200 AMPfllOXUS. 



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 consists 

 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. 191). Prom the central canal a narrow verti- 

 cal slit extends upwards to the dorsal surface of 

 the cord, both the canal and the slit being bordered 

 by large nerve-cells. In young specimens 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 punc- 

 tate 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. 



