CHAPTER XVII. 



AUDITORY ORGAN, OLFACTORY ORGAN AND SENSE 

 ORGANS OF THE LATERAL LINE. 



Auditory Organs. 



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'. 

 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 



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

 they sometimes act as dampers. 



