PHYLUM CHORDATA 



325 



the pharyngeal wall sepa- 

 rating the clefts from one 

 another, are covered by 

 an epithelium composed, 

 except on the outer face, 

 of greatly elongated and 

 ciliated cells. Each sep- 

 tum is supported towards 

 its outer edge by one of 

 the chitinoid gill-rods 

 (br. r.) already referred 

 to. 



The gill-clefts lead into 

 a wide chamber occupy- 

 ing most of the space be- 

 tween the body-wall and 

 the pharynx and called 

 the atrium (Fig. 191, 

 atr. ). It is crescentic 

 in section, surrounding 

 the ventral and lateral 

 regions of the pharynx, 

 but not its dorsal portion. 

 It ends blindly in front ; 

 opens externally, behind 

 the level of the pharynx, 

 by the atriopore (atrp. ) ; 

 and is continued back- 

 wards by a blind, pouch- 

 like extension (atr . ) lying 

 to the right of the intes- 

 tine. As in Ascidia the 

 cilia lining the gill-clefts 

 produce a current setting 

 in at the mouth, entering 

 the pharynx, passing 

 thence by the gill-slits 

 into the atrium, and out 

 at the atriopore. The 

 current, as in Tunicata 



