358 HOWARD AYERS 
form structure which does not appear to be duplicated in other 
peripheral territories. 
The velum is a part of the jaw apparatus; it is the organ about 
which has centered all this structural activity. It is lodged 
between two folds of the mucosa which fuse to form the velar 
lip and the coverings of the tentacles, and is continuous with 
the lining of the buccal cavity anteriorly and the foid which is 
continuous with the lining of the branchial cavity posteriorly 
(figs. 1, 3, 4, 8). The velum of Amphioxus is a muscular dia- 
Fig. 12 Left side of jaw apparatus of Amphioxus. The dotted outline marks 
the position of the dorsal wall of oral hood. The relations of the first seven 
myotomes and the nerves 2 to 7 are accurately drawn. Nerves 1 to 6, the jaw 
nerves. The heavy dashes mark the position of the ventral spinal roots. 
phragm. It varies much in shape, depending on the degree of 
contraction. When fully expanded it forms a thin circular 
ring with a large central hole, from the anterior rim of which 
twelve or more velar tentacles project forward into the cavity 
of the oral hood. In this state the radial muscles are contracted 
and the circular muscles are relaxed. This is the condition of 
the velum when the oral hood is wide open and the jaw apparatus 
fully expanded. When fully contracted the velum has the shape 
of a thick doughnut minus the hole and the tentacles project 
backward into the velar chamber or are folded across the posterior 
face of the muscular lump. When Amphioxus is killed with 
