SECTION THROUGH PHARYNX 189 



layer of columnar cells. Immediately within this 

 is the skeletal rod, deeply grooved along its inner 

 surface. The rod is ensheathed in connective 

 tissue, which runs inwards from it as a narrow 

 plate clothed on each side by the pharyngeal 

 epithelium, a single layer of very long columnar 

 flagellate cells. 



The branchial bars are alternately large and 

 small, the difference being especially marked at 

 their outer edges. The large ones are the primary, 

 and the small ones the secondary bars. 



In the large primary bars a small space is 

 visible between the atrial epithelium and the 

 skeletal rod. This space, the branchial coelomic 

 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. 41). 



b. The hyperbrancMal 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 a flat or slightly 



convex plate, the hypobranchial band, covered by 

 a layer of columnar epithehal cells. The hypo- 

 branchial groove does not extend into this part 

 of the pharynx. 



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 



