Water 



25 



are more permeable to iodide than the weakly homoiosmotic Cancer (Fig. 

 15). '-^^ Investigations of salt transfer will be greatly facilitated by the use 

 of radioactive tracers. 



The fact that Cancer swells less than Maja also indicates a lower permea- 

 bility to water. Huf ^^ ♦ fastened a tube to a hole in the carapace of Cancer 



as 



15 



A, 



2.5 



3.0 



Fig. 14. Blood concentrations (Ai) as a function of external concentrations (Ao) in 

 a series of crabs which regulate in both hypertonic and hypotonic media: O Heloecius 

 (Edmonds"*); 3Uca crenidata (Jones'"); X Eriocheir (Scholles"'"); A Leander (Panik- 

 kar""); • Palaemonetes (Panikkar'"'); and C an osmolabile crab, Cancer (Duval""). 



I B LOOP 

 I MEDIUM 



100 



PORTUNAS 



HYAS 



CANCER 



CARCINUS 



POTAMOBIUS 



ERIOCHEIRfoo)! 



20 



40 



80 



Fig. 15. Ratio of iodine in the blood to that in the medium after two and a half hours 

 in sea water containing Nal. From Nagel.'*' 



and measured the rise in water level in this tube when the crabs were put into 

 dilute sea water. In 50 per cent sea water he recorded a pressure rise of 1.16 

 cm.; in crabs with kidneys closed the pressure rise was 2.3 cm. The relation 

 between excretion and osmotic stress has not been examined in this group of 

 crabs; it would be of interest to know what regulating mechanism fails as the 

 medium becomes more dilute. 



