ize) THE FROG 
ments of (a) the floor of the mouth, (b) the nares (nasal open- 
ings), and (c) the body. The glottis, through which air passes 
from the throat into the lungs, may be opened or closed as 
occasion demands. 
Rotate a frog in a horizontal plane. Observe that the head 
turns in a direction opposite to that of the movement of the plate 
on which the animal is resting; often the body also turns about 
in the same way. Rotate the frog back and forth about a hori- 
zontal axis and observe that the head is moved up and down 
contrary to the direction of rotation. These movements are 
called compensatory movements, and they serve to maintain a 
fixed orientation in space. . 
II, ExTERNAL CHARACTERS. 
The following study should be made chiefly on the living frog. 
Preserved specimens in addition are desirable. 
Notice the division of the entire animal into head, trunk and 
limbs. Is there a distinct neck? Is the frog characterized by 
bilateral symmetry? antero-posterior differentiation? dorso-ven- 
tral differentiation? Observe the “hump” on the frog’s back; 
examine a skeleton and discover how the hump is produced. ; 
Observe carefully the color and color pattern of the dorsal and 
ventral sides respectively. Is the coloration such that it would be 
likely to afford protection to the animal in its natural environ- 
ment? Observe that the skin is smooth, moist and slimy; of 
what advantage is this to the animal? Besides mucus, the skin 
of the frog, when irritated, secretes a small amount of a whitish, 
slightly poisonous fluid; in the toad the glands secreting this fluid 
are very well developed. ‘In the frog observe: 
1. The eyes; observe that they are ordinarily protruded, but 
are retracted when the lids are closed. There are short upper 
and lower eyelids, and in connection with the lower eyelid a 
broad semi-transparent nictitating membrane, which functions as 
a supplementary lower eyelid. (The nictitating membrane 
reaches its highest development in certain reptiles and in birds, 
while in mammals, including man, it is represented by a rudi- 
mentary fold at the inner angle of the eye.) The outer or ex- 
posed portion of the eyeball is covered with a transparent mem- 
