THE FROG I/ 



would be likely to afford protection to the animal in its 

 natural environment? 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 : 



I. 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 cer- 

 tain reptiles and in birds, while in mammals, including man, 

 it is represented by a rudimentary fold at the inner angle of 

 the eye.) The outer or exposed portion of the eyeball is 

 covered with a transparent membrane known as the cornea. 

 The iris is a pigmented ring situated behind the cornea and 

 showing through; it acts as a diaphragm, limiting the 

 amount of light that enters the eye. The pupil, through 

 which light enters the eye, is an oval aperture surrounded by 

 the iris. Watch the eye carefully to see if the pupil changes 

 in size through the expansion and contraction of the iris. 



2. The tympanic membrane of the ear, a short distance 

 behind the eye. 



3. The brow spot, a very small light spot midway be- 

 tween the anterior ends of the eyes. The brow spot is the 

 vestige of a median eye which was functional in ancestral 

 forms. 



4. The mouth. This is kept tightly closed, except in the 

 act of taking food. 



