NERVOUS SYSTEM 147 



described four types of nerve cell lying in the muscle layers. Two 

 of these types may be of interest here. The first of these is a large 

 cell lying interposed betw^een the ventral and dorsal chaetae of 

 each side in each segment, and apparently ideally placed as tension 

 receptors since axons leave these cells and run tow^ards the 

 chaetae. Dawson, however, was unable to find evidence for 

 axons running centrally to the nerve cord and therefore believed 

 these cells to be concerned with eft'ector reactions. The second type 

 of cell lies in the circular muscle layer, and is similar in many ways 

 to sensory cells found on the surface of the body. "Just what 

 function these deep-lying cells perform is difficult to surmise. Their 

 position in the circular muscle layer and their possible restriction 

 to the ventral region indicate a probable role in connection with the 

 initiation or maintenance of the creeping movements of the worm", 

 Dawson (1920). This percipient viewpoint was expressed long 

 before the physiological demonstration of such cells as stretch 

 receptors which are now known to be widespread among arthropod 

 and vertebrate muscles. It would be interesting to examine the 

 electrical properties of these earthworm cells by electrophysiological 

 means, though a rhythmically discharging cell that may be of 

 similar type has been discussed by Laverack (1960b), and Prosser 

 (1935) found that pulling the body wall gave rise to action poten- 

 tials in the segmental nerves. The assumption of a tension or 

 stretch receptor function for these cells provides an adequate basis 

 for an explanation of the phenomena of co-ordinated movement 

 occurring in animals in which the continuity of the nerve cord has 

 been interrupted. The normal course of impulses runs through the 

 nerve cord and via the segmental nerves to the body wall, but if the 

 CNS is severed the passive tensile and tactile stimulation caused 

 by dragging the rear segments across the rough substratum will be 

 sufficient to start up fresh reflex activity in these segments. It 

 should be noted moreover that the normal pattern of movement is 

 obtained only when the body touches the ground, and not when it 

 is suspended (Moore, 1923a). 



It is possible to demonstrate that the tension reflex of pieces of 

 the body wall have a distinct threshold which ranges from 0-1 to 

 1-0 g. That is to say the smallest weight needed to evoke regular 

 peristaltic movements is of this order, though it naturally varies 

 from preparation to preparation and from time to time. This can 



