BEHAVIOUR 177 



low temperatures lowered the responsiveness of earthworms to 

 light. Section of the ventral nerve cord midway along the body, 

 leaving the anterior portion under the control of the cerebral 

 ganglion, leads to different responses of the anterior portion, 

 which is photonegative, and the rear portion, which is photopositive 

 (Hess, 1924). In all the reported cases (Hess, 1924; Howell, 

 1939 ; Prosser, 1934b ; Nomura, 1926) the cerebral ganglion appears 

 to be the ultimate controlling and co-ordinating centre. 



Although it is obvious from above that the nervous system is 

 intimately concerned with the reactions to light only one study 

 has been made of the actual electrical activity occurring within the 

 sensory nerves in response to such stimulation. Prosser (1935) 

 recorded some small potentials from the sub-oesophageal ganglion 

 which were of low frequency initially but increased to a maximum 

 after 2 to 4 seconds. They were then overshadowed by proprio- 

 ceptive impulses generated during contraction of the animal. Dark 

 adaptation affects this response, no potentials being observed upon 

 stimulation after 1 5 minutes in the dark, a small burst after 3 minutes, 

 and a large burst after one hour. These nervous responses should 

 be compared with the behavioural reactions mentioned above. 



Chemoreception 



It has been known since the time of Darwin (1881) that earth- 

 worms are able to distinguish between various food substances, 

 demonstrating preferences for certain plants above others. The 

 ability to select the material to be eaten seems to reside in the sense 

 of taste. Chemoreception was indicated by Smith (1902) in her 

 work on the reactions of E. foetida to chemical stimuli. This 

 faculty of determining external chemical conditions probably 

 plays a major role in the life of the earthworm. First it may be 

 used as mentioned above, in the detection and gathering of food, 

 secondly it may be used to give information regarding the other 

 environmental conditions, such as soil acidity, and thirdly there is a 

 possibility that it plays a part in mating by detecting the mucus 

 secretions of other earthworms. 



Earthworms usually come to the surface of the ground at night to 

 graze upon leaves in the litter layer. They are able to draw leaves 

 over the ground and down into the burrow. Not all leaves are 

 utilized at the same rate, however, and selective feeding goes on. 



