THE ATRIAL CAVITY. 



83 



atrial folds meet and fuse, converting the groove into a tube (Fig. 

 40, ac). This tube, of which the roof is formed by the ventral 

 wall of the body, the sides by the metapleural folds, and the floor 

 by the fused subatrial folds, is the atrial chamber. The formation 

 of the floor of the chamber proceeds from behind forwards. In 

 the larva shown in Fig. 37, in which there are fourteen primary 

 slits, and the secondary slits are just commencing to form, the 



Fig. 41. — A diagrammatic transverse section through an advanced Amphioxus 

 Larva with fully formed atrial cavity. (Slightly modified from Lankester 

 and Willey, and from Boveri.) 



A, aorta. AC, atrial cavity. AP, floor of atrial cavity, formed by fusion of the 

 subatrial folds. CH, notochord. CM, myocoel. CJX", diverticulum of rnyoooel lying 

 between notochord and myotomic muscle. CS, splanclmoccel. CTJ, cutis layer. DF, 

 cavity of dorsal fin. HS, skeletogenous layer. I, spinal cord. MD, metapleural ridge. 

 MF, muscle-fascia layer. ML, myotomic muscle. MV, metapleural canal. OR, 

 nencing reproductive organs. TI, intestine. V, subintestinal vessel. 



atrial tube is completed to about the level of the ninth primary 

 gill-slit ; and at a stage shortly before that shown in Fig. 38 

 the tube is completed along the whole length of the pharynx. 

 The anterior end of the tube ends blindly, but the posterior end 

 remains open as the atrial pore. 



The atrial tube is at first very narrow, and of nearly equal 



G 2 



