ORIGIN OF JAW APPARATUS 343 
the long axis of the body. ‘This angle is greater in both Ammo- 
eoetes and Bdellostoma than it is in Amphioxus. 
The jaw bars extend the entire length of the buccal aperture, 
which is not quite as long as the buccal chamber, owing to the 
pocket which lies behind the base of the jaw bars and ends in 
front of the velum. The bases of the jaw bars are tied together 
by tendons and muscle. 
Fig. 1 Ventral view of the anterior end of Amphioxus to show relation of the 
jaw apparatus to the body wall, myotomes, and velum. 
The extension of the muscles forward to the tips of the bars 
and out onto the tentacular projections ties all skeletal parts 
into a unit structure for the work it has to do. 
The chondral jaw bars which frame the buccal opening in 
Amphioxus are the simplest expression among living forms of 
the vertebrate mandible. But they include much more than 
definitive mandible, e.g., of the cartilaginous fishes. 
The jaw bars rotate 180° from their position when the buccal 
cavity is wide open to their position when the buccal aperture 
