THE CEPHALOCORDA 



183 



It will be noticed that in addition to the cavities described 

 above, there is a considerable cavity in each metapleural fold. 

 These are not part of the ccelom, but lymph spaces. There was 

 a ccelomic cavity in each atrial fold at its first appearance, 



S.GO. 



Fig- 44 



Diagram illustrating the anatomy of the pharyngeal region of Amphioxus. A 

 segment has been cut out of the pharyngeal region, and a portion of the right 

 wall removed to show the atrial cavity, the pharynx and its gill-slits, and the 

 dorsal ccelom. ao, dorsal aorta ; atr, atrium ; dco, dorsal coelom ;' en, endo 

 style ; ep. epipleur ; fr, fin ray ; go, gonads ; hy, hyperbranchial groove 

 mp, metapleur ; -nipc, metapleural canal ; my t myotomes ; nch, notochord 

 nph, nephridium ; /, neural tube ; pb, primary gill-bar ; tb, tongue-bar 

 S.co, sub-endostylar ccelom. 



but it disappeared in larval life, its walls being converted into 

 the muscle fibres of the metapleural and epipleural folds. 



