1. Of the four main types of locomotion observed in snakes, three (serpentine, concertina and crotaline) can be elicited from the common grass snake (Tropidonotus natrix) by appropriate modification of the animal's environment.
2. Serpentine motion depends on three factors, (i) The body must be thrown into one or more curves each of which exhibits an increase of curvature when measured towards the head of the animal, (ii) Active muscular tension must develop in the axial muscles which lie on the same side of the body as that in which the curvature is increasing, (iii) The body must be subjected to at least three external resistances acting normally to the surface of the body. The propulsive force is the resultant of the reactions exercised by all these external resistances.
3. A snake cannot propel itself by serpentine movement along a straight or circular path. Under such conditions Tropidonotus progresses by concertina movements, the nature of which are described.
4. The muscular cycle of a snake exhibiting ‘crotaline’, or side-winding, movements is essentially the same as that during serpentine motion; the difference in the type of movement relative to the ground is due to a difference in the nature of the external resistances offered by the animal's environment. The mechanical principle of crotaline movement is, fundamentally, that of a caterpillar tractor.
5. Serpentine, concertina, and crotaline movements do not depend on active movements on the part of the ribs or scales. Rectilinear movement involving these structures has not been observed in Tropidonotus.