133 ELEMENTS OF COMPARATIVE ZOOLOGY. 



to the back part of the mouth. Hence a fish can perceive 

 odors in the water only as it swirls in and out of the nasal 

 sac. In the air-breathing forms, odors in the air are drawn 

 with the breath over the sensory surface. 



The essential part of the ear consists of a thin mem- 

 branous sac on either side of the head. In three places this 



sac is so pinched as to form 

 small tubes (semicircular 

 canals) open at either end into 

 the main sac. The whole is 

 filled with fluid in which are 

 numerous minute solid parti- 

 cles (otoliths). At one end 

 of each tube and at places in 



FIG. 64.-Diagram of mammalian the SaC are SenSOI 7 Or g anS 



ed with the auditory 

 Sound-waves entering 



by 



waves to the inner parts. the sensory organs and thus 



to stimulate the nerve. 



In the sharks this ear-sac is placed behind and medial to 

 the spiracle (p. 17). In the higher vertebrates the spiracle 

 becomes closed on the outside, but the rest of the structure 

 remains, and is known as the Eustachian tube, and as its 

 outer end comes between the ear and the external world, 

 one or more bones usually extend across the tube to convey 

 the sound-waves to the sac. In the frogs the outer end of 

 the Eustachian tube is closed by the large tympanic 

 membrane on the side of the neck. 



In the higher vertebrates an external ear occurs. This 

 consists of a tube leading inward to the tympanic mem- 

 brane, and to this tube are frequently added structures to 



