THE CEPHALOCORDA 173 



and when alarmed will disappear into the sand at the bottom 

 of an aquarium in the twinkling of an eye. 



The external features of the animal are shown in fig. 42. 

 The body is about one and a half inches long, compressed from 

 side to side, and tapering to a point at either extremity (hence 

 its name dfiipio^vs, sharp at both ends). There is no trace of 

 a specialised anterior head. Examination with a lens shows 

 that the walls of the body are divided into a number of muscle- 

 segments or myotomes, separated from one another by parti- 

 tions or septa of connective tissue. These myotomes are not 

 simple rings like the annuli of a worm, but are bent at a sharp 

 angle, the apex of the angle pointing forwards, and one result 

 of this arrangement is that several of them are cut through 

 in any transverse section of the body. The most anterior and 

 posterior myotomes are very small, those in the middle of the 

 body are large, but do not extend to the ventral surface. As 

 many as seven or eight myotomes lie in front of the mouth. 

 It should further be observed that the myotomes of the two 

 sides of the body do not correspond, but alternate with one 

 another. 



At either end of the body the tip of an elastic skeletal rod 

 may be seen projecting beyond the myotomes. This rod is 

 the notochord or chorda dorsalis, and in a very young tran- 

 sparent specimen, or in one that has been cleared in turpen- 

 tine, it may be seen extending through the dorsal region of 

 the body in the form of a rod lying above the alimentary 

 canal, but below the more dorsally placed central nervous 

 system. The notochord, a structure which recurs in the 

 embryos at least of all higher vertebrata, is one of the most 

 important features in the anatomy of Amphioxus. 



The anterior end of the notochord is enclosed in a vertical 

 membranous fin. This fin is continued along the mid-dorsal 

 line as a low ridge, expands posteriorly into a lancet-shaped 

 caudal fin, surrounding the posterior myotomes, and is con- 

 tinued forward as another low ridge on the ventral surface for 

 about a quarter of the length of the body. The dorsal and 

 ventral fins are strengthened by a number of little skeletal 

 rods of cartilaginous consistency, called fin-rays. There is no 

 correspondence between the number of fin-rays and the 

 number of myotomes. There are sixty-two of the latter in 

 hioxus lanceolatus, and as many as 250 to 260 dorsal 



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