86 



Mr. A. Willey. On the 



[May 22. 



FIG. 6. 



FIG. 7. 



s.ar. 



FIG. 6. Transverse section through a larva with eleven or twelve unpaired 

 gill-slits and with lateral mouth, showing the minute sub-atrial ridges. 



d.m. Dorsal division of myocoel in which the fin-ray will lie when it is 



developed. 

 no. Nerve -cord. 

 nch. Notochord. 

 m. Muscle-plate. 

 my. Cavity of myoocel. 

 d.a. Dorsal aorta. 

 Int. Intestine. 

 d.l.m. Double-layered membrane separating the myoccel from the splanch- 



nocoal. 



sp. Primitive splanchnocoel. 

 v.a. Ventral vessel. 

 met. Metapleur. 

 s.a.r. Sub-atrial ridges. 



FIG. 7. Transverse section through a slightly older larva. The sub-atrial ridgrs 



(s.a.r.) have fused for a short distance between atriopore and pharynx ; biit in the 



pharyngeal region the atrium is unclosed, and consequently the gill-slits still open 



directly to the exterior. 



at. Atrium. 



The atrium is formed by a small horizontal growth (s.a.r. in 

 fig. 6), which starts from the inner i r ace of each metapleur and floors 

 in the deeper half of the groove or area between the two meta- 

 pleura (fig. 7, at.). 



These horizontal growths may be called the sub-atrial folds. 



