350 HOWARD AYERS 
along a horizontal line of insertion lying below the dotted outline 
of the dorsal border of the oral hood, as shown in figure 12. When 
the muscles of the jaw apparatus are fully contracted, the rectus 
abdominis section of each myotome contracts in unison to assist 
in withdrawing the jaw apparatus up into the boat-shaped cavity 
formed by the myotomes on each side and the notochord in 
Se 
MINIS j 
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\ ViVi AYN WAN 
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Fig. 7_ Dorsal view of dissection of jaw base and its muscles. Amphioxus. 
the middle and at the same time the metapleural folds are folded 
inward, the right over the left, so that the buccal opening is 
entirely closed and covered over and protected by these folds 
of the skin. The buccal cavity at such time is entirely filled by 
the tentacular projections of the jaw bars, crowded together 
like so many straws. 
