Size as Structure and Function 11 



of contractile vacuoles, it is evident that in most pro- 

 tozoa tremendous volumes of water are taken into the 

 body and excreted again. The fastest turn-over be- 

 longs to Cryptochilum (Maupas, '83); while Parame- 

 cium caudatum puts out its own volume of water in 

 15 to 50 minutes. The slowest turn-over recorded is 

 for an Ameba which required 31.5 hours to put out its 

 own volume (Adolph, '26). 



Surface permeabilities. Intimately related to the 

 question of vacuole formation are the permeabilities of 

 the superficial boundaries of unicellular organisms. 

 The distributions of materials which they maintain 

 are to a small extent known, but practically nothing is 

 certain with respect to their means of regulating the 

 passage of substances. That the body's boundaries are 

 important in this regulation is illustrated by the older 

 observations of Brandt ('95) upon the specific gravity 

 of the large marine colonial radiolarian Thalassicola. 

 He observed that the specific gravity of the body or its 

 vacuoles was much less than that of the sea-water, 

 and so the salt content of its fluids must have differed 

 from that of sea-water. More extensive observations 

 of Harvey ('17) on Noctiluca 1 show that the specific 

 gravity may readily change in the body, and in a direc- 

 tion opposite to that which would be attained if the 

 salts became equally distributed between body and 

 medium. Changes of permeability, presumably at the 

 surface, have been demonstrated in a great many spe- 

 cies of unicellular organisms. 



It must be fairly evident that all sorts of properties 

 of the surfaces and of the internal structures of unicel- 

 lular organisms are directly concerned with the regula- 

 tion of the passage of those substances that constitute 

 the body. But at present almost no correlations among 

 these properties can be drawn. The passage of sub- 



1 Also of Ludwig ('28c). 



