CHEMISTRY OF THE SEA 165 



biologically not unimportant problem as to whether the sea was 

 formerly less saline than at present. The assumption that the sea salt 

 is derived from the land is not necessarily true. Seas without outlet 

 whose salinity is exclusively derived from the river water do not have 

 the composition of the sea water, and, for example, may contain Epsom 

 salts or borax. The rivers, furthermore, contain only very small 

 amounts of chlorides, which predominate in the sea water. Salt masses 

 from the sea bottom itself may have conditioned the composition of 

 the sea water. 



The salts of the sea are important in the life of its animals. Indi- 

 rectly, animals profit by the substances taken up directly by the 

 plants; in addition to potassium, sodium, calcium, magnesium, sulphur, 

 and chlorine, which are present in excess, plants use among others 

 phosphoric acid, nitrates, nitrites, and ammonia. Animals are able to 

 draw directly upon a number of the inorganic materials of the sea 

 water to build up their bodies, especially sodium, magnesium, and 

 calcium, and also silicic acid. Occasional animals may use rarer 

 elements, as, for example, the radiolarians of the suborder Acantharia, 

 which use strontium sulphate; horn corals store bromine and iodine 

 in their skeletons; and ascidians contain vanadium as a coloring mate- 

 rial in their blood. The differential withdrawal of NaCl is fatal to 

 most marine animals, even when it is accomplished gradually; but 

 this is a condition that does not concern such animals in nature. 



The osmotic properties of sea water are of high importance to 

 marine life. Osmotic pressure rises with increasing concentration and 

 falls with decreasing concentration. It amounts to no less than 26.7 

 atmospheres in the water of the Red Sea, of 40%c salinity, at 30°. 

 In the waters of the Baltic at Bornholm, of 7.5%o salinity, at 18°, it is 

 only 4.9 atmospheres. Marine animals, with the exception of the bony 

 fishes, are adapted to the medium in which they live in having their 

 body fluids isotonic with sea water, so that these are not altered by 

 diffusion. This is probably the reason why some marine animals are 

 very sensitive to variations in the salinity of the surrounding water. 

 Others, on the contrary, are able to live in waters of widely different 

 salinities, and some can even withstand relatively rapid change from 

 weak to strong concentration, and vice versa. The former animals are 

 characterized as stenohaline, the latter as euryhaline. There are all 

 possible degrees of the sensibility to variation of salt content. 



Stenohaline animals are subject to certain restrictions in their dis- 

 tribution. They are found primarily in the open ocean, where they are 

 little exposed to this variation. Even the surface forms, for which the 

 rains may dilute the sea water, can escape this variation at the depth 



