CHAPTER III 

 BLOOD AND ITS CIRCULATION 



Since the time when Harvey first watched the heart of a shrimp 

 there has been a continual advance in our knowledge of the blood 

 and its circulation in the Crustacea. The composition of the blood 

 and the mechanism controlling its propulsion vary. The study of 

 these variations has produced information on such diverse topics 

 as the largest known molecules and the similarities of water fleas 

 and men. 



The concentration of salt in the blood, and its ionic composition, 

 are important because animal tissues can only function properly 

 when bathed in a liquid which maintains a fairly steady balance 

 in its ionic composition. Excess of any one ion leads to changes in 

 permeability., and to changes in the physical state of protoplasm, 

 which, if severe enough, can completely disorganise it. The ionic 

 composition of the blood is more important than the total con- 

 centration of salts, but the concentration of the fluid bathing cells 

 cannot fall to a very low level without the danger of the cells 

 swelling to the point of bursting. 



Most Crustacea live in water, and if the concentration of the 

 blood is higher than that of the water (as it is in freshwater 

 Crustacea) then there will be a continuous tendency for water to 

 flow into the body, and for salts to be lost from the body. Many 

 freshwater Crustacea minimise these tendencies by having the outer 

 covering almost completely impermeable, and restricting such sur- 

 faces as are necessary for exchange between blood and water to a 

 limited area, such as the gills. Crustacea living in the sea do not 

 have the same problem; the concentration of salt in their blood is 

 similar to that of sea water, although some species (e.g. the prawn 

 Palaemon serratus) keep their blood slightly less concentrated than 

 sea water. The ionic composition of the blood often differs from 

 that of sea water, indicating that the animals are actively regulating 

 the proportions of the various ions, though it is not known exactly 

 how they do this. One ion which varies from species to species is 

 magnesium, and its concentration seems to vary inversely with the 



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