176 THE PHYSIOLOGY OF EARTHWORMS 



waveband, is non-stimulatory since earthworms can be studied 

 comfortably when illuminated with red light and it has no apparent 

 effect upon the normal activities of the animals. If blue light is 

 flashed upon them they withdraw rapidly into their burrows 

 (Walton, 1927). 



This reaction to blue light is most pronounced at a wavelength of 

 about 483 m^it (Mast, 1917). Although the intensity of the various 

 wavelengths was not thought to affect these experiments it is none 

 the less not possible to state definitely that earthworms are able to 

 distinguish different wavelengths of light. Ultra-violet Hght may be 

 lethal to earthworms, and it has been suggested that the many 

 earthworms found lying dead upon the surface of the ground after 

 rain have been killed by the u.v. of the sunlight (Merker and 

 Braunig, 1927). 



Regional sensitivity. The experiments detailed above show that 

 earthworms are capable of receiving light stimuli and will respond 

 to such stimulation. Such sensitivity is not a unifoFm property of 

 the body surface, however. If, instead of a large generalized source 

 of light, a narrow pencil beam of light is projected onto the skin 

 it can be shown that although light stimulation can be detected 

 over the entire surface of the animal there are certain regional 

 specializations. The prostomium is the most sensitive area of all, 

 and the ability to react to light decreases as illumination passes 

 rearwards along the body. The anterior end is more sensitive than 

 the mid-region, but then sensitivity again rises towards the rear end 

 (Hess, 1924, Howell, 1939). Within each individual segment the 

 dorsal aspect is the most sensitive, and the edges of the segments at 

 the intersegmental furrow are least reactive (Hess, 1924). 



Light and the Nervous System 



Interference with the nervous system in some form such as the 

 removal of the cerebral ganglia, or the severance of the circum- 

 oesophageal commissures, causes at least a partial reversal of the 

 normal responses to light. L. terrestris becomes photopositive in 

 moderate light intensities, and Pheretima agrestis becomes positive to 

 weak light, though it still remains photonegative in moderate inten- 

 sities (Hess, 1924; Howell, 1939). Prosser (1934b) obtained similar 

 results with E. foetida and also noted that depressant drugs and 



