82 THE PHYSIOLOGY OF EARTHWORMS 



of aestivation, or diapause, is associated with low soil humidities, 

 and it is possible to prevent its appearance by keeping earthworms 

 in a humid atmosphere. A. longa kept at 18 °C in high humidity 

 with plenty of food shows cyclic clitellar development and repro- 

 duces normally. Slow dehydration and lack of food is followed after 

 4-6 weeks by diapause, the animals coiling up, becoming colour- 

 less and isolating themselves in a pocket of earth. For the next 2 

 months the animals cannot be awakened, spontaneous wakening 

 occurring at the termination of this state. A great deal of weight is 

 lost during this period, presumably due to loss of water by 

 evaporation and food reserves by metabolic breakdown. 



It is also possible to induce aestivation in the winter by pro- 

 longed desiccation, but although humidity conditions are of 

 great importance in controlling diapause, other factors play a 

 considerable part. In view of the recent research into neurosecretion 

 associated with reproductive and regeneration capacity it is highly 

 likely that internal cyclic factors are concerned in the regulation of 

 aestivation (Michon, 1949, 1954). 



Summary 



Earthworms freshly dug from the soil can take up water, losing 

 weight again when returned to the habitat. Many species can 

 remain alive in waterlogged soil under standing water. Severe loss 

 of water can be withstood and the prostomium appears to be the 

 site of response for a dehydration tropism. Water passes rapidly 

 across the body wall in both directions, and there is evidence that 

 mineral salts may do likewise. In its normal water relations the 

 earthworm functions as a fresh-water animal with one or two 

 modifications. It excretes a copious hypotonic urine and this is 

 modified in its composition during its passage through the 

 nephridium. Ciliary activity within the nephridial tubules increases 

 when the animal is immersed in water of low salt concentration, 

 thus diluting the coelomic fluid. Some earthworms undergo a 

 period of aestivation when the external humidity drops. This may 

 be governed by a neurosecretory process. 



