402 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 31 



By these epithelia some selective action is, no doubt, exercised over 

 the materials entering the organism. Their situation alone is such as 

 to prevent access to them of any but selected parts of the environ- 

 ment. But as a characteristic feature of organisms possessing this 

 mechanism is the freedom rather than the restriction of its exchanges 

 with the outside world, the main regulatory mechanism must be 

 sought elsewhere. 



All organisms which have the power of regulating the osmotic 

 pressure of their body fluids are provided with an excretory organ 

 corresponding to a kidney. This mechanism exercises its control over 

 the composition of the circulating fluid by eliminating from the 

 organisms those constituents passing through it which are present 

 in excess. It contributes to the independence of the organism of its 

 environment, or, in other words, to its adaptation thereto, by placing 

 another means at its disposal for keeping within suitable limits the 

 immediate environment of its cells. 



Even in those vetebrates in which the osmotic pressure of the body 

 fluids is close to that of the surrounding medium there is a con- 

 siderable degree of control exercised over the proportions of the 

 various materials present. While the proportions between the vari- 

 ous ions may be close to that of the sea water, their total concen- 

 tration may be only a fraction of this. The remainder of the osmotic 

 pressure in these cases is contributed by excretory products, prin- 

 cipally urea, which are allowed to reach relatively high concentra- 

 tions. This fact has been taken as evidence that the primary func- 

 tion of the kidney is not to excrete end-products of metabolism, but 

 to adjust the composition of the immediate environment of the cells. 



The possession of such a mechanism enables the organism to un- 

 dertake more active measures to adapt itself to its environment and 

 in some degree to adapt its environment to its needs. The environ- 

 ment of such an organism has already been modified before it gains 

 access to any but specialized portions of the living substance. While, 

 then, the cells of the organism are enabled to continue their existence 

 under more or less primitive conditions, the organism as a whole is 

 able to carry on its activities but little affected by the vicissitudes 

 of a changing environment. 



CONTROL OF ENVIRONMENT BY ORGANISM 



(A) FOOD SUPPLY 



In discussing the means taken by certain primitive organisms to 

 adapt themselves to their surroundings, reference was made to the 

 process of encystment and the performance of tropic movements. It 

 was pointed out that in protecting the organism against unfavorable 



