TECHNICAL NOTES AND BIBLIOGRAPHY 24 1 



tion and Add-Base Equilibrium. Little, Brown and Com- 

 pany, Boston, 2nd ed., 1959. 



Unicellular organisms and very small plants and ani- 

 mals that live in rivers and lakes containing only traces 

 of salts do so by virtue of the regulatory powers of the 

 exposed, bounding membranes, aided by subsidiary cel- 

 lular devices such as contractile vacuoles. These vacu- 

 oles are contractile' in the sense that they fill and empty 

 periodically as they accumulate water (and perhaps 

 waste products) in the form of a droplet which is then 

 bodily extruded from the cell. Such vacuoles serve to 

 maintain both the volume and the composition of the 

 cell, and they represent the most primitive type of ex- 

 cretory operation. Contractile vacuoles are present in 

 all fresh-water protozoa, and in some species the amount 

 of fluid thus baled out of the body in a few minutes may 

 equal the body weight. They are generally absent in 

 marine forms, but exposure of marine forms to fresh 

 water or diluted sea water may bring vacuoles into op- 

 eration de novo or, if pre-existing, speed up their excre- 

 tory activity. But the protovertebrate, no matter along 

 what lines it is reconstructed, was too big and complex 

 to rely on this device. 



At a higher level, many invertebrates of typically 

 marine nature have invaded brackish or fresh water: a 

 few coelenterates and, in order of increasing numbers of 

 species, mites, clams, crabs, crayfish, annelid worms, and 

 snails; all of them have faced the plethora-of -water prob- 

 lem, and have solved it with varying degrees of success 

 in a variety of ways: by actively absorbing salts (notably 

 sodium chloride) through the respiratory membranes, 

 or by incorporating contractile vacuoles for the excretion 

 of water into a compacted excretory organ. But where 

 osmoregulation has attained a fairly advanced degree of 

 control, as in the fresh-water crayfish, the entire body, 

 except for the oral and respiratory membranes, is cov- 

 ered with waterproof insulation preadaptively evolved 



