CEPHALOCHOEDA 



133 



protruding. It feeds on suspended matter, mostly diatoms 

 and minute animal life. At night it becomes more active, 

 and then has been observed to emerge from the burrow and 

 swim about. 



It has a symmetrical fish-like shape, tapering at both 

 ends. The sides of the body are occupied by the < -shaped 

 myotomes or muscle segments ; the angle of the < is 

 directed anteriorly. The numbers of the myotomes between 

 the head and the atriopore and between the head and the anus 

 are employed to distinguish species. In the lanceolatus group 

 the preatrioporal myotomes number about thirty-six, and the 

 preanal fifty to fifty-six. The species found on the west coast 

 of America and at the Cape have myotomes which number 

 respectively about forty-five and sixty. 



Notochord 



Dorsal fin 



Oral cavity Metapleure Gonad Atriopore Ventral fin Anus Caudal fin 

 FIG. 61. Amphioxus. The adult from the left side. 



The body is laterally compressed, and the margin is pro- 

 duced to form a median fin. The dorsal fin runs from end to 

 end of the body, and is expanded at the posterior end to form 

 a caudal fin ; and below, the median fin sinks again to form the 

 ventral fin, which reaches forwards to the atriopore. Between 

 the atriopore and the oral cavity paired folds of the body wall 

 occur, subtending a ventral groove. These are termed meta- 

 pleural folds. Between them and the myotomes are square- 

 shaped masses which are the gonads. 



Below the anterior extremity is the wide opening of the 

 oral cavity. The cavity narrows internally to an opening 

 which is the true, or at all events the original, mouth. The 

 margin of the oral cavity is produced into numerous processes 

 which are called cirri. The anal opening is on the left side of 

 the ventral fin. The atriopore is the median opening at the 

 posterior end of the metapleure and in front of the ventral 

 fin ; it discharges the contents of the atrial cavity. 



