72 THE BIOLOGY OF MARINE ANIMALS 



tein is very high in certain prosobranchs and in cephalopods, largely owing 

 to the occurrence of haemocyanin (Table 2.13). Mechanisms of ionic 

 regulation in marine forms are imperfectly understood. Examination of 

 fluid from the renal sac of cephalopods shows that resorption of potassium, 

 calcium and magnesium ions, and secretion of sulphate ion take place in 

 the formation of urine (Table 2.16). As a result, levels of the former ions 

 are raised, and levels of the latter lowered in the blood. The low level of 

 sodium in the renal fluid of Sepia, compared with plasma, is ionically 

 balanced by secretion of ammonia (146 m-equiv./kg water). As far as 

 absorption from the medium is concerned, it would appear that potassium, 

 magnesium, etc., are taken up against a concentration gradient, while 

 sodium and sulphate enter along a diffusion gradient. Gills and general 

 integument are probably involved. The body wall of Aplysia (tectibranch) 

 is freely permeable to the ions of sea water (106, 1 12, 113). 



Tunicates. The body fluids of tunicates are in osmotic equilibrium with 

 sea water and ionic regulation is feebly developed (Table 2.10). All ions 

 except sodium are kept at different values from those of sea water: potas- 

 sium is significantly raised in Salpa, and the divalent ions — calcium, mag- 

 nesium and sulphate — are reduced in both Thaliacea and Ascidacea. The 

 pattern of ionic regulation is not dissimilar from that of Amelia (cf. 

 Table 2.10). Excretory tubules being absent in tunicates, the whole burden 

 of ionic regulation is borne by the external surfaces (114). 



Fishes. Marine elasmobranchs tend to be almost isosmotic with sea 

 water owing to high internal concentrations of urea. From the fluids taken 

 into the alimentary canal, magnesium and sulphate ions are absorbed to 

 only a slight extent, compared with potassium, calcium and chloride ions. 

 Some magnesium and sulphate are lost in the urine and extrarenal excre- 

 tion of chloride is believed to take place. Blood protein levels are rather 

 high in elasmobranchs and teleosts (1-8%), and the proteins interact with 

 inorganic ions. 



The mechanism of ionic regulation is somewhat similar in marine 

 teleosts except that the blood is hypo-osmotic. Monovalent ions, sodium, 

 potassium and chloride, are absorbed from the sea water which is drunk, 

 while calcium and especially magnesium and sulphate are concentrated in 

 the intestinal fluid. Excess magnesium and sulphate are excreted in the 

 urine, while sodium, potassium and chloride are excreted extrarenally via 

 the gills (Table 2.16). Branchial excretion of chloride has been measured 

 by Keys in the heart-gill preparation of the eel Angui/Ia (77). 



MINOR ELEMENTS IN TISSUES AND SKELETONS 



In addition to the principal elements which we have just reviewed, we find 

 various minor elements in many species. These show a random distribution, 

 and their functional significance is not always known. We exclude here the 

 common constituents of the three main groups of organic compounds — 

 carbohydrates, fats and proteins. 



Apart from chlorine, the three halogens, bromine, iodine and fluorine, 



