340 MECHANICAL AND ACOUSTICAL SENSES 



are specialised and form desmosomal associations with neighbouring cells, 

 presumably to give the epithelium some mechanical rigidity. 



The base of the hair cell makes contact with the lateral-line nerves, which 

 are of two types, one afferent in function and the other efferent. The af- 

 ferent ending is always present and is associated with clusters of vesicles in 

 the base of the hair cell which are concentrated around an electron dense 

 body called the synaptic body, of which there may be more than one per 

 cell. The efferent ending, which is usually smaller than the afferent terminal, 

 is filled with vesicles; where it forms a synapse there often develops inside 

 the hair cell a closed membranous sac, the subsynaptic cistern. In contrast, in 

 the afferent fibre there is little obvious postsynaptic specialization. So far in 

 fishes, the interesting condition in which the efferent synapse is made on the 

 afferent fibre has been reported only in the neuromast of the goldfish sac- 

 culus (Nakajima and Wang 1974). 



Most of these standard features can be recognised in those elasmobranch 

 hair cells that have been studied with the electron microscope— in the laby- 

 rinth of Raja (Lowenstein, Osborne and Wersall 1964); in the pit organ of 

 Mustelus (Hama 19696); in the lateral lines of Scyliorhinus (Roberts and 

 Ryan 1971) and Mustelus (Hama and Yamada 1977), and in Savi's vesicles of 

 Torpedo (Nickel and Fuchs 1974). 



The cupula has been demonstrated in shark lateral-line organs by Tester 

 and Kendall (1968), who described it as a delicate structure, poorly preserved 

 in conventional histology but seen clearly in fresh-frozen sections when it 

 fills the lumen of the canal almost completely. The surface is irregular, has 

 no limiting membrane, and stains metachromatically, presumably because it 

 contains mucopolysaccharides. It has a striated appearance and includes cel- 

 lular debris thought to come from the sensory epithelium. As described by 

 Tester and Kendall the cupula is to be considered a dynamic structure that 

 "grows" at the neuromast surface by incorporating material derived from the 

 supporting cells, while other material is shed distally into the canals, which are 

 kept clear by a slow, head-to-tail endolymph flow. The cupula of Necturus, 

 which was visualized with vital dyes and china ink particles, grows at about 

 15 jum/h (Frishkopf and Oman 1972). 



A notable feature of all fish hair-cell systems is that the cilia, although 

 varying in number from fewer than 20 to more than 70, are always arranged 

 in a very characteristic way, with the kinocilium positioned on one side of 

 the ciliary group (Figures 3 and 4). The orientation of the ciliary bundle 

 with respect to its neighbour, and to the axis of the fish, differs in different 

 sense organs. Thus in the teleost lateral line the kinocilia of adjacent hair 

 cells face each other, but in the labyrinth whole clusters of hair cells in the 

 same neuromast have the kinocilium situated on the same side of the ciliary 

 group. In ultrastructural studies on the ray labyrinth, Lowenstein and 

 Wersall (1959) observed that the kinocilia were on the side of each ciliary 

 group that faced towards the utriculus, in the case of the hair cells of the 

 horizontal canal, but faced away from the utriculus in the crista of the 

 anterior vertical canal. By relating this striking morphological arrangement to 

 the well-established electrophysiological properties of these canal organs (see 



