346 MECHANICAL AND ACOUSTICAL SENSES 



stimulus for the semicircular canals, while in both the utricular and saccular 

 neuromasts the cilia are embedded in a gelatinous mass that is loaded with 

 calcium carbonate particles that render the sense organ sensitive to linear 

 accelerations (including gravity). The modality of the sensory systems built 

 around the hair cells depends on the relationship between the sense organs 

 and the ancillary structures. 



Free Neuromasts: the "Pit Organs"— Scattered over the head and 

 body of all elasmobranchs are neuromasts that project into the outside 

 world rather than into canals. These free neuromasts lie in slits in the skin of 

 rays and primitive sharks (e.g. Heptanchus) and in pits between specifically 

 modified scales in sharks. A description of these organs in various elasmo- 

 branchs is provided by Johnson (1917), Budker (1958), and Tester and 

 Nelson (1967), where the modified scales and patterns of pit organ distribu- 

 tion are particularly well described. 



The sense organs in each pit form a round (e.g. Sphyrna) or elongate (e.g. 

 Squalus) domed structure, 100-200 jitm diameter, which is set among the 

 modified scales. The dome is covered by a mucilaginous plug, but Tester and 

 Nelson (1967) were uncertain whether this was really a cupula, as the usual 

 striations are absent. Budker (1958) was impressed by the apparent simi- 

 larity in form between the pit organ and the teleostean taste bud and postu- 

 lated a chemoreceptive function for the pit organs. He thought he had evi- 

 dence when he found behavioural responses to food extracts applied directly 

 to pit organs. However, in Tester and Nelson's view the pit organ closely 

 resembles the lateral-line neuromast, an observation supported by the elec- 

 tron microscopic description of Mustelus pit organs, where the sensory cells 

 are seen as hair cells with stereocilia and kinocilia (Hama 1969b). 



So far the only neurophysiological attempt to determine pit organ func- 

 tion has been that of Katsuki, Yanagisawa, Tester, and Kendall (1969) in 

 Ginglymostoma; recordings were taken from filaments of the posterior 

 lateral-line nerve while mechanical and chemical stimuli were applied to the 

 caudal fin receptors. Although both canal and pit organs are innervated by 

 the same nerve bundle, their responses could be distinguished; for although 

 both were spontaneously active, the canal organ had a much lower threshold 

 to mechanical stimulation. In addition, the pit organ stopped discharging 

 when it was exposed to distilled water, responded vigorously to monovalent 

 cations (e.g., 1 M NaCl) and was inhibited by calcium and magnesium; anions 

 and sugar however were ineffective. It was also found (Katsuki and Hashi- 

 moto 1969) that monovalent cations enhanced the mechanosensitivity of 

 these sense organs. 



Canal Organs— The neuromasts of the canal system are housed in a 

 series of canals distributed over the head and along each side of the trunk in 

 a pattern that forms a conspicuous feature of fish anatomy. The history of 

 their discovery is well described by Ewart (1892), who reviews the earlier 

 literature. The system appears to have been recognised first, in the elasmo- 

 branchs, as openings discharging mucus (by Stenonis in 1664). Lorenzini 



