558 



CEPHALOCHOBDA. 



retained is the atriopore (Fig. 294, />). The lateral folds are also 

 found in the adult as projecting ridges of the body- wall, running from 

 the mouth to the atriopore. 



The atrial cavity thus farmed is at first a tube with a compara- 

 tively narrow lumen (Fig. 295 B) which only secondarily widens ; 

 this widening, which is accompanied by a shifting apart of the meta- 

 pleural folds, is brought about by the lateral growth of the atrial 

 cavity between the wall of the intestine and the body-wall pressing 

 into the coelom so as almost completely to grow round the alimentary 

 canal (Fig. 311, p. 572), the coelom being proportionately reduced as 



FIG. 295. Three diagrammatic transverse sections through older larval stages of 

 Amphioxus to illustrate the development of the atrial cavity (after RAY LATSKESTER 

 and WILLEY). ao, aorta ; c, cutis ; ch, chorda ; d, enteric canal ; f, muscle-fascia ; 



- fh, dorsal fin-cavity ; m, myomere ; n, neural tube ; p, atrial cavity ; sf, metapleur ; 

 sfh, metapleural cavity; si, sub-intestinal vein; sk, sclera-layer ; si, sub-atrial 

 ridge ; sp, coelom. 



the atrium enlarges. Part of the outer wall of the atrial cavity (the 

 epipleura of KAY LANKESTEE) therefore does not arise through the 

 formation of folds, but is a modified portion of the body- wall which 

 has undergone no displacement. [See also MACBEIDE, No. VIII. a.] 



The formation of the atrial cavity in Amphioxus recalls to some extent 

 that of the similarly placed cavity in the Ascidians which, as we have seen, 

 originated in the form of paired invaginations (p. 366) which also only 

 secondarily grow round the pharynx. 



The outer wall of the atrial cavity cannot be homologised with the oper- 

 culum of the fishes, the latter being a fold which belongs exclusively to the 

 hyomandibular arch. 



