190 AMPHIOXUS 



through the gill-shts. The atrial epithelium consists 

 of a single layer of pigmented columnar cells, and is 

 often much folded on the ventral wall. 



7. The coelom is much subdivided in this region : its 



principal divisions are as follows. 



a. The dorsal ccelomic canals are a pair of large ir- 



regular spaces at the sides of the hyperbranchial 

 groove, above the suspensory folds of the pharynx, 

 and between these and the atrial folds. 



b. The branchial coelomic canals are the cavities in 



the primary branchial bars, between the atrial 

 epithelium and the skeletal rods. They all open 

 at their dorsal ends into the dorsal coelomic canals. 



8. The metapleural canals are a pair of large spaces, 



triangular in section, lying in the metapleural folds. 



9. The circulatory system. The vessels can be readily re- 



cognised in sections, owing to the coagulated blood 

 which they contain. 



a. The cardiac aorta is a median vessel lying in the 



floor of the pharynx. 



b. The dorsal aortae are a pair of vessels lying one on 



each side of the hyperbranchial groove. 



c. The aortic arches lie in the branchial bars, both 



primary and secondary, along the inner sides of 

 the skeletal rods. 



10. The nervous system is the same as in A. 



C. Transverse Section passing through the Hinder Part of 

 the Pharynx. 



1. General characters. The section is larger thanB, pass- 

 ing through the thickest part of the body. It is 

 triangular in shape, the angles being formed by 

 the dorsal and lateral fins respectively. The section 

 passes through the laterally compressed pharynx, on 

 the right side of which is the liver. The reproduc- 

 tive organs project into the atrial cavity from the 

 inner sides of the atrial folds. 



