SENSORY ORGANS AND RECEPTION 343 



(< 32 c/s), by the skin receptors alone. When a connexion exists between 

 the air-bladder and the inner ear, the bladder acts as a hydrophone and 

 the pressure waves which it picks up are transmitted to the vestibule. This 

 arrangement increases auditory sensitivity, lowering threshold and ex- 

 tending the frequency range. Auditory thresholds are about 30 db lower 

 in fishes having the air-bladder coupled to the ear, compared with fishes 

 lacking this connexion. Absolute threshold in the catfish (Ameiurus nebulo- 

 sus) for the range 60-160 c/s is about 0-01 dyn/cm 2 . This animal has a 

 Weberian apparatus (43, 55, 56, 122). 



Further information has been revealed by electronic recording. Micro- 

 phonic effects (generator potentials) have been recorded from the inner 

 ear of various species of teleosts in response to tapping, loud speech and 



TABLE 8.1 

 Upper Frequency Limits of Hearing in Certain Fishes 



1 Maximal frequency tried. 

 (Data from Griffin (55); Dijkgraaf (42, 43); Jones and Marshall (76).) 



sound of a tuning-fork (60 c/s). The microphonic potential is generated 

 in the macular region of the sacculus (54, 124, 170). 



More recently Lowenstein and Roberts (100) have demonstrated vibra- 

 tional responses in the ear of the ray {Raja clavata) by recording from 

 branches of the auditory nerve. The regions responsive to vibrational 

 stimuli are the lateral part of the macula sacculi, the macula neglecta 

 (dorso-medial aspect of the sacculus) and the lacinia utriculi (an uncovered 

 portion of the utriculus macula). In the absence of vibrational stimuli 

 many of the receptors display resting activity, but quiescent units, when 

 stimulated, become recruited to take part in vibrational responses. 

 Vestibular microphonics were observed at stimulating frequencies as high 

 as 750 c/s, but vibrational responses were restricted to frequencies below 

 120 c/s. 



Not much is known about the importance of sound as sensory cues for 

 fish. The continuous production of low frequency (infrasonic) vibrations 

 by waves and moving bodies may be utilized to a considerable extent by 

 many species. Marshall (103) describes unusually prominent lateral line 

 organs in some bathypelagic fishes, especially blind species (brotulids, 

 macrourids). These may be concerned especially with detection of low- 

 frequency pressure waves generated by moving prey. Sudden sounds 



