CHAPTER 2 



WATER, SALTS AND MINERALS 



The stability of the milieu interiew is the primary condition for freedom 

 and independence of existence; the mechanism which allows of this is 

 that which insures in the milieu interiew the maintenance of all the 

 conditions necessary to the life of the (tissue) elements. 



Claude Bernard 



The total amount of water contained in the body varies greatly from 

 species to species, but for each kind of animal there is usually a charac- 

 teristic value, or range of values for different functional states. The water 

 content is high in many transparent pelagic forms, amounting to about 

 96 % in the common jellyfish Aurelia aurita. Generally the water content 

 of animals lies between 70 and 85% (Table 2.1). In any given animal 

 there are differences in the amounts of water contained in the various 

 tissues. Nevertheless, the osmotic concentrations throughout an organism 

 are approximately equal and animals with greatly different water contents 

 may have similar osmotic concentrations. This is explained by the fact 

 that only a proportion of the substances composing an organism are 

 osmotically active; bone, connective tissue and chitin, for example, may 

 be considered as osmotically inert (14). 



The osmotically active substances found in animals comprise, broadly, 

 electrolytes, organic non-electrolytes and colloids (proteins). Within the 

 organism the cells are in osmotic equilibrium with their surroundings — 

 blood, lymph and tissue fluids. The concentrations of individual ions 

 within the cells, however, are markedly different from the concentrations 

 existing in the circumambient tissue fluids, and the presence of selectively 

 permeable cellular membranes bars the passage of certain ions, or retains 

 diffusible organic molecules at relatively high levels (48). 



OSMOTIC ADJUSTMENT AND REGULATION 



In the normal environment characteristic of the species, animals tend to 

 maintain their water content at a constant level referable to some basic 

 relationship such as water/protein ratio. In the maintenance of a steady 

 and optimal water content, osmotic adjustments and regulations are 

 fundamentally concerned. Marine animals, vertebrates excepted, generally 

 are in osmotic equilibrium with their environment. This is true of most 

 members of predominantly marine groups (e.g. Scyphomedusae, Poly- 

 chaeta, Mollusca, Crustacea, Echinodermata, Ascidiacea). Marine 

 animals have repeatedly invaded fresh waters and the land by various 

 routes, through estuaries, up the shore or across brackish reaches of 



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