PHYLUM CHORDATA 



395 



since they are attached to the connective tissue partitions, are able 



to produce the lateral movements of the body used in swimming. 



The mouth opening is at the bottom of a funnel-shaped cavity 



in the ventral surface near the anterior end, called the vestibule 



Fig. 342. — Diagram illustrating the anatomy of the pharyngeal region of 

 Amphioxus. ao., dorsal aorta; atr., atrium; d.co, dorsal coelom; en., endostyle; 

 ep., epipleur; Jr., fin-ray; go., gonads; hy., hyperbranchial groove; mp., meta< 

 pleur; mpc, metapleural fold; my., myotomes; nch, notochord; nph, nephrid* 

 ium; nt., neural tube; p.b., primary gill-bar; tb., tongue-bar; S.co, subendo- 

 stylar coelom. (From Bourne.) 



(Fig. 341, ves.). The anus {an.) is situated on the left side of the 

 body in myotome fifty- two (m 52 ) . Just in front of the ventral fin 

 opposite myotome thirty-six (m 36 ) is the atriopore (atp.), an open- 

 ing through which water used in respiration passes to the outside. 



