ORIGIN OF JAW APPARATUS 361 
is marked by the tentacle-covered surface which leads back into 
the buccal opening between the vertical parts of the jaw bars. 
Thus the posterior part only of the ancestral buccal aperture 
is functional in Ammocoetes (figs. 14, 18). 
Fig. 14 Upper lip of a 15-em. Ammocoetes seen from ventral surface. The 
positions of jaw bars are in part outlined by two of the rows of tentacles and the 
long muscle of the jaw between the rows of tentacles. In front of the functional 
buccal aperture is included the remains of the anterior part of the buccal aperture 
of the amphioxine ancestor. The number of rows of tentacles and consequently 
the width of this space varies in different stages of growth. The jaw muscles 
shown are those belonging to the jaw bars and tentacles and the muscles at the 
base of the jaw apparatus. On the right side of the figure the forward extension 
of the parietal muscles is shown. They attach to the upper and lower lips. In 
front of the buccal aperture only the two main rows of tentacles are indicated. 
In this 15-em. Ammocoetes there are four additional rows of smaller tentacles 
on each side. The dotted outlines show the boundary of the tentacular region 
in an 8-cm. Ammocoetes and the rows of dashes the position of rows of small 
tentacles of this stage. The median line of dots marks the position of a median 
line of small tentacles sometimes present. 
