Some of the vertebrae may have a median hypophysis.
(iv) Transverse processes are rod-shaped and well-developed. These processes at the caudal region are bifid.
(v) The ribs are single headed and present in all vertebrae except atlas.
(vi) Distally the ribs are attached to ventral scales by ligaments to help in locomotion.
(vii) The girdle and girdle bones are completely absent except Python, only vestige of pelvic girdle is represented.
