The mechanism of locomotion in snakes.

Among snakes four main types of terrestrial progression have been described: (i) serpentine movement which is observable in all, or nearly all, genera; (ii) concertina movement as briefly described by Wiedemann (1932) in Viper a and Coronella; (iii) crotaline or 'side-winding' as seen in Crotalus or Cerastes (Mosauer, 19320,6); and (iv) rectilinear movement as exhibited by boas and other large types (Home, 1812). The observations described in the present paper have all been made on the common grass snake [Tropidonotus natrix); the evidence to be presented shows that serpentine, concertina or crotaline movements can all be elicited in this genus by appropriate modification of the external forces operating on the body of the moving animal. So far as is known, rectilinear movement—in which the animal can progress whilst its body is orientated along a straight line—does not occur in Tropidonotus. This type of locomotion depends on active movements on the part of the ribs and ventral scales; such movements have not been seen in Tropidonotus whose progression does not depend on active movements of either scales or ribs.