172 THE PHYSIOLOGY OF EARTHWORMS 



relations of the sense organs in the skin were described as long 

 ago as 1895 by Langdon, but only within the last few years have 

 modern electrophysiological techniques been appHed to a study of 

 the actual sensitivity of such organs. 



Prosser (1933) has followed the development of the characteristic 

 behaviour of the embryos of E. foetida and has shown that as the 

 structural differentiation of the body proceeds so does the activity 

 and response to external stimuli become more complex. The first 

 responses to tactile stimulation are local contractions of the body 

 wall which are correlated with the appearance of cells of the circular 

 and longitudinal muscles. Later on flexion of the anterior end in 

 response to touch requires not only the presence of the muscle 

 layers, but also fibres in the CNS and lateral nerves in the sensitive 

 area. The more complex behaviour of extension, peristalsis, 

 exploration and co-ordination of setae only occurs when the 

 circular, longitudinal and transverse fibres of the nervous system 

 are complete. The response to illumination of the anterior end 

 is correlated with the development of prostomial nerves bearing 

 enlargements which probably contain photoreceptors, and with the 

 appearance of central nervous connections. This is followed by the 

 development of swift end-to-end contractions mediated via giant 

 fibres. 



The macroscopic behaviour of Eisenia (Allolobophora) foetida 

 was studied by Smith (1902). This species is a well-known inhabi- 

 tant of piles of rotting vegetation and manure where the organic 

 content is high, and very often the temperature also is higher than 

 in the adjacent soil. The temperature in such a pile of rubbish can 

 sometimes rise as high as 20 °C. Smith found that if she put E. 

 foetida on a glass plate which she proceeded to warm gradually the 

 worms showed no response as the temperature rose from 20 to 

 30 °C, but that if the temperature continued to rise above 30 °C the 

 animals moved away from the source of heat. Heat death occurred 

 at a temperature of 36-40 °C. This compares with a heat death tem- 

 perature of 29 it 0-5 °C when aUied with desiccation (Hogben and 

 Kirk, 1944). It seems unlikely from these observations that environ- 

 mental temperatures play much part in governing the activity of 

 this species since the temperatures encountered in field conditions 

 rarely rise high enough to affect overt activity. At such times as 

 intense summer heat may occur it is always possible for earthworms 



