LOCOMOTION' IN GENERAL. 105 



opposite to the intended movement; this is the more usml mode 

 of locomotion ; walking, running, leaping, belong to this first 

 form. For this purpose the limbs serving for locomotion are 

 composed of a series of rigi 1 levers, susceptible of change in 

 length ; they can be shortened by the angular flexion of the 

 articulations, nnd they gro\v longer, by being drawn up. If 

 the leg when bent touches the ground at its extremity, and if 

 a muscular effort be made to produce the extension of the 

 limb, this can only be effected by removing 1 to a greater dis- 

 tance from each other the ground on which the extremity of 

 the leg rests and the body of the animal which is united 

 to the base of this limb; the ground offers resistance, and the 

 body, yielding to the impulse, is displaced. Sometimes the 

 displacement in terrestrial locomotion is effected, not by a 

 change in length, but by a simple change of the angle formed 

 between the limb which causes the motion and the body of 

 the animal. 



In the second type, namely crcc]>iny, a tractile effort is pro- 

 duced ; the animal lays hold by a part of its body on an ex- 

 ternal fixed point, and then drags the mass of its bulk towards 

 this point. Let us take a snail, and place it on a piece of 

 transparent glass ; at the end of a few moments the animal 

 begins to crawl. If we turn the glass over, we shall see 

 through the plate the details of its movements. Throughout 

 all the length of its body appears a series of transverse bands, 

 alternately pale and deeply coloured, opaque and transparent. 

 These bauds are transmitted by a continual motion, from the 

 tail to the head of the animal; they seem like the spirals of 

 a screw which turns incessantly in the same direction. It' we 

 fix our attention ou one of these b.uids in the neighbourhood, 

 of the tail, we sse it pass towards the head, which it 

 teaches in fifteen or twenty seconds, but it is followed by 

 a continued series of bands which seem to spring up behind 

 it as it advances. These bands remind us of the muscu- 

 lar wave and its progress through a contracting tibi'e, only 

 with greater dimensions. E ich time that a wave arrives 

 at the cephalic region of the animal, it disappears, producing a 

 forward motion of the head, which slips a little on the surface 

 of the glass and advances slightl}' without any retrogression. 



