receptors, may indicate the best ways of solving this 

 problem. 



4) How are the immediate effects of chemoreceptor stimulation 

 coded into patterns of afferent impulses which enable sharks 

 to discriminate various chemical stimuli? And how many 

 types of chemical structure or modalities of stimuli, are in 

 fact detected? 



Probably the most powerful tools to study mechanisms of chemical 

 stimulation include both electrophysiological methods, to detect receptor 

 responses, and radio-isotope techniques, to label and follow the distri- 

 bution of chemical stimuli. The first tests with labeled stimuli have 

 recently been completed and suggest that neural mechanisms of behavior 

 are somehow automatic, once initiated, and play out a sequency of re- 

 sponses that is independent of the initial stimulus (E.S. Hodgson, 

 R.F. Mathewson and A. Karsten, personal communication). This opens up 

 additional new perspectives in the study of innate, more or less auto- 

 matic responses "wired into" the CNS of the shark; it is possible that 

 many feeding and attack behavioral patterns fall into this category, and 

 consequently they deserve further study. 



Many physiological studies of the chemical senses are handicapped by 

 lack of knowledge of the fine structure of the tissues involved. To 

 our knowledge, no electron microscope studies have been made on the ultra- 

 structure of either olfactory or gustatory sensory cells in elasmobranchs. 

 It would be worthwhile to determine more precisely how an olfactory cell 

 differs from a gustatory cell, whether cilia or villi are present on the 

 exposed surface, the nature of organelles, the nature of innervation, 

 etc. The barbels of bottom feeders, such as Ginglymostoma , should be 

 studied for the presence of external taste buds and how they compare in 

 structure with those of teleosts. 



A further limitation has been that most physiological studies on the 

 chemical senses have been done with four or five species in the genera 

 most readily maintained in captivity: Mustelus , Squalus , Negaprion , 

 Ginglymostoma , and (very recently) Sphyrna . It is obvious that with some 

 250-300 species of sharks, each specially adapted to its own environment, 

 it sometimes involves unwise extrapolations to attempt predictions about 

 the physiology and behavior of other species, or even of these better- 

 studied species in other areas of the world. Careful physiological 

 studies require adequate laboratory facilities and with the development 

 of new marine laboratories there may be opportunities for highly desirable 

 comparative studies on additional species of sharks. 



3. Ac oustico lateralis System 



This is a complex sensory system which is deeply involved in shark 

 predation and defense. It is comprised of the sensory canals of body and 

 head, the pit organs, the ampullae of Lorenzini and the ear. Basically, 

 the sensory organ is the neuromast and the actual receptor of the organ 

 is the hair cell (Dijkgraaf, 1963). 



a. Sensory Canals 



The gross morphology and histology of the sensory canals, their 



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