WATER, SALTS AND MINERALS 59 



problem, namely threatened desiccation. An example is Cancricola, a 

 harpacticid copepod found on the gills of littoral and land crabs. These 

 parasites feed on blood and mucus, and benefit from the fact that the host 

 must keep its gill surfaces moist. 



External parasites which feed on the blood of marine vertebrates take 

 in a fluid of much lower osmotic pressure than the surrounding medium, 

 and tend to become flooded until the surplus water is eliminated. A peculiar 

 degree of osmolability is displayed by the parasitic copepod Lemaeocera 

 branchialis from the cod. Lemaeocera is normally isotonic with sea water 

 but becomes hypotonic when living upon a fish and feeding upon the latter' s 

 blood (99). 



MINERAL SALTS AND IONIC REGULATION 



Osmotic relations are a function of the total concentrations of solutes in 

 the media. Although osmotic pressures of external and internal media are 

 usually equal in marine invertebrates, the concentrations of individual 

 ions are frequently dissimilar. In this regard it is necessary to distinguish 

 between intracellular and extracellular fluids. All cells appear to differ in 

 inorganic composition from the surrounding media, whether tissue fluids 

 or sea water (Table 2.8). The cell sap of the large unicellular alga Valonia 

 differs greatly in ionic composition from surrounding sea water. Eggs of 

 the sea urchin Paracentrotus show high levels of potassium and relatively 

 low concentrations of sodium, chloride and sulphate ions. Muscles gener- 

 ally contain large amounts of potassium and low levels of sodium and 

 chloride ions. Calcium and magnesium ions are present in greater (Ap/ysia) 

 or lesser amounts (Pecten) than in sea water. Blood corpuscles of Caudina 

 concentrate ionic potassium and sulphate, but reduce sodium, magnesium 

 and chloride. Axoplasm contains particularly large amounts of potassium 

 and low levels of sodium and chloride ions, in conjunction with special 

 excitability characteristics of nerve fibres (Chapter 10). Cells both within 

 the body and freely exposed to sea water can regulate individual ions at 

 concentration-levels differing from sea water or tissue fluids. There is a 

 general tendency for cells to concentrate K and, to a lesser extent Ca + + , 

 and to reduce Na + and Cl~. Muscle fibres of Carcinus regulate Na + and 

 K + , Na + being secreted, and K^ retained by the fibre. Much variation 

 exists in the way in which Ca ++ , Mg +4 and S0 4 = are treated (123tf, 139). 

 In lower forms the intracellular osmotic concentration is largely due to 

 inorganic electrolytes, but in certain higher marine invertebrates a tendency 

 exists for a large proportion of these ions to be replaced, particularly in 

 muscle cells, by organic substances. Considerable quantities of amino- 

 acids occur in molluscan and crustacean muscle, where they are believed 

 to be important in regulation of intracellular osmotic pressure (20, 34). 



Ionic Concentrations in Body Fluids 



Turning now to internal fluids (plasma, haemolymph, coelomic fluid) 

 we find certain disparities in the ionic concentrations of different species. 



