RENAL FUNCTION 261 



long periods of time if toxic materials are absent (cf. van Brink and Riet- 

 sema, 1949), so we may assume the worms were in good physiological 

 condition. 



TABLE 2. THE COMPOSITION OF THE BLOOD AND COELOMIC 



FLUID IN COMPARISON TO THAT OF THE URINE OF 



PHERETIMA POSTHUMA 



Let us assume that a blood filtrate enters the closed nephridia of Phcre- 

 tinia. Because of the immersion there might be an unusually large osmotic 

 inflow of water, with the result that the nephridia would handle an unusual 

 quantity of fluid, as much as half the volume of the body in 24 hours accord- 

 ing to Bahl's measurements. To conserve salts the electrolytes would be 

 pumped from the filtrate back through the nephridial cells. But, since the 

 nephridium is bathed in coelomic fluid, the electrolytes would arrive not 

 in the blood from which they originated but in the coelomic fluid ; and 

 only after an interval of time without intense water uptake would iso- 

 tonicity be re-established between blood and coelomic fluid. In Liimhricus 

 there are present only open nephridia into which coelomic fluid can drain ; 

 but the initial assumption that more blood filtrate enters the nephridia of 

 Pheretinia must be regarded with reserve, for Bahl believes there may be 

 filtration into the open nephridia of Lumbricus. He states (1947, p. 141 ) : 



In several earthworms there are characteristic vascular dilations or ampullae on 

 the nephridial capillaries, the meaning of which has not so far been clear. Benham 

 (1891) described them in Lujnbricus and I have described them in Lampito diibius 

 and again in Hoplochaetella khandalensis. It is probable that these dilations are 

 really tiny "blood filters" through which the blood plasma is filtered out into the 

 nephridia for excretion. No data are available on the blood pressure in the nephridial 

 "artery," but apparently the rhythmically contractile dorsal vessel and "hearts" supply 

 the necessary filtering force driving the blood plasma (minus the colloids) out of the 

 blood capillaries and dilations into the nephridia. 



