426 EVOLUTIONARY MORPHOLOGY 



None of these forms, however, exhibit the typical chordate 

 segmentation of the body, which enables them to swim in 

 definite directions instead of being carried aimlessly about at 

 the mercy of currents . The immediate ancestors of Amphioxus 

 and of the higher chordates were elongated, compressed from 

 side to side and deep from dorsal to ventral edge. As a 

 consequence, they were able to bend the body from side to 

 side, and perform undulatory movements. The body was 

 made up of several segments. Each segment was separated 

 from the ones in front and behind by septa or partitions, and 

 stretching from septum to septum were the myotomic muscle- 

 fibres. When the myotome on one side of a segment contracts, 

 the septa bounding that segment come closer together, and 

 the body becomes concave on that side. The advantage of 

 having several segments made it possible for the body to bend 

 in several places. By bending alternately right and left in 

 successive regions of the body, and making the bends pass 

 down the length of the body by throwing the myotomes into 

 contraction in succession, the undulatory movements are 

 produced which enable the organism to swim. These move- 

 ments were made still more efficacious by lengthening the body, 

 which was accomplished by the development of an extension 

 behind the anus forming the tail. After bending in any 

 place, the body became straight (before bending in the opposite 

 direction), and this was effected not only by the relaxation of 

 the myotome on that side, and the contraction of the myotome 

 on the opposite side, but by the possession of a stiff yet elastic 

 rod running along the whole length of the animal : the 

 notochord. This is the typical primitive method of chordate 

 locomotion which persists not only in the fish, but also in the 

 lowest land-vertebrates. 



The fact that the animal moved in a definite direction had 

 the consequence that the front end was further specialised by 

 a concentration of sense-organs, which ultimately was to bring 

 about the formation of a head. 



It has been held, with some degree of probability, that the 

 habit of swimming in a definite direction was evolved in re- 

 sponse to the constant direction of flow of water in rivers or 



