352 



MECHANICAL AND ACOUSTICAL SENSES 



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Figure 10 Directional responsiveness of lateral -line organs. (A) A single 

 efferent fibre in Scyliorhinus, in this case responding to body movements, 

 showing an accelerated discharge to one direction of movement (indicated 

 by the upper trace of each record) and a cessation of activity to a move- 

 ment of opposite direction (Roberts 1972). (B) Impulse frequency (s _1 ) of 

 a unit in Raja responding to the flow of Ringer solution along the canal 

 (beginning at arrow and lasting for 10 s. In one direction the flow is 

 excitatory (o); in the opposite direction it is inhibitory (•) (redrawn from 

 Sand 1937). 



Savi's Vesicles— These organs, named after their discoverer, are vesic- 

 ular structures 2-3 mm in diameter, found in rows on the dorsal and ventral 

 surface of the anterior edge of the disc of electric rays. Coggy (1891) and 

 Norris (1932) established a good case for believing that these organs are 

 vestigial remnants of the ventral canal system which otherwise is absent from 

 these fishes; more recent studies on structure and neurophysiology support 

 this interpretation (Szabo 1968). Each vesicle consists of a sensory surface 

 containing three neuromasts, totally covered by a cupula; at the ultrastruc- 

 tural level these neuromasts all seem to consist of slender hair cells (with 

 dual innervation) and adjacent supporting cells (Derbin and Szabo 1966; 

 Nickel and Fuchs 1974). 



Fessard and Szabo (1958) (and Szabo and Fessard 1965) have made 

 recordings from the afferent supply of these receptors and found tonically 

 discharging units that responded to stimulation by an increase or decrease of 



