DISTRIBUTION 137 



The table shows that the non-regulator Portunus had allowed the 

 concentration of iodine in the blood to become almost equal with 

 that in the medium, while in Eriocheir the amount which pene- 

 trated was too small to be measured. 



The crabs which regulate their internal concentration are known 

 to be able to take up salts from media which are more dilute than 

 the blood. This seems to be done mainly through through the 

 gills. If the gills of Eriocheir are removed from the body they will 

 still actively take up salts from dilute media. The active nature of 

 this uptake of salts by the gills has been demonstrated by the fact 

 that it stops in the absence of oxygen, and is inhibited by various 

 substances which interfere with the chemical processes involved in 

 respiration. 



Another method of controlling the concentration of the blood 

 is to regulate the flow of urine from the excretory organ. The 

 antennal gland of Carcinus produces more urine when the crab 

 is in half-strength sea water than when it is in undiluted sea water. 

 This crab copes with the tendency for the blood to become diluted 

 by increasing its output of urine to get rid of excess water, and, like 

 Eriocheir, it takes in salt through its gills. Surprisingly it has been 

 found that Eriocheir produces the same amount of urine in both 

 sea water and fresh water. The low permeability of Eriocheir, com- 

 bined with its ability to take in salts from dilute solutions, seems 

 to have made unnecessary any increase in urine production in fresh 

 water. 



Table 4 

 Rate of urine flow in certain decapod Crustacea. 



