WATER, SALTS AND MINERALS 65 



groups, embracing coelenterates, echinoderms, polychaetes, lamelli- 

 branchs, gastropods and ascidians, regulation is slight. There is some regu- 

 lation of potassium and, to a lesser extent, of calcium ions; ionic sulphate 

 is reduced slightly, except in Aurelia and Phallusia, where it is halved. 

 Sodium, magnesium and chloride ions are in equilibrium with sea water. 

 In contrast to these forms, the two active invertebrate groups, cephalopods 

 and decapod crustaceans, show well-marked regulation of all ions in the 

 haemolymph. Cephalopod bloods show a range (expressed as percentage of 

 concentration in dialysed plasma): Na L , 93-8; K + , 152-219; Ca ++ , 

 91-107; Mg + ^, 98-103; CI", 101-5; S0 4 =, 22-81. Potassium is held at 

 high levels, magnesium is accumulated slightly and sulphate is greatly 

 reduced. The range in decapod crustaceans is: Na + , 94-113; K + , 77-156; 

 Ca ++ , 84-137; Mg ++ , 14-99; CI", 87-104; S0 4 =, 32-135. Potassium is 

 usually accumulated (anomurans and brachyurans) and calcium is main- 

 tained at higher levels than sea water in most species examined. The most 

 striking feature is the great reduction in magnesium, ranging from equi- 

 librium with sea water in Maia and Lithodes, to 14% of sea-water values 

 in Homarus. Sulphate is likewise reduced. A peculiarity of decapod blood 

 is seen in the accumulation of sodium, in correlation with a reduction of 

 magnesium. Hermit crabs {Eupagnrus) are unusual among decapods in 

 accumulating sulphate, and in correlation with this fact have reduced 

 levels of chloride to preserve the balance of anions. The isopod Ligia 

 oceanica likewise shows well-marked ionic regulation (Table 2.9). Sodium, 

 potassium, calcium and chloride are all more concentrated than in sea 

 water, whereas magnesium and sulphate are much reduced. 



Data relating to ionic regulation in lower vertebrates are presented in 

 Table 2.10. The serum of Myxine is isosmotic with sea water (449 mM 

 when external Cl~ is 483 mM). Salt levels of lampreys and all higher forms 

 are only about one-half sea-water values (280 mM in selachians and 200 mM 

 or less in teleosts). In lower vertebrates sodium, and often potassium, are 

 accumulated in the plasma relative to chloride, to a small extent in Myxine, 

 and at relatively high levels in marine selachians and fishes. Magnesium 

 is greatly reduced relative to chloride in correlation with the increase of 

 sodium and sulphate is held at very low levels. Ionic regulation is more 

 highly developed in vertebrates than in the most advanced invertebrate 

 phyla (114). 



Mechanisms of Ionic Regulation. There are two aspects of ionic regula- 

 tion which have excited much interest, namely the historical basis for the 

 ionic similarity of body fluids, and the mechanisms by which particular 

 concentrations are maintained. The similarities in ionic composition of 

 the body fluids of animals are more striking than the differences. Thus, all 

 fluids show relatively high levels of Na + and CI", and low levels of K + , 

 Mg ++ and S0 4 = . Moreover, the relative concentrations are roughly of the 

 same order as those found in sea water. This general similarity between 

 body fluids and sea water has been regarded as a relic of the probable 

 marine origin of animals. Macallum (85) has pointed out that the fluids of 



M.A. — 3 



