SECTION THROUGH ATRIAL PORE 



191 



The lateral fins are present, but are much smaller 

 than in B or C, and between them in the median 

 plane is the atrial pore. 



The skin, and the skeletal, muscular, and nervous 

 systems are practically the same as in B, except that 

 the ventral muscles are absent. 



FIG. 34. 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. TT, intestine. O- ccelomic 

 space surrounding intestine. P, metapleural coslomic canal. Q, dorsal aorta. 

 TJ, portal veins. V, atrial pore. "W, metapleural folds. 



2. The intestine is circular in section. Its walls are thick 

 and, like those of the liver, consist of a single layer 

 of very long and slender epithelial cells, resting on 

 a thin -connective-tissue basement membrane. 



