XIII 



PHYLUM CHORDATA 



43 



growth takes place from 

 the dorsal edge of the 

 aperture, forming, as in 

 Balanoglossus (p. 3), a 

 tongue which extends 

 downwards, dividing the 

 original cleft into two, 

 and itself becoming a 

 secondary lamella. A 

 further complication is 

 produced by the for- 

 mation of transverse 

 branchial junctions con- 

 necting the primary 

 septa with one another 

 at tolerably regular 



\J CD 



intervals. 



The Atrium.- -The 



gill-clefts lead into a 

 wide chamber occupy- 

 ing most of the space 

 between the body-wall 

 and the pharynx and 

 called the atrium (Fig. 

 698, atr. ; Fig. 697, A, 

 1).). It is crescentic in 

 section, surrounding the 

 ventral and lateral re- 

 gions of the pharynx, 

 but not its dorsal por- 

 tion. It ends blindly 

 in front ; opens extern- 

 ally, behind the level 

 of the pharynx, by the 

 atriopore (atr p.) ; and 

 is continued backwards 

 by a blind, pouch-like 

 extension (atr'.) lying to 

 the right of the intes- 

 tine (Fig. 697, B, atr.). 

 The whole cavity is 

 lined by an atrial epi- 

 thelium of ectodermal 

 origin. As in Ascidia, 

 the cilia lining the gill- 

 clefts produce a current 

 setting in at the mouth, 



- 



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