ORIGIN OF JAW APPARATUS 351 
There are six pairs of muscles which move the jaw apparatus 
of Amphioxus. Based on their action they may be listed as 
follows: 
I. Transversus abdominis, the parent muscle of them all. 
Is attached by tendons to basal part of jaw and supplies the 
bundles which compose muscles 2, 3, 4, 5. Assists in retracting 
the jaw apparatus and contracting the buc¢al cavity. 
II. Retractor of the jaw apparatus, arises from transverse 
muscle and inserts into base of jaw. Draws the jaw apparatus 
backward and, in connection with the transversus, Inward and 
upward into the buccal cavity. 
III. Adductor of the jaw bars, arises from transverse muscle 
and inserts into basal part of jaw dorsolateral of the retractor. 
Rotates the mandible inward through an angle of 180° and 
adducts the jaw. 
IV. Extensor of the jaw bars, arises from transverse muscle 
and inserts into basal part of jaw ventrolaterad of retractor. 
Rotates the mandible outward through an angle of 180° and 
extends the jaw. . 
V. Extensor of the jaw bars, arises from each segment of 
jaw and inserts into the more distal segments. The powerful 
extensor of the mandible. 
VI. Adductor of the tentacles, arises from base of tentacles 
and inserts into next anterior tentacle. 
All of these muscles, which have become largely longitudinal 
in direction, do not appear to have distinct sheaths; but from 
the clear-cut motions of opening and closing the buccal opening, 
retracting and in-folding the jaws and tentacles, it is certain that 
they have sharply defined mechanical pulls and are subject to 
an equally distinct nerve control. The whole jaw apparatus 
is anchored by tendons which extend from the muscles to the 
mesial surfaces of the body wall in front of the velum. Both 
tendons and slips of muscle extend from the velum to the jaw 
base. 
The jaw apparatus as a whole is anchored by tendons to the 
ventral transverse muscle and to the tough aponeurosis covering 
the mesial surface of the myotomes. It functions in existing 
