SECTION THROUGH PHARYNX. 203 



canal, which is part of the coelom, is much 

 larger in the dorsal than in the ventral bars of 

 the section, and in the most dorsa ones it is a 

 space of considerable size, the atrial epithelium 

 being produced outwards for some distance. The 

 uppermost two or three of these outward pro- 

 longations 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. 41). 



b. The epibranchial groove is a deep median groove 



in the dorsal wall of the pharynx immediately 

 beneath the notochord. It is lined by columnar 

 epithelium. 



c. The ventral wall of the pharynx is formed by a flat 



or slightly convex plate, the endostyle, covered 

 by a layer of columnar epithelial cells. 



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 gill-slits. The atrial epithelium consists 

 of a single layer of pigmented columnar cells, and is 

 often much folded on the ventral wall. 



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 epibranchial 

 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. 



0. The endostylar coelomic canal is a median space in 



