WHITE SUCKER 
161 
As the fish grows, the relative positions of eyes, nares, and mouth are greatly 
altered in a manner peculiar to this family of fishes. Probably the eyes and nares 
are the more fixed in position, while the mouth descends to its inferior posi- 
tion. At 14 millimeters the mouth opening is on a level with the lower rim of the eyes 
(fig. 24), and by 16 millimeters it is entirely below this level. Its position now 
apparently interferes with feeding on floating particles, for before attaining a length 
of 18 millimeters the fish adopts the habit of feeding on the bottom. This change of 
habit will be discussed in more detail later. 
In the 43-millimeter larvae papilla? are present on the lips, which are now thick 
and swollen. In fish exceeding 16 millimeters in length the mouth may be described 
as still a crescent but lying more and more in a plane approaching the horizontal. 
In an adult 12 inches in length this crescent is 20 millimeters wide and 15 millimeters 
from front to back, including the lips. The lips are covered with dome-shaped 
papillae, five rows of them on the anterior lip and nine on the posterior. This pos- 
Fig. 29. — Adult sucker, showing relative size of fins and relative position of the eye, nostril, and mouth. 10 inches long 
terior lip is deeply notched behind and is flattened out at its posterior margin and also 
laterally. 
These lips are capable of much protrusion, assuming the form of a funnel, extend- 
ing downward and forward. When the mouth is closed the anterior end of the head 
is decidedly blunted in side view. The eye in the adult is relatively quite small and 
high up in the head. (Fig. 29.) The nares lie just anterior to the eyes and are divided 
by a vertical earlike flap 2 millimeters high. 
HISTOLOGY OF THE MOUTH. ORAL EPITHELIUM 
At 10 millimeters the mouth and pharynx are lined with a stratified epithelium, 
the cells of which are rounded, some of the superficial cells being flattened. There 
are but two to four layers of cells in this epithelium, which is 20 microns thick. 
The nuclei are basal in the cells. Taste buds of striking size are found here and there 
throughout the mouth and pharynx. They are conical or mammiliform and extend 
above the surface of the epithelium for another 20 microns. (Fig. 31.) 
These flasklike taste buds show, in each section, about six columnar nuclei in 
the basal portion and a clear upper neck, which, being finely striated, doubtless 
contains processes of the sensory cells. In one case seventeen of these sense papillae 
