WATER RELATIONS 81 



relation to such concentration and dilution of body fluids as the 

 earthworms may encounter in nature save in extreme conditions. 

 Consequently Roots (1956) attributes little importance to the 

 resistance of the nephridia to changing osmotic conditions. 

 None the less it has been mentioned above that the earthworm 

 taken fresh from the field is not completely hydrated, but can 

 become so under conditions when free water is available such as 

 heavy rain, or lose water when soil humidity decreases so that 

 some strain must be imposed upon the organs of excretion and 

 volume control. For example Zicsi (1958) has shown that 60% of 

 the body water is lost from many species of worms living in soils 

 containing less than 10% water. This is an exceptionally low 

 quantity of soil water, but such humidity levels can be reached 

 during long dry spells of weather. 



The hydrostatic pressure within the coelom of earthworms 

 amounts to approximately 15 cm w^ater, and the total number of 

 nephridia to about 300. On this basis Chapman (1958) calculates 

 that between 1/100 and 1 /4 of a millilitre of fluid will be lost per hour 

 if the nephridia are considered simply as open tubes connecting the 

 coelom to the exterior and the animal maintains a constant internal 

 pressure. The nephridia can of course, be sealed by a sphincter 

 muscle when necessary, as in the passage of a peristaltic wave. The 

 amount of fluid escaping in this calculation agrees closely with the 

 volume of urine production observed by Wolf (1940) and by 

 Ramsay (1949b). Thus the coelomic pressure could account for 

 much of the urine appearing at the nephridiopores, but Ramsay 

 noted that collection of urine occurs in pipettes. This involved an 

 active excretion of fluid so evidently some other force combines 

 with coelomic pressure to account for the formation of urine in the 

 normal animal. 



Aestivation 



In some species of earthworm more than others, the variations in 

 external water have pronounced eflFects upon the behaviour of the 

 animals. Such worms as A. longa and E. rosea undergo a period of 

 aestivation. This occurs during the summer months when the 

 animal becomes isolated, curls into a small knotted ball in a 

 pocket of air in the soil and surrounds itself with mucus secretions 

 which dry to form a coating on the inside of the pocket. The onset 



