tubules and their neuromasts are reasonably well known for sharks and 

 rays from the works of older authors, augmented by more recent studies, 

 e.g., Tester and Kendall (1969) for Carcharhinus . The orientation of 

 the tubules and tapering caudad bore of the canal provide for a slow 

 tailward flow of seawater through the canal. The rate of flow should be 

 determined for several species. The function of peculiar vesiculated 

 cells adjacent to the neuromast in the canal epithelium, greatly elaborated 

 in the anterior head canals, should be investigated. The recent study of 

 Liddicoat and Roberts (1972), showing the canal fluid of dogfish to have 

 the same ionic content as seawater, should be repeated for other species. 

 These latter studies would contribute to our knowledge of ionic regulation 

 of body fluid, and the part played in establishing a sodium-potassium ratio 

 optimal for electrogenesis in the hair cells. 



The fine structure and orientation of hair cells of the canal organ 

 has been studied extensively in teleosts but not in sharks. Roberts and 

 Ryan (1971) have made Electron Microscope (EM) studies in Scyliorhinus . 

 Similar studies should also be undertaken in other common sharks, including 

 both bottom-dwelling and pelagic species in order to contribute to our 

 knowledge of how a hair cell functions as a receptor of displacement waves. 

 Items of interest are (1) the orientation of hair cells along the canal 

 with respect to the position of the kinocilium (movement in one direction 

 produces excitation and in the oposite direction, inhibition); (2) the 

 presence of different types of hair cells; (3) the nature of organelles; 

 and (4) the presence of both afferent and efferent synapses. 



Roberts and Russell (1970) and Roberts (1972) , again working with 

 Scyliorhinus , have made the only study in sharks of the source of efferent 

 activity that modulates the afferent firing of the hair cells in the canal 

 organ. Since the results are not clear cut, the work should be repeated 

 and expanded, preferably with other species. This work is essential to a 

 complete understanding of how the canal organ functions as a receptor of 

 displacement waves. 



Given that the ordinary lateral-line organs are displacement receptors, 

 the following questions arise (cf. Dijkgraaf's lateral-line review, 1963): 

 (1) What are the displacements that sharks detect in their daily life? 

 Here we should distinguish between (a) displacements imposed upon the 

 animals from external sources, in which case the receptor system operates 

 in a passive mode, and (b) displacements resulting from the animal's own 

 activity relative to the environment with the receptor system operating in 

 active mode. (2) What may be the biological significance of the various 

 displacements (e.g. detection of other animals by the displacements re- 

 sulting from their swimming or ventilation movements, detection of objects 

 by the distortions they cause in the sharks' own* "underwater bow waves")? 

 (3) How do sharks mechanically relate to their environment? How do they 

 integrate the information from their spread-out lateral-line organs, and 

 how do they analyze the spatial and temporal aspects of natural displace- 

 ment fields? To answer these questions, the physics of the pertinent 

 displacement fields should be studied more rigorously, the biological 

 relevance should be determined in well-planned behavioral tests, and the 

 central processing of the total receptor input should be investigated by 

 recording the electrophysiological responses of peripheral nerves and 

 central nuclei. 



17 



