ORIGIN OF JAW APPARATUS 365 
bars. The head has also acquired a large muscular upper lip 
which carries on its undersurface the anterior part of the terri- 
tory of the oral hood, or correctly put, the distal part of the 
oral hood has acquired a muscular roof which has suppressed 
the cavity. The muscles of the mandibular bars consist of two 
tapering fusiform bundles of fibers which accompany the bars, 
flattening and thinning out on the horizontal part of the jaw bars. 
Fig. 17 Ventral view of superficial muscles of 10-cm. Ammocoetes, to show 
the transverse muscle and its derivations that lie in the lower lip. 
At the base of the jaw bars they are thicker than the bars and 
arise from the lateral borders of the median bar and the floor of 
the buccal chamber. They pass foward, outward, and upward 
along the outer border of each jaw bar. Where they originate they 
interdigitate with the circular muscles of the head. They furnish 
the tentacles with muscles, sending off shoots into the larger ones 
(fig. 16), which attach to the walls. The jaw muscle is attached to 
the skeletal tissue at the base of all tentacles, both large and small. 
The motions of the jaw bars in Ammocoetes are not as varied 
