228 CHEMICAL SENSES 



Receptor cells sensitive to chemicals are among the most important compo- 

 nents of elasmobranch sensory systems. Such cells are chemoreceptors. By 

 convention, chemoreceptors are divided into gustatory (taste) receptors and 

 olfactory (smell) receptors. A third "common" or "general" chemical sense 

 has been postulated (Parker 1922, Tester 1963), but it is involved in re- 

 actions to relatively high concentrations of chemicals (often so-called irri- 

 tating compounds) and it appears to lack the specificity and specialized 

 end organs of olfaction and gustation. This review will be limited to the 

 olfactory and gustatory receptors, with special emphasis on the modern 

 techniques of electrophysiology that are being used increasingly for their 

 study. 



HISTORICAL DEVELOPMENT OF STUDIES 

 ON CHEMORECEPTION 



Scientific understanding of the chemical senses of elasmobranchs, and most 

 animal sense organs, has developed through three general stages. The first 

 is characterized by observations of the animals in their natural surroundings. 

 The information gleaned from such observations is at first anecdotal but may 

 become organized and codified into understanding that can be valuable in 

 pointing the ways to appropriate experimental analyses. Examples of such 

 insights concerning the sensory biology of sharks, incorporated into the 

 traditions of seagoing Pacific cultures, are presented elsewhere in this volume 

 (Hodgson 1978). 



A second stage of understanding is reached when the structures of the 

 sense organs that mediate particular behavioral patterns are determined. 

 Anatomical investigations of the olfactory and gustatory sense organs of 

 elasmobranchs dominated the studies in this field from the 1860s until well 

 into the present century, and have been extensively reviewed by Parker 

 (1922), Tester (1963), and Kleerekoper (1978). 



The third stage is physiological analysis. This level of understanding 

 relies heavily on electrophysiological techniques. 



It is clear from any survey of progress in this field that these stages of 

 historical development are not mutually exclusive. For example, some types 

 of anatomical studies, particularly those at the level of cellular ultrastruc- 

 ture, are extremely important today. Moreover, the electrical techniques 

 of neurophysiology have not preempted the approaches essential for modern 

 physiologists working in this area. There has been a retreat from an initial 

 assumption that nerve recordings from chemosensory organs would corre- 

 late exactly with behavioral reactions of intact animals stimulated by chemi- 

 cals. This revision of viewpoint is not confined to studies of elasmobranchs, 

 but applies to other forms studied by electrophysiological methods as well 

 (Hodgson 1965). Thus, it is clear that new physiological studies of the 

 chemical senses require a return to (and more precise experimental design of) 

 continuing behavioral studies on whole animals. 



In the future there may be a fourth stage of analysis, concentrating on 

 the "coding" of afferent impulses from chemoreceptors and on their inte- 



