452 



E. EAY LANKESTER AND AllTHUU WILLET. 



That belonging to the animal's left side keeps a more or less 

 median position, and can be traced (though but small in eleva- 

 tion) when twelve gill-slits are present as a ridge situated at 

 the lower or ventral margin of the gill-slits, and dying out in 



Fig. 6. 



Fig. 7. 



s.a 



Fig. G. — Transverse section through a larva with eleven or twelve unpaired 

 gill-slits, showing the minute sub-atrial ridges, d.m. Dorsal division 

 of myocffil in which the fin-ray will lie when it is developed. nc. 

 Nerve-cord. nch. Notochord. m. Muscle-plate, mi/. Cavity of niyoccel. 

 d.tt. Dorsal aorta. Int. Intestine. d.l.m. Double-layered membrane 

 separating the myocoel from the splanchnocoel. sp. Primitive splanch- 

 nocoel. v.a. Ventral vessel, met. Aletapleur. s.a.r. Sub- atrial ridges. 



Fig. 7. — Transverse section through a slightly older larva. The sub-atrial 

 ridges {s.a.r.) have fused for a short distance between atriopore and 

 pharynx ; but in the pharyngeal region the atrium is unclosed, and conse- 

 quently tiic gill-slits still open directly to the exterior, at. Atrium. 



the anterior region of the pharynx (PI. XXIX, fig. 6). The 

 right-hand ridge, or metapleur, takes a course to the right of 

 the gill-slits (which, it will be remembered, are on the right 

 side of the body), and overhangs the upper limit of the slits to 



