BEHAVIOUR 



175 



dim illumination sensitize L. terrestris so that it reacts positively 

 to more intense light than when it is dark-adapted. 



Another species of earthworm, Pheretima agresttSy shows only one 

 response to light, avoidance. The photonegativity becomes more 

 pronounced as the stimulus intensity increases, and although 

 adaptation occurs in dim light so that the threshold illumination 

 necessary to stimulate the animal rises, the response remains 

 photonegative. Other types of stimulation, mechanical and thermal, 



Fig. 58. Nerve enlargements carrying photoreceptor cells in 

 prostomium. E = epidermis; L = photoreceptor cells; NE = 

 nerve enlargements; other letters as in Fig. 57 (from Hess, 1925). 



for example, decrease the sensitivity of the light response (Howell, 

 1939). 



The quality (wavelength) of the light, as well as the quantity 

 (intensity), also plays a part in the light stimulation of earthworms. 

 Bretnall (1927) passed light through a prism and projected the 

 resulting spectrum onto a sheet of white paper. He then dropped 

 worms on the paper and noted that all except one moved away from 

 the blue colours and travelled across the spectrum to pass out at the 

 red end. The dissenting individual lay full length in the green 

 band. It would seem that red, rather than being an attractive 



