XIII 



PHYLUM CHORD ATA 



49 



growth takes place from 

 the dorsal edge of the 

 aperture, forming, as in 

 Balanoglossus (p. o), a 

 tovgue 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 or 

 synaptvn(hi\ supported 

 by rods connecting the 

 primary septa with one 

 another at tolerably 

 regular intervals. 



The Atrium. — The 

 o-ill- clefts lead into a 

 wide chamber occupy- 

 ing most of the space 

 between the body-wall 

 and the pharynx, and 

 called the atrium (Figs. 

 742, B, and 743, atr.). 

 It is crescentic in sec- 

 tion, 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 (atrj/.) ; and 

 is continued backwards 

 by a blind, pouch-like 

 extension (atr.) lying to 

 the right of the intes- 

 tine (Fig. 742, 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 



VOL. II 



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