No. 419.] THE ANATOMY OF AXOLOTLS. 893 
The folds at the side of the mouth reduce the diameter of its 
opening. This difference in the size of the mouth is another 
marked difference between the axolotls and the terrestrial 
forms. It is indicated in Fig. 3, where the angle of the jaws 
is seen in 3 a to be anterior to the eyes, while in 3 c it is 
considerably posterior. This 
smaller size of mouth is per- 
haps correlated with the proc- 
esses involved in taking water 
into the mouth chamber for the 
purpose of forcing it back and 
out of the sides of the throat 
during aquatic respiration. 
The gills (in No. 2) do not 
differ essentially from the ac- 
counts and figures of Baird and 
others. They are shorter than 
the head. They are flattened 
outgrowths of the sides of the 
throat. Proximally they are 
continued in the floor of the 
throat to the hyoid bone in 
thecenter. On the underside 
Me the opercula, thin flaps of Eme 
skin, which merge distally into : 
the under margin of the gill. 
The gill plates are borne on 
the posterior side of the free T 
Portion of the gill, which by . pre. 3.—Side views of bed. a siredon Xo, 3 
1G. 3 
from Colorado ; 4, siredon No. 10 
m E 
fans of a muscle running rado ; c, salamander from St. Paul. 
ns length can be drawn back 
against the side of the body, thus protecting the plates. The 
latter are thin and placed in rows crossing the long axis of the 
gill, They are so arranged that two longer plates next the edge 
of the gill are followed by two smaller plates, set nearer to each 
Other in the middle, as shown in Fig. 4 4. Many of the sp ü id 
Mens in the collection show the gills in a state of degeneration, 
the gills being much shorter and the plates reduced in number. 
