Inorganic Ions 



93 



is not known. Possibly the contractile vacuole in marine Protozoa has an ion- 

 regulating function. 



Evidence lor active excretion of selected ions is given by the ratios of urine 



TABLE 14. RATIOS OF CONCENTRATIONS OF IONS IN URINE TO 

 CONCENTRATIONS IN PLASMA (u/p ratios). 



to plasma concentrations of various elements (Table 14). These ratios are 

 subject to much variation with dietary changes, etc., but they indicate the 

 efficiency of the kidney in salt regulation. In marine crustaceans the concen- 

 trations of Mg++ and S04= are much higher in urine than in blood; the 

 lower concentration of K+ in urine indicates some fixation of this element by 

 tissues and also raises the possibility of extrarenal salt excretion. The kidneys 

 favor the retention of K+, Na + , and Cl~, and the elimination of Mg^ "*^ and 

 S04==. Nagel-'^ injected iodide into Carcinus and Hyas and found a con- 

 centration in the urine higher than in the blood; the amount of injected iodide 

 excreted was greater in Hyas than in Carcinus. In fresh-water arthropods the 

 urine is hypotonic but not salt-free. It is essential to fresh-water life to have 

 a kidney which either reabsorbs salt or secretes water. Bialaszewicz ^^ injected 

 large amounts of isotonic salt solutions into the crab Maja and measured the 

 amount excreted in urine over a day or two. He found the time required for 

 return to normal level was less for KCl (1.5 hr.) than for CaCl. (15 hr.), and 

 this was less than for MgClo (23 hr.) or i\a2S04 (more than 120 hr.). When 

 mixtures were injected, various elements were eliminated at different rates. 

 The amount of the different salts eliminated by the kidneys represented only 

 a small fraction (6 per cent for Mg) of what disappeared from the blood over 

 a period of 24 hours. Bialaszewicz concluded that salt is stored in tissues and 

 that the kidneys are not important in salt balance. This conclusion is hardly 

 justified from his nonphysiological experiment, and it becomes important to 

 search for possible extrarenal routes of salt excretion. 



