WATER RELATIONS 



79 



urine is carried out along the entire length of the wide tube. It is 

 also possibly a function of the middle tube since there is a certain 

 amount of variability of the results. The bladder seems to be solely a 

 collecting and storage organ for the formed urine; no changes in 



Nephridiostome 



First loop 



Third loop 



1^^ 



Narrow tube, ciliated portion 

 Narrow tube, non-cilioted portion 

 Middle tube 



Wide tube, proximal segment 

 Wide tube, middle segment 

 Wide tube, distal segment 



Muscular duct or bladder 



Fig. 23b. Diagram of nephridium of Liimhricus (from Ramsay, 

 1949b) showing regions sampled with micropipettes. 



the concentration of the contents being found in relation to the 

 wide tube. 



There is no knowledge available at present to say whether the 

 changes in urine concentration noted in the wide tube are due to an 

 influx of water into the tube, to removal of salts from the lumen by 

 active resorption into the blood system, or whether it is a combina- 

 tion of both of these phenomena. There is some evidence given by 



