166 THE PHYSIOLOGY OF EARTHWORMS 



animal, although the initial input is probably governed by the 

 sensory connections (Fig. 53). 



Sense Organs and Sensitivity 



As late as 1955 Hodgson, reviewing the gaps in knowledge of 

 invertebrate chemoreception reported that nothing was known of 

 the sense responses of Lumbricus. Although the histology of the 

 sense organs of Lumbricus had been described in detail by Langdon 

 (1895) and Smallwood (1926) ''experimental verification of the 

 presumed chemosensory function is lacking". 



Prosser (1935), however, has given us some information on the 

 sensitivity of earthworms to a number of chemical substances as 

 well as to Hght and touch. The segmental nerves of the earthworm 

 are mixed nerves, there being no division into sensory and motor 

 nerves amongst the segmental nerves. Large spike potentials are 

 obtained in response to tactile stimulation of the epidermis with a 

 fine needle, and also in response to pulling and stretching the body 

 wall. This latter observation indicates that stretch receptors are 

 indeed present as suggested by earlier experiments on locomotion. 



The touch receptors of the earthworm segments serve discrete 

 areas on the surface of the body. Each of the three nerves in each 

 segment covers particular regions of the segment of origin, and also 

 the segment immediately before and after. The anterior nerve in 

 each segment has a greater receptive area on the segment ahead, 

 whilst the posterior nerve has a greater sensory area on the seg- 

 ment behind. These results have been confirmed by Laverack 

 (1960b). 



Light also evokes potentials in the segmental nerves. This 

 response is only noticeable when the earthworm has undergone a 

 considerable period of dark adaptation, illumination after 15 

 minutes darkness causing no potentials to appear, after 30 

 minutes, such treatment a small burst is noted, and after an hour of 

 dark adaptation a large volley is seen. 



Prosser (1935) found no nervous activity was elicited by the 

 application of sucrose solutions to the body wall, but obtained 

 records after stimulation with HCl, NaOH, and NaCl. N/50 HCl 

 provoked a short burst of potentials. N/20 HCl gave a larger burst 

 and N/10 killed the sense organs of the skin. 



These results have since been extended by Laverack (1960b) 



