II. 4 



LOCOMOTION OF AMPHIOXUS 



27 



is usually stated that this is a 

 'supporting structure', but, of 

 course, an animal such as a fish in 

 water needs no 'support'. Nor is 

 the notochord a lever to which 

 muscles are attached, as they are 

 to the bones of many higher forms. 

 No muscles pull on it directly, 

 though the myocommas are at- 

 tached to its sheath. Its function 

 is to prevent the shortening of the 

 body that would otherwise be the 

 result of contraction of longitudi- 

 nal muscles. In fact, it serves to 

 make that contraction efficient in 

 bending the body; its elasticity 

 may also play an important 

 part. 



The notochord is composed of 

 a series of flattened plates sur- 

 rounded by a fibrous sheath. The 

 plates are arranged in a regular 

 manner with their flat surfaces in 

 the transverse plane of the body. 

 They are of two sorts, fibrous 

 and homogeneous, which alternate 

 with each other. Each plate de- 

 velops as a highly vacuolated cell, 

 the nuclei being later pushed aside 

 to the dorsal or ventral edge. This 

 structure is well suited by the 

 turgidity of its cells enclosed in 

 the sheath to resist forces tending 

 to shorten the body. The cord of 

 amphioxus is peculiar in that it 

 extends from the very tip of the 

 head to the end of the tail, pro- 

 jecting, that is to say, beyond the 

 level of the myotomes, a condition 

 presumably associated with the 

 burrowing habit. 



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