ORIGIN OF JAW APPARATUS 363 
which are mainly directed forward and upward. When the 
tentacle is fully erected the branches stand nearly vertical to 
the long axis of the body. About this bush are grouped ten 
other similar bushes, the crowns of ten lateral tentacles, five on 
each side. These bushes are so set that their branches inter- 
digitate, forming a screen which separates the functional part 
of the buccal cavity from the outside (figs. 14, 18). 
The posterior part of the median bar projects caudad in the 
ventral wall of the body as far back as the branchial region, 
where it tapers to a point in the connective-tissue framework 
of the body wall. Below the base of the jaw bars and the median 
bar is a sheet of procartilage which thins out laterally as it passes 
upward in the body wall on each side and also as it runs caudad 
below the median bar. This sheet of procartilage gives rise 
to the hyoidean apparatus of Petromyzon (figs. 15, 21). 
As in Amphioxus, the jaw bars bear tentacles their entire 
length. Present as a single row about the functional part (buc- 
cal aperture) they appear in a varying number of rows on the 
surface of the lip, the rows being more numerous in younger than 
in the older stages of growth. The tentacles vary from short 
finger-shaped projections, such as are given off at the distal 
extremity of the rows, through all gradations to the bushy struc - 
tures given off from the base of the jaw bars (fig. 16). The 
branching is dichotomous. As Prof. 8. H. Gage has explained, 
this group of tentacles forms a strainer or sieve in front of the 
buccal cavity. Along the horizontal part of the jaw bars the 
tentacles project from the ventral concave face of the upper 
lip, due to the spreading out of the skin cover of the jaw bars 
as part of the lip covering. They decrease in size toward the 
tip of the bars and also from the mesial rows laterad. They 
form converging rows of fungoid outgrowths, coursing laterad, 
the more posterior ones terminating in the border of the ten- 
tacular area, the anterior (mesial) ones converging toward the 
middle line near the front border of the lip (fig. 14). The median 
tentacle mentioned above is the largest of all and surmounts a 
bulbous structure which after the metamorphosis appears as 
part of the skeleton of the mandibular mechanism of Petromy- 
