CHAPTER XVII. 



AUDITORY ORGAN, OLFACTORY ORGAN AND SENSE 

 ORGANS OF THE LATERAL LINE. 



Auditory Organ*. 



A GREAT variety of organs, very widely distributed amongst 

 aquatic forms, and also found, though less universally, in land forms, 

 are usually classed together as auditory organs. 



In the case of all aquatic forms, or of forms which have directly 

 inherited their auditory organs from aquatic forms, these organs are 

 built upon a common type; although in the majority of instances the 

 auditory organs of the several groups have no genetic relations. All 

 the organs have their origin in specialized portions of the epidermis. 

 Some of the cells of a special region become provided at their free 

 extremities with peculiar hairs, known as auditory hairs; while in other 

 cells concretions, known as otoliths, are formed, which appear often to 

 be sufficiently free to be acted upon by vibrations of the surrounding 

 medium, and to be so placed as to be able in their turn to transmit 

 their vibrations to the cells with auditory hairs 1 . The auditory 

 regions of the epidermis are usually shut off from the surface in 

 special sacks. 



The actual function of these organs is no doubt correctly described, 

 in the majority of instances, as being auditory; but it appears to 

 me very possible that in some cases their function may be to enable 

 the animals provided with them to detect the presence of other animals 

 in their neighbourhood, through the undulatory movements in the 

 water, caused by the swimming of the latter. 



Auditory organs with the above characters, sometimes freely open 

 to the external medium, but more often closed, are found in various 

 Ccelenterata, Vermes and Crustacea, and universally or all but univer- 

 sally in the Mollusca and Vertebrata. 



1 The function of the otoliths is not always clear. There is evidence to shew that 

 they sometimes act as dampers. 



