302 



SYSTEMATIC ZOOLOGY. 



membrane have only external nostrils, but in all others 

 they are placed at one side of a tube, which leads from the 

 external nostril to the back part of the mouth. Hence a 



B 



FIG. 111. Relations of the olfactory organ. A, in fishes; B, in higher ver- 

 tebrates, b, brain; i, internal nostril; n, external nostril. The sensory 

 surface is folded. 



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, the inner ear (fig. 112), con- 

 sists of a thin membranous sac on either side of the head. 



s In three places this sac 



is so pinched as to form 

 small tubes (semicircular 

 canals) open at either end 

 into the main chamber. 

 The whole is filled with 

 fluid in which are nu- 

 merous minute solid par- 

 ticles (otoliths). At one 



FIG. 112. Diagram of mammalian ear. c, , /. i > , 



cochlea; e, Eustachian tube; s, semicir- end 01 eacll tUD6 ana at 

 cular canals, connected with the central -. ,-1 n-ro 



sac and separated from the surrounding places 111 [116 

 bone (black) by a space; t, tympanic i 



cavity closed externally by membrane, and SeilSOlT OrgailS Connected 

 traversed by a bone, which conveys the ..-, -, -,. 



sound-waves to the inner parts. With the auditory nerve. 



Sound-waves entering the 



ear set the fluid in motion, causing the otoliths to strike 

 the sensory organs and thus to stimulate the nerve. 



