MECHANORECEPTORS AND BEHAVIOR 379 



only simultaneous signals, differing in amplitude along the array, have an 

 impact on the brain. 



The association of the tactile receptors and the lateral-line endings is 

 expressed in the distinction between "distant touch" and "touch" and in the 

 fact that signals detected by the lateral line appear to be interpreted by a fish 

 as arising from some external source, whereas touch is a sensation referred to 

 the skin surface. We do not know how this perceptual difference is achieved, 

 but it must result from the mode of neural organization. Pumphrey (1950) 

 emphasised that the tactile responses were generated at the segmental level, 

 whereas analysis of acoustic o-lateralis function was carried out in a closely 

 grouped medullary centre (see Figure 15). The enormous number of 

 synapses made in the lobes, the integrative, wide sampling capacity of the 

 secondary neurons, and the possibility of the auricular and the lateral-line 

 centres interacting by means of the parallel fibre pathways, are all specialisa- 

 tions that are probably important in this type of analysis (Paul and Roberts 

 19766). 



CONCLUSIONS AND PROSPECTS 



Our concluding view that the major mechanoreceptor groups provide a spec- 

 trum of sensation is not startlingly novel. More than 150 years ago Knox 

 (1825) deduced that the lateral line contains "organs of touch, so modified 

 however, as to hold an intermediate place between the sensations of touch 

 and hearing." Since then, little progress seems to have been made in the 

 analysis of fish sense organs, particularly in relation to behaviour. Many areas 

 of research await exploration, therefore, but we can be confident that the 

 elasmobranch fishes, because of their limited repertoire of behaviour when 

 compared with the teleosts, and because of their suitability for experimental 

 work, will provide useful research material. Three kinds of research problems 

 come immediately to mind and are now beginning to be examined. 



At the behavioural level an important problem concerns the significance 

 of "hearing" in shark behaviour, which has begun to be described in the last 

 decade and which should lead to vigorous research into the mechanisms of 

 hearing and into the identification of attractive and repellant sounds. 



At the neurological level, the reflexes associated with labyrinthine stimula- 

 tion, because of their simplicity and because of the ease of access to the 

 labyrinth— features that have made the elasmobranch favoured material in 

 the past— should now attract neurophysiological investigation. 



Finally, an important question in sensory physiology concerns the signifi- 

 cance of the efferent system. This is a complex problem and will require 

 comparative data from a number of species; once again, the elasmobranch 

 labyrinth should prove particularly informative. 



ACKNOWLEDGMENTS 



I am grateful to Dr. Q. Bone, Dr. E. J. Denton, and Mr. J. Montgomery for 

 discussions and to Ms. Joyce Coles and Marsha Rapson for secretarial as- 

 sistance. 



