Anatomy
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. 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.
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. 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.
Locomotion and Limblessness
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.
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 a marine environment and are descended from marine lizards.
Support for the latter view comes from recent discovery of the most primitive snake now known, a fossil specimen with two short but well-developed hind legs found in marine sediments in Israel. It lived in a shallow sea 95 million years ago.
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 a marine environment and are descended from marine lizards.
Support for the latter view comes from recent discovery of the most primitive snake now known, a fossil specimen with two short but well-developed hind legs found in marine sediments in Israel. It lived in a shallow sea 95 million years ago.
Tidak ada komentar:
Posting Komentar