ORIGIN OF JAW APPARATUS 369 
face of the upper lip, other muscles belonging to the constrictor 
group assist in the control of the jaw apparatus through their 
insertion into the base of the upper lip. The upper lip muscles 
as a whole form a concavoconvex muscular flap attached to the 
anterior end of the head—indeed, the upper lip is the muscular 
roof of the oral-hood territory and the jaw bars and tentacular 
skeleton form its main support. Muscle bundles run into it 
from the right and left sides from above and below and the 
lower lip is a unit part as far as the muscles are concerned. The 
Fig. 21 Sagittal section of the anterior end of the head of Ammocoetes, to 
show the jaw bar in the median plane with the large tentacle, the position of the 
velum and the dorsal body wall. 
lips thus form a funnel with the lower third of its wall cut away 
almost back to the jaw bars. 
The jaw apparatus is under the control of the trigeminus and 
facialis. I have not given these nerves of Ammocoetes the 
necessary analytical dissection, and cannot therefore make com- 
parison with the nerves of the jaw apparatus in Amphioxus. 
The Marsipobranch innervation will be described as it occurs in 
Bdellostoma. Here a thorough dissection shows the Marsipo- 
beanch innervation of the jaw apparatus is fundamentally in 
harmony with that of Amphioxus. 
