Water 29 



and of all of the ammonia appears to occur by way of the gills. When a 

 specimen has had its chloride content reduced by prolonged immersion in 

 frequently changed distilled water and is then put back into pond water or 

 dilute salt solutions it absorbs salt very rapidly against a concentration gradient. 

 The crab fails to reduce the concentration of the medium below 0.2-0.4 

 milhmolar. No difference in respiration in salt and in fresh water was 

 detected. --- 



In Eriocheir, then, the kidneys have no osmoregulatory function but do 

 function in ion regulation. The crab is able to live in fresh water because of 

 its low salt and water permeability and the ability of its gills to absorb chloride. 

 Possible extrarenal routes of v^ater excretion have not been investigated. 



Fresh- Water Crustaceans. The Italian crab, Telphiisa fliiviatiUs, adjusts to 

 high concentrations of its medium but regulates well at lower concentrations, 

 maintaining a Ai of 1.17 in fresh water. «- The urine of Telphiisa is isotonic 

 with blood. '-^^ Changes in blood concentration occur more slowly than in 

 marine animals. 



In fresh-water crayfish the blood freezes at about -0.8°C., which corre- 

 sponds to a salinity of 1.4 per cent. There may be a normal fluctuation of 

 10 per cent in blood concentration during a period of a week or two. '^'* 

 When crayfish are put into dilute sea water (50 per cent or less) their internal 

 concentration rises and they continue to be hypertonic, but in higher concen- 

 trations of sea water the blood concentration is isotonic with or slightly lower 

 than that of the medium. Crayfish lived for a month in 66 per cent sea water 

 and for three months in 50 per cent sea water. ■*-• '*'- 



In isotonic (1.5 per cent) saline (diluted sea water) there was no weight 

 change in crayfish, either normal or with antennal glands plugged, but in more 

 dilute media there was a considerable increase in weight in those with the 

 kidneys plugged; in more concentrated (hypertonic) media there was a loss 

 of weight by both groups.^^- Apparently, then, normal crayfish can regulate 

 their volume in dilute or isotonic media but not in hypertonic concentrations. 



The permeability of crayfish to both water and salts is low but not negligible. 

 The volume increase in fresh water of crayfish with antennal glands plugged 

 may be 5.5 per cent of body weight in 48 hours. '°- Permeability to iodine is 

 low (Fig. 15). Most of the salt and water exchange occurs through the gills 

 rather than through the body wall. Since weight remains constant the small 

 increase in concentration in dilute sea water must be due to uptake of salts. 



Crayfish urine is much more dilute than urine of marine Crustacea or of 

 Eriocheir. The total urine output of crayfish in per cent of the body weight 

 per 24 hours is 3.8 per cent, ^^- 5.2 per cent, ^-^^ or 4.0 per cent, ^-'o an 

 excretion volume not much higher than that in many marine crabs. The cray- 

 fish urine, however, is very dilute. Mean freezing points of blood and urine 

 in cravfish from fresh water (Ao=0.0l8°) are as follows: 



Caniharus clarkii'^''^ 

 Astactis fluviatilis'^ 

 Astacus fluviatili^'' 



When salt is added to the medium the urine concentration increases; in 

 isotonic and slightly hypertonic solutions the urine output declines to zero. 



