364 HOWARD AYERS 
zon. Kaensche has described the part taken by the skeleton 
of the median tentacle in forming the jaw of Petromyzon as 
well as the origin of the hyoid from the sheet of procartilage 
of the Ammocoetes which lies ventrad of the jaw bars. The 
buceal screen is figured by Professor Gage on plate VI, figure 
22, and, as he states, this tentacular sieve prevents the entrance 
Fig. 16 A. One of the larger-branched tentacles from the jaw bar. 
B. Four small tentacles to show method of branching. 
of large particles into the buccal chamber and when tightly 
closed shuts off the flow of water through the buccal aperture. 
The mandibular bars are not so conspicuous superficially 
as in Amphioxus, but they occupy the same relative position 
in the head. Due to the shortening of the notochord, the for- 
ward overgrowth of the brain, and the housing of the nasal 
organs, the head does not project beyond the boundary of the 
buccal chamber marked out by the proximal part of the jaw 
