208 AMPHIOXUS. 



b. Other much smaller muscles are seen in connection 

 with the buccal skeleton and tentacles. 



5. The buccal cavity. 



The epithelium lining the buccal cavity con- 

 sists of a single layer of cells, which in the anterior 

 and ventral part are short and columnar, but in the 

 posterior and dorsal region are much elongated, 

 slender and flagellate. 



6. The nervous system. 



a. The spinal cord lies in a connective-tissue sheath 



immediately above the notochord. In transverse 

 section it is somewhat triangular, with rounded 

 angles, and is rather wider from side to side 

 than it is dorso-ventrally. 



The central canal lies nearer the ventral than 

 the dorsal surface : it is lined by short columnar 

 ciliated cells, and immediately below it are the 

 large deeply pigmented cells already mentioned 

 (p. 199). From the central canal a narrow 

 vertical slit extends upwards to the dorsal surface 

 of the cord, both the canal and the slit being 

 bordered by large nerve-cells. In young speci- 

 mens the slit is open above, but in adults it is 

 closed by connective tissue, and by processes of 

 the nerve-cells growing across it from side to 

 side. The greater part of the cord has a finely 

 punctate appearance, caused by the cut ends of 

 the nerve-fibres. 



b. The nerves can only be recognised in sections 



which happen to pass through their roots. 

 i. The dorsal nerves arise by large single roots from 



the sides of the cord, about midway between 



the dorsal and the ventro-lateral angles, 

 ii. The ventral nerves arise by very minute and 



multiple rootlets from the ventro-lateral angles 



of the cord. 



