344 HOWARD AYERS 
is tightly closed, and they carry the tentacles with them in 
this semicircular sweep. When fully expanded the tentacles 
form a widespread fringe encircling the buccal opening. When 
fully contracted the tentacles form an interdigitated and usually 
jumbled mass crowded into the buccal cavity and entirely fill- 
ing it, and then the apparent buccal opening is formed by the 
contracted right and left metapleural folds. 
There is thus evidence of abundant and varied muscular 
action, and the skeletal parts are so arranged as to give free and 
extensive change of motion of the parts. The jaw apparatus 
of Amphioxus has thus the motions of lateral biting Jaws, but 
Amphioxus does not use them as such. It is important to note 
that the jaws can be and are frequently withdrawn inside the 
buccal cavity. 
While the Amphioxus buccal skeleton is developed and ad- 
justed to opening and closing the oral aperture, since the food 
of Amphioxus consists of minute organisms suspended in the 
respiratory current, no biting or comminuting function is 
needed. It is apparent how readily adaptable this apparatus 
is to such function, and when change of food habit occurred 
the jaws could be readily modified to meet the requirements 
of the new conditions. 
Rising from the dorsal borders of the jaw bars, the right and 
left walls of the oral hood arch upwards to fuse together in the 
median line below the notochord; posteriorly they pass into 
the velar curtain which is perforated by the velar mouth (figs. 
PNB ANS attr) 
The tentacular rods are generally single, but branched rods 
occur in forked tentacles. The tentacles from the second quarter 
of the jaw bar reach to the tip of the buccal cavity when retracted 
and the short ones project outside (laterad) of them. 
The tentacles are covered by extensions of the tissues of the 
jaw bar, consisting of a thick layer of surface epithelium and 
subepithelial connective tissue in which the nerves pass to their 
endings and the vascular channels ramify. The epithelium is 
produced at intervals into hillocks containing sensory cells. 
