Water 31 



Gammarns pzdex is much longer than in the marine G. locusta, but there is 

 httle or no difFerence in kidney size in fresh-water and salt-water varieties of 

 Palaemonetes varians. --- Thus the entire crustacean kidney is, in principle, 

 like one unit of the vertebrate kidney. By means of a capillary pipette Peters 

 removed samples of fluid from each of the regions of the crayfish kidney, and 

 then measured their chloride content. His results in mM/1- (recalculated by 

 Krogh) are as follows: 



Main End of Neph. 



Blood Coel. Sac Lahyrinth Lahyrinth Canal Bladder 



CI- mM/1. 196 ±3 198 ± 2 209 ±: 7 212 ±: 7 90 ± 6 10 ±0.6 



The fluid from the coelomic sac and labyrinth may be a blood filtrate, essen- 

 tially isotonic with the blood, and in the nephridial canal chloride may be 

 reabsorbed, leaving a dilute urine to enter the bladder. 



Evidence for filtration was given by Picken, ^"" who measured a hydro- 

 static pressure of 20 cm. H^O in the first leg of a crayfish. The colloid osmotic 

 pressure of the blood was found to be 15 cm. HoO and that of the urine, 2 

 cm. HoO. Hence filtration might occur from blood vessels to tubules. 



The histological picture is not compatible with filtration, however.'"'- ^"•''' 

 The coelomic sac and tubules lie in hemocoelic spaces, so that the pressure 

 must be similar on all sides, and it is difficult to see how unidirectional pressure 

 could exist as it does in a vertebrate kidney glomerulus. The epithelium of the 

 coelomosac may show vacuoles; the nephridial tubule cells of a crayfish from 

 fresh Vv'ater contain many vacuoles which can be seen being extruded into the 

 lumen. Crayfish kept in saline for several days lose these vacuoles and regain 

 them when returned to fresh water.^"'- ^"■"' The labyrinth cells can accumu- 

 late and secrete dyes such as cyanol, phenol-red, and indigo carmine. This 

 evidence argues against filtration and reabsorption of salt but demands secre- 

 tion of water. 



In no condition has a crayfish been observed to excrete totally salt-free urine. 

 A crayfish weighing 50 gm. loses 600 mM of CI daily. '^"^ Some salt is normal- 

 ly obtained from food, but crayfish can be kept for weeks without food. The 

 chloride content of the urine of starved crayfish is less than that of fed 

 ones. "^ In 8 days in distilled water the chloride content of the blood fell 

 from 6.8 mg./gm. to 4.5 mg./gm., i.e., a loss of about one third, and the crayfish 

 eventually died. ^^"^ When crayfish were removed from distilled water to fresh 

 water before they died, recovery was complete. Krogh '^'^ washed out crayfish 

 bv leaving them' in distilled water for 3 days and found that when they were 

 then put into 0.02 Ringer solution they took up chloride at the rate of 2.3 

 /xM/hr. over 3 hours, even though the external concentration was 2 mM and 

 the internal concentration was 100 mM. From pure sodium chloride solution 

 of the same concentration the uptake was initially greater (6.0 ^M/hr.); the 

 rate of uptake later slowed, but at all times the active absorption was at a lower 

 rate than in Eriocheir. Astaais took up bromide at about the same rate as it 

 did chloride. The gills take up silver salts, and washed-out crayfish take up 

 CI- from KCl, Na+ and CI" from NaCl, and Na+ from Na.SOj. 



By its low permeabilitv and by two energy-requiring processes-salt reab- 

 sorption or water secretion in the kidney, and active salt absorption by the gills 

 -the crayfish is able to maintain a high internal concentration while living 

 in fresh water. 



