ORIGIN OF JAW APPARATUS 315 
defined, and they only grade into the ventral transverse muscles 
where the bases of the jaw bars attach to the parent muscle. 
In Ammocoetes we have the system of constrictors quite com- 
plete as such, forming a broad girdle of muscle bands encircling 
the branchial pharyngeal and velar region, attaching to the 
axial skeleton above and holding the base of the jaw bars in 
their grasp. With many interruptions of the continuity of the 
ring muscles in the branchial region these groups of muscular 
girdles send out offshoots—to the velum, to the jaw apparatus, 
and to the lip flaps. The phylogenetic history of these ring- 
Fig. 26 Side view of dissection of head of 37-mm. Bdellostoma embryo, to 
show relation of jaw apparatus at this stage. The dentigerous jaw is omitted. 
The abrupt bend ventrad of the nasal region is shown, as also the folding of the 
‘skin on the dorsum of head. 
—s 
muscles is not clear. We find in Ammocoetes the hyoidean 
apparatus laid down in procartilage within the ring-muscle 
system and extending from below the jaw bars backward. It 
cares for stresses beyond the function of the muscles, but the 
ring-muscle bundles still largely retain their continuity as rings. 
In Bdellostoma, however, a larger and more active fish, the 
ring-muscles have been broken into segments of rings by the 
very great development of the hyoidean apparatus in both 
cartilaginous and membranous form and the segments of the 
ring muscle of the Ammocoetes stage now insert, by muscle ends 
