SECTION THROUGH ATRIAL PORE. 



215 



D. Transverse Section passing through the Atrial Pore. 



1. General characters. The section is smaller than the 

 preceding one, and the myotomes reach much 

 further down the sides. The intestine is cut across, 



Fig. 45. Amphioxus lanceolatus. Transverse section of an adult 

 specimen, passing through the atrial pore. The boundary of the 

 atrial cavity is indicated by a thick black line. (a. m. m.) 



A, skeleton of dorsal fin. B, spinal cord, c, notochord. D, connective- 

 tissue sheath of notochord. H , atrial cavity. K, intestine. O, ccelomic space 

 surrounding intestine. P, metapleural canal. Q, dorsal aorta. U, portal 

 veins. V, atrial pore, represented diagrammatically. W, metapleural folds. 



but the section lies entirely behind the reproductive 

 organs. The metapleural folds are present, but are 

 much smaller than in sections B and C, and between 

 them in the median plane is the atrial pore. 



The skin, and the skeletal, muscular, and nervous 



