EVOLUTION AS SEEN IN EMBRYONIC DEVELOPMENT S7 



contracted. Since the contraction shortens them, the result is necessarily 

 the bending of the head to one side, as shown. Now this "first flexure" 

 starts to move posteriorly down the row of segments, by successive con- 

 traction of one somite after another down the length of the animal. At the 

 same time myotomes between the flexure and the head of the animal relax 

 in corresponding sequence. Thus the flexure "travels" down the length of 

 the animal. When the first flexure nears the tail a second flexure forms on 

 the right side just behind the head (Fig. 4.7, third diagram). Then this 



exure 



secon 

 flexure 



first flexure 



FIG. 4.7. Action of segmental body muscles in producing swimming movements. (After 

 Coghill, Anofomy and fhe Problem of Behaviour, Cambridge University Press, 1929.) 



second flexure "travels"" down the length of the body. The combination of 

 the two flexures throws the body into an S curve, as shown. The bends of 

 the body travel backward, pressing against the surrounding water and 

 sending the body forward. The result is comparable to that obtained from 

 the thrust of a propeller blade. In rapid locomotion the successive right 

 and left flexures follow each other with great rapidity. 



