WATER RELATIONS 77 



than in the medium and at the same time eHminate a hypotonic 

 urine. 



There are a few conflicting facts affecting the simphcity of the 

 picture of passive entrance of water under osmotic pressure, 

 passive and active entry of salts with or against the osmotic 

 gradient and eHmination of hypotonic urine. For example it is 

 known that when earthworms recover in water after considerable 

 desiccation, the rate of intake of water increases many-fold at 

 deficits that do not even double the osmotic pressure of the body 

 fluids (Adolph, 1927). The volume of the body also holds an 

 anomaly when compared with the O.P. of the medium for 

 Stephenson (1945) found that the body volume is at a maximum at 

 an external concentration of 0-35% NaCl {A of body fluids = 

 0-31%) and declines as the medium becomes more or less con- 

 centrated. This is unexpected on the simple system outlined above. 

 Ramsay (1949a) suggests that further work will indicate where the 

 modifications in this picture are to be found. 



The Nephridium 



By the use of still more refined techniques Ramsay (1949b) 

 collected fluid from the various tubules of the nephridium with the 

 object of determining the site of production of the hypotonicity of 

 the urine. The osmotic pressure of urine samples from the ampulla, 

 the ascending and descending sections of the narrow tube, the 

 middle tube, the mid and distal portions of the wide tube, and the 

 bladder of the nephridia of Lumhriciis were estimated (Fig. 236). 



Figure 2Za shows the results Ramsay obtained. The narrow 

 tube probably contains fluid isotonic with the coelomic fluid that 

 normally surrounds it. The diameter of this tube becomes greater 

 almost immediately if the bathing fluid is diluted, suggesting that 

 ingress of water is very rapid, but whether it goes through the 

 nephridial walls or down the nephrostome at an increased clearance 

 rate is not obvious. The middle tube also swells when the external 

 fluid is diluted, but does so more slowly than the narrow tube. The 

 wide tube and the ampulla do not change in diameter. 



There is almost certainly an increased ciliary clearance rate in 

 animals with diluted coelomic fluid. Worms kept in 14% NaCl for 

 one week had nephridia that were small and shrunken and with the 

 ciliary tracts almost stationary. The bladders were empty. When 



