184 AMPHIOXUS 



its sides being formed by growth forwards of the 

 anterior ends of the atrial folds. The true mouth of 

 Amphioxus is the aperture in the velum leading into 

 the pharynx, and not the entrance to the buccal cavity. 

 The epithelium hning 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 abcfve the notochord. In transverse 

 section it is somewhat triangular in shape, 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 seen in the 

 whole animal. Prom the central canal a narrow 

 vertical slit extends upwards to the dorsal surface 

 of the cord, bordered along each side 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. Large nerve-cells 

 also occur around the central canal. The greater 

 part of the cord has a finely punctate appearance, 

 caused by the cut ends of the nerve-fibres. 



b. The nerves will only be recognised if the section 



happens to pass through their roots of origin. 



i. The dorsal nerves arise by large single roots 

 from the sides of the cord, about midway 

 between the dorsal and ventro-lateral angles. 



iij The ventral nerves arise by very minute and 

 multiple rootlets from the ventro-lateral angles 

 of the cord. 



