366 HOWARD AYERS 
or extensive as in Amphioxus, owing to the fixation of the anterior 
part of the jaw apparatus to the upper lip. The contraction 
of the vertical portions of the jaw muscles closes the buccal 
aperture and they are aided in this action by the circular muscles 
of the head, which form a special sphincter surrounding the 
buccal aperture outside the jaws. The expansion of the buccal 
aperture seems to be largely due to the elasticity of the chondral 
jaw bars aided by the ventrolateral retractors of the jaw bars. 
Jo 
Fig. 18 Composite picture of sagittal sections of head of Ammocoetes. The 
position of the eye is indicated, as well as the circular muscle which begins in 
front of the eye and runs ventrad to insert on the jaw apparatus. The left jaw 
bar with its tentacles and the median tentacle are shown as though lying in the 
plane of the section. The tentacles are outlined as though solid bodies. 
The head muscles of the Ammocoetes consist of the parietal 
or trunk muscle, the branchial, velar, buccal, jaw, and lip mus- 
cles. The buccal and velar muscles are composed of two groups, 
the ring and the longitudinal. ‘The ring or transverse muscles 
consist of two layers of fibers separated by a layer of procartilage. 
They are the continuation forward of the muscles of the branchial 
basket. The group of buccal muscles attach to the base of the 
cranial skeleton, both the cartilaginous and membranous cranium, 
and to the nasal capsule. The general direction of the fibers is ven- 
trad in the sidewalls of the head, to curve inward from right toleft 
to the fusion of the muscles of the two sides along the median ven- 
