Locomotion

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 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.