174 



THE PHYSIOLOGY OF EARTHWORMS 



The behavioural reactions of earthworms to light depends to a 

 certain extent upon the immediate past history of the animals. For 

 example Hess (1924) kept L. terrestris in the dark for some hours, 

 and then exposed either the right or left side of the body to a beam 

 of light, recording the movements of the anterior end. In all 

 except the weakest light (0-0018 metre-candles) the animals 



Fig. 57. Epidermis of prostomium, showing photoreceptor cells 



with two optic nerves. B = basement membrane; C = cuticle; 



L = lense; N = nucleus; ON = optic nerve; R = retinella; 



S = subepidermal nerve plexus (from Hess, 1925). 



responded by moving away from the light ; in the weakest light the 

 earthworms were photopositive. In other words earthworms 

 crawl towards very dim light, but away from more intense sources. 

 If, however, L. terrestris is subjected to long periods of moderate 

 illumination they often do not react at all to a sudden increase in 

 the intensity of the light (Hess, 1924), due perhaps to adaptation 

 or to complete saturation of the light receptors. Long periods of 



