MECHANORECEPTORS AND BEHAVIOR 333 



temperature, and possibly pain sensation and is located near the skin surface. 

 Because the skin of most elasmobranchs contains many hard scales, which 

 are unamenable to conventional histological procedures, most studies on skin 

 innervation have been carried out on whole mounts, stained with methylene 

 blue, or on sections cut through the softer skins of embryos or rays. Such 

 studies (Weddell 1941; Murray 1961) have demonstrated that the cutaneous 

 innervation, which is arranged in the standard vertebrate pattern and consists 

 of two layers of connecting networks, shows little development of spe- 

 cialised endings. In his illustration of part of this plexus, Murray shows 

 myelinated axons (3-6 jum in diameter) of a subdermal network of parallel- 

 running fibres, dividing and losing their myelin before terminating as 'free- 

 endings'. In teleosts such endings measure less than 1 jum in thickness and 

 show no evident distinctions from other small nerve fibres, even at the 

 ultrastructural level (Whitear 1971). 



The relative simplicity of this plexus and the skin's resistance to drying 

 provided Murray with a good experimental preparation from which it was 

 possible to record electrical activity by simply placing a wire electrode on 

 the skin surface. In this way he obtained slow, positive-negative impulses that 

 appeared to be initiated some distance from the nerve terminal, into which 

 they then propagated antidromically. The endings were fairly sensitive, giv- 

 ing discharges to movements of less than 20-fim amplitude, but they adapted 

 rapidly to sustained stimuli (75% complete in 5 s). The latency of the spikes 

 (2.2-7.0 ms) depended on the intensity of the stimulus. 



The Wunderer Corpuscle— At present only two forms of morpho- 

 logically specialised nerve terminals have been described in elasmobranch 

 fishes— the corpuscles of Wunderer, found in the deeper layers of the skin, 

 and the endings of Poloumordwinoff, associated with certain muscle fibres 

 of the paired fins of rays. 



The corpuscular endings of the fins of a variety of elasmobranchs, first 

 reported by Wunderer (1908) after whom they are now named, have since 

 been seen in the body by Bone (1964), Tester and Kendall (1967), Roberts 

 (19696), and Bone and Chubb (1976). The latter authors provide a detailed 

 description of this ending. In the body the corpuscles are nearly always 

 located among the collagenous fibre bundles of the stratum compactum of 

 the dermis, although Tester and Kendall (personal communication) have 

 found some endings deeper, on myotomal septa and close to the vertebral 

 column. Each ending, a whorl of fine unmyelinated fibres, terminates a large 

 axon (Figures la, lb), the myelin sheath of which usually ceases before the 

 ending is formed, although sometimes it penetrates a little way; quite fre- 

 quently a single axon branches to give a pair of endings. 



From their position one might suspect that these end organs would be 

 stimulated not only by touch but also by body and fin movement; this view 

 has been confirmed by Lowenstein (1956). In an electrophysiological study 

 in the pectoral fins of Scyliorhinus he demonstrated that these endings re- 

 sponded not only to mechanical pressure but also to fin movement. A study 

 (Roberts 19696) of the properties of these endings in the body (also in 



