THE PHYLUM CHORDATA 



39 



a voluminous water-filled chamber, the atrial cavity. The atrium is 

 lined with ectoderm and has but one opening to the exterior, a poste- 

 riorly directed atriopore, which carries off the water that comes 

 through the pharyngeal clefts. The atrium is a protection for the 

 delicate pharjmx while the animal is in its sandy burrow and helps 

 to maintain an uninterrupted current of water. 



sn 





JM 



s>li 



T~*^Af 



B 



Fig. 11. — A. Lateral view of brain of Amphioxus. cv, central vesicle; dd, dorsal 

 dilatation of the neural canal; es, eye spot; of, olfactory funnel; np, neuropore; 

 /, first cranial nerve, olfactory; //, second (optic) cranial nerve, showing dorsal 

 and ventral roots. 



B. Dorsal view of brain and spinal cord of Amphioxus. oes, accessory dorsal 

 eye spots, some median, some paired, es, eye spot; I and //, first and second 

 cranial nerves; sn, spinal nerves. (Redrawn from WiUey, after Hatschek and 

 Schneider.) 



3. General Characters. — The alimentary system consists of the 

 phamyx, a short, straight stomach intestine terminating in a ventral 

 anits, which opens to the left of the ventral fin. The stomach gives 

 off a ventral diverticulum or liver, which is directed forward. 



The circulatory system (Fig. 14) consists of a ventral pulsating vessel 

 with no specialized heart enlargement, which pumps the colorless 



