b. Pit Organs 



The gross morphology, histology and innervation of these organs have 

 been described in several species of sharks by Budker (1938), Tester and 

 Nelson (1967) and Tester and Kendall (1967). They resemble the canal 

 organs and, in fact, have been called free neuromasts by some authors. 

 However, there is doubt as to whether they actually have a cupula or 

 whether the sensory surface is merely coated with an undefined mucus layer. 

 There is still some question as to whether they have cilia: only a brush- 

 like fringe can be seen under the light microscope. The only EM work to 

 date is referred to by Katsuki, Hashimoto and Kendall (1971) who state 

 "Hama and Yamada (in preparation) confirmed electron-microscopically the 

 ciliary structure of the hairlike process in these sensory cells." There 

 is need for further EM studies of the pit organs of sharks to further 

 elucidate the fine structure of the sensory cell and to compare it with 

 that of the lateral line hair cell. Suitable preparations are difficult 

 to make because of protective modified scales. 



The function of the pit organ is still uncertain. Onada and Katsuki 

 (1972) imply that it has the dual function of mechanoreception and chemo- 

 reception. In earlier papers, Katsuki and his co-workers have implied, 

 from electrophysiological experiments, that it is primarily a salinity 

 detector. How its true function or functions (from the shark's point of 

 view) can be determined is problematical. Since it apparently does have 

 mechanoreceptive functions it may contribute to the sharks ability to 

 orient and home in on a near field sound source, or it may serve to detect 

 changes in flow pattern. Further electrophysiological experiments, 

 augmented by behavioral experiments, should be undertaken to clarify its 

 alleged mechanoreceptive and/or chemoreceptive function. For behavioral 

 experiments, some method will have to be devised to occlude or otherwise 

 nullify either the sensory canals or the pit organs, both of which are 

 serviced by the same nerve, in order to investigate their functions 

 separately. It may also be necessary to occlude the ampullae of Lorenzini 

 and the ear when studying the response to displacement waves. 



c. Ampullae of Lorenzini 



These unique organs, located on the dorsal, ventral, and lateral 

 surfaces of the shark's head, have been subject to many morphological, 

 electrophysiological, and behavioral studies over the years. The organ is 

 a flask-like tube with an opening on the head and is filled with a mucoid 

 jelly. Its blind end has a central plate and several out-pouchings lined 

 with sensory epithelia. EM studies by Waltman (1966) show that the pear- 

 shaped sensory cells have a cilium or hair with a supporting structure that 

 differs from that of the "ordinary" lateral-line haircell. Despite this 

 and other differences, the ampullae are still considered to be "specialized" 

 lateral-line organs. 



Excised ampullae of Lorenzini have been shown by many authors (see 

 Murray, 1974) to respond to a variety of stimuli: thermal, mechanical, 

 chemical, and electrical. Nerve recordings from free-swimming sharks 

 ( Scyliorhinus ) revealed, however, that in situ ampullae, though still very 

 sensitive to weak electric fields, do not appreciably respond to mechanical 

 perturbations nor to thermal gradients in the water (Kalmijn, 1972, 1974). 

 These results accord with the location of the sensory epithelia deeply 



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