332 



MANUAL OF ML£M£NTAKY ZOOLOGY 



tongue bars, because they arise in development by the down- 

 growth of a tongue-shaped process from the top of a slit, thus 

 dividing it into two secondary openings which become the 

 permanent slits. This process may be seen in all its stages 

 at the hind end of the pharynx, where new slits are con- 

 tinually being added as long as the animal is growing. 1 



The rods of the primary bars are really double, consisting of two 

 strips which lie side by side touching one another in the bar, but 

 separate to form the forks above and below. The rods of the 

 secondary bars are single, though each parts into two to become 

 forked at its upper end. 



Fig. 240. — Transverse sections of gill bars of Amphioxus. 

 — Partly after Benham. 



A , Primary bar ; B t tongue bar. 

 «/., Atrial side ; at.ep. t atrial epithelium ; /»/., main blood vessels ; 

 &l'., capillaries; ctt.ep., ciliated epithelium; coe/., ccelom ; 

 pk.y pharyngeal side; pig., pigment cells; sk. t skeletal rods; 

 s&'., additional skeletal piece. 



The gill bars are connected by horizontal bars or synapti- 

 of which two or three cross each slit. These contain 

 skeletal rods. Along the mid-dorsal line of the pharynx is 

 a deep epibranchial groove lined by ciliated cells. The mid- 



. 1 In the larva the primary slits correspond with the myomeres, but 

 afterwards they become more numerous. 



