3f2- HOWARD AYERS 
with the cartilaginous jaw bar, while the other two (the second 
and fourth) are connected only by tendons to the jaw bar. The 
cartilages of the tentacle, two on either side, are fused together 
where they meet in the middle line below the nasal tube, and 
through this bar they are connected with the subnasal cartilage. 
The cartilages of tentacle four lie in the heavy lip folds at the 
sides of the ventral part of the mouth opening, and are so held 
by tendons and muscles as to serve as a firm support for the pair 
of thick flaps of skin which operate as a mouth guard and jaw 
cleaner, as well as a palp. The other three tentacles are sensi- 
a B Mj 
es ee ee 
Fig. 23 Composite figure from sagittal sections of head of adult Bdellostoma 
showing outlines of important parts of jaw apparatus. The tentacular part 
of the jaw bar, of course, lies in the side of the head. The tooth-bearing jaw 
cartilage is also shown, but it also lies at one side of the median plane. 
tive fingers used in connection with both nasal and buccal 
apertures. 
The important relation is that the jaw bars are the skeletal 
framework of the buccal aperture just as they are in Amphioxus 
and Ammocoetes. We found in Ammocoetes that a new skeletal 
structure had been added ventrad of the base of the jaw bars and 
that this sheet of procartilage of Ammocoetes developed into the 
submandibular cartilage of Petromyzon. In Bdellostoma we 
find this element larger and assuming proportions unusual, so 
much so that Prof. G. B. Howes called it “the dominant mon- 
ster of the Hag.’”’ It is however, admirably adapted to its 
functions. It serves as a traveling floor for the jaws in their 
extensive excursions out of and into the buccal cavity. Its 
