678 EVOLUTION OF ANIMALS Part V 



the basis of certain trials, it appears that a fish can tell a blue fly from a red 

 one when either one is submerged, but not when they are on the surface. One 

 version of what a fish may be able to see above the surface is set forth in 

 The Story of the Fish, by Curtis Brian (Suggested Reading). 



Hearing. Bony fishes must hear better than sharks and others with carti- 

 laginous skulls since bone is a better resonator than cartilage and because fishes 



A. B. 



ACCOMMODATION IN EYE 

 OF A BONY FISH 



A. Position for near sight 



B. Limited for sight 



Fig. 33.13. Eye of a blenny. A, usual resting position for near sight. B, the 

 lens pulled backward, in the position for limited far sight. Fish are nearsighted 

 and probably their eyes and the condition in water do not allow vision to extend 

 more than fifty feet. As any trout fisherman knows the trout can see above the 

 water surface. Blennies (Blennius) live among the mussel beds on reefs of the 

 Pacific coast. (After Walls: The Vertebrate Eye. Bloomfield Hills, Mich., Cran- 

 brook Institute of Science, 1942.) 



lack eardrums and middle ears. They have parts of the inner ear (utricle and 

 sacculus), but they do not have the cochlear duct so important in the human 

 ear (Fig. 17.9). Experiments have led to the conclusion that goldfishes can 

 hear, although within a very small range of sound. Care must be taken that 

 responses to vibrations received by the skin cells are not mistaken for hearing. 

 The sounds used to test the goldfishes were produced by a telephone inside a 

 submerged balloon. 



Pressure. The lateral line is a tube that lies just below the skin and runs 

 along each side of the body from the gill openings to the tail. It often con- 

 tinues with several branches onto the head. The tube is filled with mucus and 

 at frequent intervals opens by a pore over a group of sensory cells in its floor 

 (Fig. 33.14). Slightly deeper in the body wall is a long branch of the tenth 

 cranial nerve which supplies a branch to each lateral line organ. These organs 

 are extremely sensitive to changes of pressure. They react to the sUght changes 

 of pressure that come from a passing fish and they doubtless initiate the shift- 

 ing of position that can be seen so often in a school of fishes idling in a slow 

 stream. 



Touch. Sense organs of touch essentially like our own are spread over the 

 surface of the skin. 



