82 AMPHIOXUS. 



its walls becoming ultimately converted into the tract of colum- 

 nar ciliated epithelium, which in the adult Amphioxus lines the 

 posterior part of the buccal cavity. 



7. The Atrial Cavity. 



The atrial chamber begins to form in larvge which have from 

 nine to ten primary gill-slits, but in which the secondary gill- 

 slits have not commenced to develop. A narrow longitudinal 

 groove appears along the ventral surface of the body of the larva, 



Fig. 40. — A diagrammatic transverse section across an Amphiosus Larva with 

 eleven or twelve primary gill- slits, but no secondary ones. (Slightly 

 modified from Lankester and Willey.) 



A, aorta. AG, atrial cavity. AP, subatrial fold. CH, notocliord, CM, myoccel. 

 C~N, diverticulum of myoccel lying between notocliord and myotome. CS, splancliiuoccel. 

 CU, cutis layer. DF, cavity of dorsal fin. US, skeletogenous layer. I, spinal cord. 

 MD, metapleural ridge. MP, muscle-fascia layer. ML, rnyotoniio muscle. MV, 

 metapleural canal. TI, intestine. V, subintestinal vessel. 



behind the region of the pharynx. The groove is bordered by 

 two folds, which become later the metapleural ridges of the adult 

 (Fig. 40, md) : on reaching the pharyngeal region, the two meta- 

 pleural ridges are deflected towards the right side of the larva, 

 and run forwards one on each side of the row of primary gill-slits. 

 From the inner side of each metapleural ridge a horizontal 

 shelf-like outgrowth, the subatrial fold, arises ; and the two sub- 



