450 PROTOPLASM 



this are certain crabs. The Httle red fiddler crab, Portunus 

 depurator, when put into dihited sea water rapidly changes the 

 concentration of its body fluids, presumably by transfer through 

 its gills, until, in a few hours, it has come into equilibrium with 

 its new surroundings. If the change is not too great — say, to 

 75 per cent sea water — it will live ; but in 50 per cent sea water, it 

 dies. The shore crab, Carcenus maenae, lives where it must be 

 ready to meet diluted sea water in estuaries or concentrated sea 

 water in pools, from one to the other, day after day. It can 

 tolerate 25 per cent sea water for a long time, slowly diluting its 

 body fluids until they reach a value isosmotic with about 60 per 

 cent sea water. The gills are presumably permeable to the salts 

 of sea water but can also resist their passage. They are able to 

 tolerate a considerable difference of concentration on their two 

 sides — a property essential to the crab's existence and probably 

 very important to some of our own cells, e.g., the kidneys and 

 alimentary canal. 



In higher animals, the absence of salts is made evident quickly 

 and strikingly. Miners' cramp is an example. Miners, in com- 

 mon with other workers in hot atmospheres, perspire profusely, 

 losing sometimes % lb. in weight per hour. If this loss is 

 replaced by drinking pure water in quantity, cramps result. 

 These may be prevented by the addition of a very small quantity 

 of salt to the water. The explanation from the point of view of 

 salt equilibrium is obvious. Perspiration contains much salt. 

 Replacing the loss by water alone disturbs the natural balance 

 between the blood and the tissues; the latter, being left in a too 

 acid condition, swell, just as does acidified gelatin, and cramps 

 result. The presence of a neutral salt in the water prevents 

 this. 



Salt concentration and salt balance have become important 

 subjects in medical practice. The salt content of the blood is of 

 diagnostic value. Proper bone formation depends upon a proper 

 calcium-phosphorus equilibrium. 



The value of salts or their metals is coming more and more to 

 the force in dietetics. We are told that whole wheat, unpolished 

 rice, baked potatoes, etc., are better food not only because 

 vitamins are in the husk and skin but because biologically 

 important metals are also there. Plants store small amounts of 

 copper, manganese, and zinc in the husks and embryos of their 



