THE CIRCULATION OF BODY FLUIDS 105 



beat in sea water. Adjustments of osmotic pressure of body 

 fluids to that of the medium probably takes place through 

 the gills. There is no known advantage in this arrangement ; 

 on the contrary, the extreme sensitivity of colloidal systems to 

 electrolytes implies the advantage of a fixed osmotic pressure 

 for the delicate adjustments of colloidal equilibrium which 

 underlie the physical processes of life in any medium. In 

 this connexion reference may be made to the fact that, 

 while there is a paucity of reliable analyses of the electrolyte 

 content of the blood in invertebrates, such figures as are 

 available indicate a rather higher percentage of magnesium 

 than is present in the blood of the higher vertebrates. It will 

 be remembered that sea water is richer in magnesium than in 

 any other kation with the exception of sodium. 



A few words may now be said about coagulation, which also 

 has some bionomic interest ; for while the greatest care should 

 be exercised in interpreting biological phenomena as protective 

 mechanisms, it is difficult to deny any utilitarian significance 

 to the fact that the phenomenon of blood coagulation is 

 exemplified in no more striking manner throughout the animal 

 kingdom than in arthropods, whose segmental structure 

 renders the loss of limbs a common occurrence. The blood 

 of molluscs has little coagulative power. That of Crustacea 

 clots with remarkable rapidity ; and the process is often a com- 

 plex one, taking place in some species in two stages. The first 

 stage in the clotting of crustacean blood corresponds more or 

 less to the coagulation process in that of the Arachnid, Limulus, 

 whose blood also clots rapidly. In Limulus the protein con- 

 tent of the blood is almost exclusively made up by the haemo- 

 cyanin and white blood corpuscles. Coagulation is essentially 

 a phenomenon of cytolysis (Alsberg and Clark), and can be 

 prevented by reagents which hinder cell agglutination. For 

 the first stage of coagulation in crustacean blood, which is 

 brought about by the cytolysis of special '' explosive " cells, 

 first recognised by Hardy (1892), immediately they come into 

 contact with foreign substance, it has been shown by L. Loeb 

 that calcium ions are not necessary, though they are necessary 

 for the second, which takes place in lobster blood about a 



