VERTEBRATA. 81 



a. What is the shape of the external portion of the 

 eyes ? Compare with the eyes of a fish. 



h. Can they be retracted to avoid danger ? (Touch 

 the eye gently.) 



c. How many eyelids are there ? 



d. How do the eyelids differ in structure and in 



use ? 



e. What is the shape of the pupil ? 

 /. What is the color of the iris ? 



4. The Ears. 



a. Observe the ear-drum (tympanum) just back of 



the eyes. 



b. Is the frog's hearing good ? (Give reasons for 



your answer.) 



5. The Nostrils. 



a. How many nostrils are there ? 



b. What is their position ? 



a. Do they open through into the mouth ? (Dem.) 

 d. Is there any regular movement of the nostrils ? 



D. The Mouth. 



1. Observe the extent to which the mouth can be 



opened. 



2. Notice its large broad cavity and the thin- walled 



throat. 



3. Observe also the movement of the throat in a live 



frog and note what the effect of this movement is 

 on the size of the cavity. 



4. The Tongue. 



a. What is the shape of the tongue ? 



b. Where is the tongue attached ? 



c. What reason can you see for this mode of attach- 



ment ? 

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