360 HOWARD AYERS 
radial muscles of the posterior face of the velum insert into the 
base of the tentacles and draw the lip caudad, thereby flexing 
the tentacles caudad and pulling the lip caudad of the muscular 
diaphragm. In expanding, the motions are the reverse of these. 
The velum is firmly tied to the surface of the myotomes on each 
side as well as to the base of the jaw apparatus in front and the 
gill basket behind. It thus forms a flexible curtain between 
the buccal cavity and the gill chamber. 
AMMOCOETES 
The jaw apparatus of Ammocoetes is laid down after the same 
plan as that of Amphioxus. It consists of two lateral jaw bars 
of soft cartilage and procartilage, which are largest at their 
union in the midventral line and taper to their anterior dorsal. 
ends located on the ventral surface of the upper lip. They 
form the boundary of the buccal aperture with their proximal 
vertical portions and inclose between them a remnant of the 
ancestral buccal aperture of Amphioxus in their horizontal 
distal portions. They bear tentacles throughout their entire 
length (figs. 14, 15, 18, 19, and 21). 
In the midventral line they fuse, and from this junction is 
given off dorsally a flask-shaped body which is crowned by the 
median tentacle, and ventrally the jaw bars are continued cau- 
dad in the form of a large spike of cartilage or median projec- 
tion of the jaw bars (figs. 15, 18, 19, 21). 
A reduction of the distal part of the jaw bars together with 
an extension of their proximal portions has taken place since 
the Amphioxus stage was passed, but the amphioxine charac- 
ter of the jaw bars is plain to see. Viewed from in front the 
jaw bars curve outward and upward nearly vertically from the 
midventral line, then inward, but do not meet in the middorsal 
line, thus forming an incomplete ring with the opening above. 
Up to this point they are inclosed in the lips of the buccal aper- 
ture. The ends of the open ring lie at the junction of the upper 
lip with the head. Here the jaw bars bend forward nearly at 
right angles and lie on the ventral surface of the upper lip, and 
the position of the anterior part of the ancestral buccal chamber 
