142 THE PHYSIOLOGY OF EARTHWORMS 



(1935) discovered that tactile stimuli, giving rise to action potentials 

 within a segmental nerve, could be perceived as giving rise to 

 activity when touch was applied in the segment of origin or in the 

 segments to either side as well, though the areas on the segmental 

 wall to fore and aft were rather more restricted (Fig. 44). Thus, at 

 least for the touch receptors, the sensory nerves ramify over a 

 numoer of segments beside that in which they originate. At the 

 same time it is thought that, whilst the sub-epidermal nerve 

 network is organized on a segmental basis, the various forces applied 

 to the body wall by movement of contiguous segments is translated 

 into nervous activity in this system, thus enabling peristalsis to 

 continue uninterrupted from segment to segment even when the 

 ventral nerve cord is severed. 



(a) (b) (c) 



Fig. 44. Sensory fields served by (a) first, (b) second, (c) third 

 segmental nerves. Dotted areas are those from which no responses 

 were obtained when explored by a fine needle (from Prosser, 



1935). 



There are many sense organs of various types within the body wall 

 of Lumbriciis and these are associated with numerous intra-epider- 

 mal nerve fibres that end freely between the epidermal cells 

 (Langdon, 1895). Langdon beHeved that each sense cell had its 

 own individual nerve fibre that ran as a separate entity into the 

 ventral nerve cord, but as there are thousands of sense organs per 

 segment, and only a few hundred fibres in each segmental nerve 

 (150 in L. terrestriSy Smallwood, 1930; 1500 in Perichaeta, Ogawa, 

 1927), and in the ventral nerve cord ganglia, it is obvious that this 

 cannot be so. Smallwood (1926) suggested that the sense organ 

 fibres synapse with the sub-epidermal network of fibres and are 

 collected together from there to form the segmental nerves. By the 

 same token the motor fibres of the ventral nerve cord, relatively 



