52 THE PHYSIOLOGY OF EARTHWORMS 



Unfortunately Haggag and El-Duweini (1959) do not give details 

 of the nitrogenous content of urine in conditions of no starvation, 

 but figures for excretory values for periods of up to 27 days without 

 food reveal that no systematic changes in the proportion of 

 ammonia or urea occur. In animals starved for 5 days the ammonia 

 fraction accounts for 54-2% of the total excreted nitrogen, whilst 

 urea amounts to 6-25% and uric acid 0-26%. No indication, 

 however, is given of the actual quantities involved. Prolonged 

 inanition leads to alterations in the amounts of these substances 

 represented in the urine but in no case does the urea content 

 exceed that of ammonia. This is in direct contrast to the results 

 of Needham (1957) on the same species in England. 



The availability of water in the environment may obviously vary 

 considerably from time to time and such variations may be of 

 great importance in the excretory mechanisms utilized by oligo- 

 chaetes. The change to a terrestrial habitat made by many oligo- 

 chaetes means that water is always at a premium. We shall see later 

 that the earthworm is rather like a fresh-water animal in its water 

 relationships. In animals living in a great excess of water the more 

 highly soluble nitrogenous products such as ammonia and 

 trimethylamine oxide are very often the sole excretory products, 

 but with the changeover to the land and a decreasing volume of 

 available water first urea and then uric acid becomes the dominant 

 substance, each less toxic than the preceding one. 



The earthworm hasn't settled upon one excretory substance to 

 do the job, relying mainly on ammonia, which is removed into the 

 soil moisture usually found in excess around the body, but also 

 excreting urea. The amount of the latter that is excreted seems to 

 depend upon the metabolic water available since starvation leads to 

 a decrease in ammonia and an increase in urea content of the urine. 

 Needham (1957) immersed earthworms in varying quantities of 

 water, for varying periods of time, and concluded that the volume 

 of water was unimportant unless accumulation of toxic products 

 was allowed to occur over a long period. Again this is unlikely to 

 occur in the field. No experiments have so far been carried out on 

 the effect of dehydration upon the excretion of these animals, a 

 state of affairs just as likely to happen in nature, but obviously 

 involving technical difficulties in the collection of adequate 

 samples for analysis. The nephridiopore collection method of 



