THE CEPHALOCORDA 



i83 



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 coelom, but lymph spaces. There was 

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



Fig. 44 



Diagram illustrating the anatomy of the pharyngeal region of Ainphioxus, A 

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

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

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

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

 tnpt metapleur; mpc, metapleural canal; my, myotomes; nch^ notochord ; 

 «/i, nephridium ; nt, neural tube ; j>b^ primary gill-bar ; ib^ tongue-bar ; 

 a.co, sub-end ostylar ccelom. 



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

 the muscle fibres of the metapleural and epipleural folds. 



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