WATIiR, SALTS AND MINERALS 29 



meadows and swamps, and many instances of such transitions may be 

 found on the margins of the oceans at the present time. But the sea has 

 also been reinvaded from estuaries and the land, as witness the existence 

 of marine pulmonates, insects and vertebrates. In these transitional reaches 

 between different environments the osmotic conditions present special 

 difficulties to organisms, and these have been solved in various ways. 



On entering aqueous media of different salt concentration, or emerging 

 into the atmosphere where there is risk of desiccation, organisms are 

 subjected to osmotic stress. Under the former conditions some animals 

 adjust themselves osmotically by passive alteration of the concentration 

 of their body fluids towards that of the external medium, and are said to 

 be poikilosmotic. Others possess powers of osmotic homoeostasis to greater 

 or lesser degree, and are able to regulate the osmotic level of their internal 

 fluids within certain limits independently of the environment, and these 

 organisms are termed homoiosmotic. Other terms in common use are 

 stenohaline referring to animals which can tolerate only limited changes in 

 salinity, and euryhaline referring to animals which can stand wide fluctua- 

 tions in salinity. Krogh (77), Prosser (107) and Robertson (114a) have 

 reviewed the field of osmotic relations in animals, and have presented 

 extensive bibliographies. 



Physico-chemical Relations 



In experiments dealing with osmoregulation the osmotic pressures of 

 the body fluids are usually expressed as depressions of the freezing point, 

 although chloride concentrations sometimes are given as a rough measure 

 of osmotic pressure. The salt concentrations of sea water are expressed 

 variously as salinities (% ), chlorinities (% ), grammes per litre, or as per- 

 centages (grammes per 100 c.c). Sea water of chlorinity 19% (salinity 

 34-325% ) has a freezing-point depression of — 1-872 C C. This is equivalent 

 osmotically to 0-56 molal NaCl (32T5 g/1). The depression of the freezing 

 point of sea-water samples can be calculated from the chlorinity by means 

 of the equation 



A = - 0-0966 CI - 0-0000052 CI 3 

 Values of A for sea water of different salinities are plotted in Fig. 2.1, 

 and the graphs in Fig. 1.5 show the variation in specific gravity of sea 

 water at five selected temperatures as a function of salinity and chlorinity. 

 Some workers, on occasion, have expressed salt concentrations in terms 

 of osmotically equivalent solutions of NaCl. For the convenience of the 

 reader concentrations of NaCl and sucrose are related to freezing-point 

 depressions in the graphs of Fig. 2.2 (for further details see Appendix). 



Osmotic Adjustment in Poikilosmotic Animals 



Most marine invertebrates are isosmotic with their environment and 

 some of the poikilosmotic forms can endure a certain amount of dilution 

 of the medium and still function effectively. When the outer coating is 

 permeable, water flows in or out of the animal according to the external 



