112 



ANIMAL BIOLOGY 



the fish immovable, slightly movable, or freely movable ? 

 Can it look with both eyes at the same object? Is the 

 range of vision more upward or downward ? To the front 



or side ? In what 

 direction is vision 

 impossible ? Can a 

 fish close its eyes 

 in sleep ? Does 

 the eyeball appear 

 spherical or flat- 

 tened in front ? 

 FIG. 210. BLACKBOARD OUTLINE OF FISH. 



The ball is really 



spherical, the lens is very convex, and fish are nearsighted. 

 Far sight would be useless in a dense medium like water. 



In what direction are the nostrils from the eyes ? (Fig. 

 211.) There are two pairs of nostrils, but only one pair of 

 nasal cavities, with two nostrils opening into each. There 

 are no nasal passages to the mouth, 

 as the test with a probe shows 

 that the cavities do not open into 

 the mouth. What two functions 

 has the nose in man ? What func- 

 tion has it in the fish ? 



There are no external ears. 

 The ear sacs are embedded in the 

 bones of the skull. Is hearing acute or dull ? When fish- 

 ing, is it more necessary not to talk or to step lightly, 

 so as not to jar the boat or bank ? 



What is the use of the large openings found at the back 

 of the head on each side ? (Fig. 211.) Under the skin at 

 the sides of the head are thin membrane bones formed from 

 the skin ; they aid the skin in protection. Just under these 

 membrane bones are the gill covers, of true bone. Which 



FIG. an. HEAD OF CARP. 



