136 A BIOLOGY OF CRUSTACEA 



of salts in the blood is similar to that in the medium around the 

 crab. It is found that whatever the concentration of the medium 

 may be the blood of Maia become adjusted to the same concentra- 

 tion, which proves fatal if it is much different from sea water. A 

 crab such as Maia is termed a non-regulator; it cannot control the 

 concentration of its blood when subjected to osmotic stress. It 

 follows that such Crustacea must necessarily be restricted to living 

 in the sea. 



Estuarine Crustacea often exhibit considerable powers of regulat- 

 ing the concentration of salts in their blood. The common shore 

 crab, Carcinus maenas, can keep its blood at a concentration well 

 above that of the diluted sea water in which it often lives. Surpris- 

 ingly it cannot keep its blood concentration down if kept in external 

 concentrations higher than sea water, and further it cannot live in 

 really fresh water. There is another crab, Eriocheir sinensis, which 

 can do both these things; it is a better regulator. The concentration 

 of its blood can be kept below that of a medium stronger than 

 sea water, and the crab penetrates hundreds of miles inland up 

 rivers. However, the adult Eriocheir must return to the sea to breed, 

 and the breeding females are not capable of living in fresh water. 



How can one species of crab withstand large changes in the con- 

 centration of its medium, while another cannot? The complete 

 answer is not known, but there are various indications of the 

 mechanisms involved. The outer covering of the regulators is much 

 less permeable than that of the non-regulators, and various species 

 can be arranged in a series according to their permeability. Such a 

 series coincides remarkably well with the dilution of the external 

 medium that can be withstood by the various species. The follow- 

 ing results from Nagel (1934) illustrate this. The permeability of 

 the cuticle was estimated by measuring the concentration of 

 iodine in the blood after the crabs had been immersed for i x /n 

 hours in a solution containing sodium iodide. 



Table 3 



Concentration of iodine in the blood, as per cent of that in 

 external medium. 



Portunus 98 



Carcinus maenas 12 



Potamobias (fresh-water crayfish) 3 



Eriocheir sinensis o-o 



