WATER, SALTS AND MINERALS 71 



placed in artificial sea water with one ion altered, there are corresponding 

 changes in the concentration of that ion in the blood. For example, 

 Carcinus in calcium-free sea water suffers a reduction of internal calcium 

 to one-fifth in 8 days. 



The gills are far more permeable than the general integument. Those of 

 Carcinus actively absorb sodium, potassium, chloride and probably 

 calcium against a concentration gradient. The gills of Eriocheir take up 

 the same ions from very dilute media. Measurements on isolated gills of 

 Eriocheir show that NaCl is absorbed at a rate of 2-5 mg g tissue hour. 



Mg 



4 + 



-► Excretory 



or^ai 



Ca+tCV 



Na* K* Water 



Ca**Cl~ 



Fig. 2.18. Diagram Illustrating the Salt and Water Exchange of Carcinus 

 Broken arrows represent movement of solutes and water brought about by diffusion 

 and ultrafiltration; continuous lines, movements (absorption and secretion) occasioned 

 by active regulation on the part of the animal. (After Webb (142).) 



Absorption of NaCl is halted by cholinesterase inhibitors, and it is likely 

 that cholinesterase is involved in the mechanism of ion-transport. The 

 accompanying diagram (Fig. 2.18) illustrates salt and water exchange in 

 Carcinus. 



Examination of crustacean urine reveals differential excretion. Sodium, 

 potassium and calcium are conserved, while magnesium and sulphate are 

 eliminated (Table 2.16). Together, the patterns of absorption and excretion 

 broadly account for the levels of individual ions in the haemolymph 

 (180,22,68,77, 102, 109,142). 



Mollusca. In lamellibranchs and gastropods potassium and calcium are 

 regulated, and in cephalopods regulation extends to all ions. Plasma pro- 



