THE LANCELET 



331 



section of the body. Each gill bar is covered with a deep 

 columnar epithelium, ciliated except on the side towards 

 the atrium, and contains a skeletal rod. At the tops of 



'atr. 



Fig. 239 — A transverse section through the pharyngeal region of 

 Amphioxus. — After Ray Lankester. 



sp.c, Nerve cord (spinal cord); nek., notochord, beneath which He the two 

 suprabranchial arteries; mym., myomere; a.c.f., brown tube lying in 

 dorsal cceloin ; C. , hepatic caecum ; G. , a genital sac with ova ; mp. , meta- 

 pleural fold; atr., atrial cavity; Ph., pharynx, with dorsal and ventral 

 grooves, and bars between gill slits. Note in the primary bars and below 

 the ventral groove the small ccelomic spaces. The ectoderm is dark 

 throughout. 



the bars these rods fork and join one another over the 

 arches. At the lower ends the rods of alternate bars fork 

 but do not join their neighbours, which are unsplit. The 

 bars which contain forked rods are known as primary bars, 

 the alternate bars, with unsplit rods, are secondary bars, or 

 22 



