MECHANORECEPTORS AND BEHAVIOR 337 



The obvious structural comparisons between the endings and the muscle 

 spindle raises the fascinating question of whether muscle spindles- 

 specialised receptors arranged in parallel with the musculature— are present in 

 fishes. So far attempts by several histologists, using a variety of stains and 

 material, have failed to reveal spindle-like endings in fish muscle, and the 

 total lack of candidates for a sensory supply has been remarked upon in 

 surveys of fish muscle innervation (Baum 1900; Barets 1961; Bone 1964). 



In more recent neurophysiologies experiments (Roberts 1969b), pro- 

 prioceptive discharges were recorded in the spinal nerves in response to 

 muscle stretch and contraction. The discharges did not conform to the rec- 

 ognisable properties of muscle spindles; moreover, they were eliminated by 

 the complete removal of the skin and were therefore attributed to the 

 Wunderer corpuscles. Roberts pointed out that the absence of muscle 

 spindles from fishes may reflect the lack of major postural problems in these 

 animals. 



The Special Cutaneous System: The Hair Cell 



In the skins of fishes direct sensory contact is made with the external 

 medium by specialised cells that are differentiated from the epithelium and 

 have a sensory role. Chemoreceptors are nearly always of this type, and one 

 specialised mechanoreceptor, the hair cell, is also of this form. In the lateral 

 lines of amphibia and some teleosts the hair cell lies in the skin with its 

 sensory hairs protruding into the water, but in elasmobranchs it recedes into 

 a pit, groove, or canal, while in the labyrinths of all vertebrates it is totally 

 withdrawn from the body surface. Although the receptor always has a char- 

 acteristic form (the hair cell), it is organized with associated structures into 

 an array of sense organs that is sensitive to several sensory modalities; this 

 collection is named the acoustico-lateralis system. 



The Structure of the Hair Cell— The hair cell, the associated support- 

 ing cells, and the peripheral secretory cells (mantle cells) form a sense organ 

 called the neuromast. Each neuromast is surmounted by a gelatinous struc- 

 ture, the motion of which displaces the sensory hairs and thereby (in an 

 unknown manner) activates the hair cells. In some cases this mucilaginous 

 cap extends as a thin sheet called the cupula, and the sense organ is then 

 called a crista; if the cap is broader and contains a dense calcareous body or 

 bodies (otoliths or otoconia) the organ is called a macula. Both types of 

 organ are found in the acoustico-lateralis system. 



A typical hair cell, with views of critical areas, is diagrammed in Figure 3. 

 The sensory hairs described by the early morphologists have been shown by 

 the electron microscope to be elongate protrusions, up to 60 per cell. One of 

 these, the kinocilium, is clearly recognisable as a typical cilium because it 

 contains internal tubules of the familiar 9 + 2 configuration and is attached 

 within the cell by a basal apparatus. The other projections, which are usually 

 shorter than the kinocilium (Figure 4) and are less obviously organized in- 

 ternally, are called stereocilia and enter the hair cell in a specialised region, 

 the cuticular plate. Near the sense organ surface the sidewalls of the hair cell 



