186 AMPHIOXUS 



compartments formed by the septa. Owing to 

 the obliquity of the myotomes, three or more- 

 are cut in each transverse section, and owing to 

 their > shape each myotome is cut twice. The 

 myotomes of the two sides are not opposite, but 

 alternate with each other. 

 b. Other much smaller muscles are seen in connection 

 with the buccal skeleton and tentacles. 



5. The atrial cavity. 



The buccal cavity is, strictly speaking, part of 

 the atrial cavity, and not of the alimentary canal, 

 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 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 seen in the 

 whole animal. From the central canal a narrow 

 vertical slit extends upwards to the dorsal surface 

 of the cord, both the canal and the slit being bor- 

 dered 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 



