378 MECHANICAL AND ACOUSTICAL SENSES 



removed the response was to particle displacement, whereas when the swim 

 bladder was present not only were the fish more sensitive but the response 

 was coupled to pressure. 



We must assume that high-frequency analysis is the function of the ear 

 because all the evidence to date indicates that the lateral line works only at 

 low frequencies. Thus, apart from the electrophysiological data already dis- 

 cussed, which demonstrated the low-pass properties of the brain centres, 

 Parker's well-known experiment (Parker 1905), in which he obtained re- 

 sponses in Mustelus to a 6-Hz tuning fork, which were lost when the lateral- 

 line nerves were cut, indicates a similar low-frequency preference. In Acerina 

 (Kuiper 1967) and in goldfish (Weiss 1969), behavioural responses evoked by 

 selective lateral-line stimulation were obtained only at low frequencies of 

 stimulation (<200 Hz). 



Myrberg et al. (1972) found that sharks sited in the far field of a vibrating 

 source were well able to localize it. This observation would be difficult to 

 explain if the sharks were responding to the pressure waves, because, as van 

 Bergeijk (1964) has emphasised, in most fishes directional localization is 

 probably only possible if use is made of an array of receptors such as the 

 lateral line. In Myrberg's experiments the frequencies used were well within 

 the range of the lateral-line organs and we must assume that the lateral line 

 was detecting the displacement from the distant source; a most interesting 

 experiment would be to test localization to far-field sounds of frequencies 

 that lie outside the lateral-line range. 



If, as we are suggesting, the ear also responds to displacement, then, in 

 the case of a complex sound with components of different frequency, it 

 should be possible to distinguish between a large distant object and a small 

 object sited nearby, by comparing the responses of the ear and the lateral 

 line and by measuring the frequency of the signal as well as the amplitude of 

 the displacements. 



The receptivity of the ear and that of the lateral line also overlap to some 

 extent with tactile sensation. The significance of cutaneous sensation has 

 been too often overlooked, despite Dijkgraaf's longstanding observation 

 (Dijkgraaf 1950) that behavioural responses in Gobius to a 100-Hz tuning 

 fork were obtained even after bilateral labyrinthectomy and with both 

 lateral-line nerves cut. Similarly, Parker (1909) found that responses of 

 Mustelus to a bang on the side of the tank, although lessened after both 

 auditory nerves had been cut, were not obliterated until the lateral lines had 

 been denervated and the skin procainized as well. 



The response of sharks to currents (e.g. Hodgson and Mathewson 1971) is 

 a good example of overlap between the senses, although the response is 

 probably mediated only by the tactile endings of the skin. Parker (1905) 

 found that both Raja and Mustelus, when exposed to a strong current, swam 

 upstream even if the lateral line was denervated. Current orientation (rheo- 

 tropism) is an example of a behavioural response in which the lateral line is 

 detecting the stimulus but is not monitoring it; its response therefore does 

 not initiate behavioural activity. Perhaps this is not surprising, for we have 

 already seen that considerable smoothing takes place in the lateral-line lobes; 



