Snakes constitute the suborder Serpentes (or Ophidia). In most snakes limbs are entirely lacking, but a few have traces of hind limbs. The skin, which is covered with horny scales, is shed, usually several times a year. The extremely long, narrow body is associated with distinctive internal features. The number of vertebrae is much larger than in most vertebrates, paired internal organs are arranged linearly rather than side by side, and only one lung is developed, except in members of the boa family, which have two lungs. The jaws of snakes are loosely jointed and extremely flexible. The pointed, backward-curved teeth are fused to the supporting bones of the head. There are no ears or movable eyelids; the eyes are covered by transparent "spectacles," or ocular scales. Snakes have good vision. They do not hear airborne sound waves, but can perceive low-frequency vibrations (100–700 Hz) transmitted from the ground to the bones of the skull. A chemosensory organ opens into the roof of the mouth; it receives stimuli from the forked tongue that constantly tastes the surroundings as the animal moves along. Snakes have no larynx or vocal chords, but are capable of producing a hissing sound.
A snake moves by means of muscular contraction, which can produce several types of locomotion, the commonest types being undulation and straight-line movement. Straight-line movement is aided by the ventral plates, elongated scales on the abdomen that overlap with their open ends pointing toward the tail. These plates can be moved forward by means of muscles attached to the ribs.
It is believed that snakes are descended from lizards, but how and why they evolved toward limblessness is uncertain. Some paleontologists have held that limblessness was an evolutionary advantage in the dense vegetation that formed the early environment of snakes, or that it developed to facilitate burrowing habits, but others believe that the earliest snakes evolved in an aquatic environment and are descended from marine reptiles related to mosasaurs. The fossil evidence for a land or marine origin is inconclusive.
Small snakes feed on insects and larger ones on proportionately larger animals. Their teeth are designed for catching and holding prey, but not for chewing. The construction of the jaws, the ribs, and the expandable skin enable them to swallow very large prey whole. Some snakes capture animals by pinning them to the ground; some—the constrictors—crush them by wrapping their bodies around them and squeezing; still others—the venomous snakes—inject poison into their victims. The poison, or venom, is produced by modified salivary glands from which it passes through either a groove or a hollow bore in the fangs, the enlarged, specialized teeth found in venomous snakes. A snake may bite a person when threatened or alarmed; if the snake is venomous the bite can sometimes prove fatal (see snakebite). Only by familiarity with the appearance of particular species, or by examination of the fangs, can the venomous snakes be distinguished from the harmless ones.
Fertilization is internal in snakes; as in lizards, the males have paired copulatory organs, either of which may be used in mating. Females of some species can store sperm for several years to insure future fertilization. In most species the female lays eggs; in some the eggs are incubated and hatched within the mother's body; in a few there is true viviparity, or live birth, with the young nourished by means of a placenta rather than an egg. Some egg-laying snakes brood the eggs, but there is no parental care of the young.
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