SECTION THROUGH PHARYNX 187 



canal, which is part of the coelom, is much 

 larger in the dorsal than the ventral bars of the 

 section, and in the most dorsally placed ones it 

 becomes a space of considerable size, the atrial 

 epithelium being prolonged outwards for some 

 distance. The uppermost two or three of these 

 outward prolongations of the atrial epithelium 

 unite together to form the suspensory folds of the 

 pharynx, which sling it to the inner surface of the 

 atrial folds, the branchial coelomic canals opening 

 here into the dorsal coelomic canals (fig. 32). 



b. The hyperbranchial groove is a deep groove, lined 

 by columnar epithelium, in the mid-dorsal line of 

 the pharynx immediately beneath the notochord. 



C. The ventral wall of the pharynx is a flat or slightly 

 convex plate, covered by a layer of columnar 

 epithelial cells. The hypobrancMal groove does 

 not extend into this part of the pharynx. 



6. The atrial cavity surrounds the sides and ventral surface 



of the pharynx, lying between it and the atrial folds, 

 and communicating with the cavity of the pharynx 

 through the gUl-slits. The atrial epithelium consists 

 of a single layer of pigmented columnar cells, and is 

 often much folded along the ventral surface. 



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



principal divisions are as follows. 



a. The dorsal coelomic 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. 



c. The metapleural coelomic canals are a pair of large 



spaces, triangular in section, lying in the meta- 

 pleural folds. 



