WATER RELATIONS 75 



from the body as fast as it diffuses inward across the body wall, 

 with the production of a copious hypotonic urine. But an earth- 

 worm finds itself between two stools, being neither completely 

 immersed in water nor living in rigidly arid surroundings in- 

 volving it in strict water conservation. The environment can vary 

 anywhere between these two extremes. 



On average, however, it must be considered that the normal 

 environment of the earthworm is midway between that of a 

 fresh-water animal and that of an air dweller. Under such con- 

 ditions Bahl (1945) considered that in Pheretima the nephridia 

 function adequately as volume and osmoregulatory organs since the 

 volume of water involved is not excessively large. But when a 

 terrestrial earthworm is kept in water like a fresh-water animal, 

 the large amounts of water passing across the skin cannot be 

 eliminated by the nephridia and the gut is then brought into 

 action, removing excess water through the anus and the mouth. 



Stephenson (1945) obtained evidence that considerable salt 

 regulation is carried out during the removal of water. He placed 

 Lumbricus in dilute salt solutions, finding that the internal chloride 

 concentration remained above the external concentration. In other 

 experiments he put earthworms into concentrated salt solutions 

 and noted that they maintained a low internal concentration 

 relative to the external medium. On the basis of this he concluded 

 that the earthworm is not like a truly fresh- water animal ''as far as 

 its osmotic relations with the environment are concerned". 



This work was repeated by Ramsay (1947, 1949) in a series of 

 very elegant and careful experiments. Ramsay utilized fine micro- 

 pipettes which he inserted into the nephridiopores of Lumbricus to 

 collect urine. He was then able to compare the osmotic pressure 

 (O.P.) of this fluid with that of the osmotic pressure of the coelom 

 and of the blood. He also confirmed that chloride accounts for 

 only 50% of the total blood O.P., and that blood is just hypotonic 

 to coelomic fluid {A = 0-31 °C for coelom). He kept his animals in 

 media having salinities between 0-025 and 1-27% NaCl, and found 

 that as the concentration of the medium increases so the osmotic 

 pressure of the body fluids also increases, remaining always 

 greater than that of the medium. The chloride content increases 

 proportionately but is less than that of the medium when the 

 latter exceeds 0-35 % NaCl. The urine obtained from the nephridio- 



