BEHAVIOUR 173 



to burrow down into the cooler depths of the soil, and thereby 

 maintain themselves in the optimal conditions. In a similar way the 

 frozen upper layers of the soil may be avoided during wintry 

 conditions. 



The factor which attracts E. foetida into a heap of manure does 

 not seem, therefore, to be the heat generated in such a site. Smith 

 (1902) found that if E. foetida is released anywhere near rotting 

 substances they take no notice of it whilst they are any distance 

 from it. Thus it seems that the animals can neither smell nor 

 detect heat from a distance. The lack of response of the nervous 

 system to distant heat has been confirmed by Prosser (1935). This 

 observation holds true even when the worms pass within a milli- 

 metre or tw^o of the material. If, however, the prostomium should 

 happen to come into contact with the manure the worm immediately 

 begins to burrow inwards, suggesting that chemo-reception is the 

 important factor here. The lack of sensitivity to manure from a 

 distance suggests that the earthworm is not capable of reacting to 

 odorous substances, but it is found to react to such things as 

 cedar oil, xylol, ether and turpentine without making physical 

 contact with any of them. Such observations would repay further 

 investigation. 



One of the best-known behavioural responses of earthworms is 

 that of thigmotaxis. When placed upon the surface of the ground, 

 or on a sheet of paper or glass, the animals are usually very active, 

 but if they should find their way into a crevice or crack which 

 means that the sides of the body are in contact wdth the substrate 

 then very often the individual ceases to move, remaining quietly in 

 this artificial burrow. This thigmotactic response, the reaction to 

 surrounding contact, overrides the usually strongly negative action 

 to light noted upon illuminating the animal. 



Light 



Earthworms, unlike some of their polychaete relations, do not 

 possess eyes as discrete structures although the oligochaete group 

 Naididae do have a pair of eye spots, about the physiological 

 function of which nothing is known. The earthworms do have 

 single sensory cells which contain a lens-like structure lying in the 

 epidermis and dermis, particularly in the prostomium, which are 

 believed to be light-sensitive (see Stephenson, 1930). 



