OSMOTIC PRESSURE IN ANIMALS AND PLANTS 565 



Osmotic Pressure in Primitive Organisms 



Since the most primitive forms of life developed in water 

 and were devoid of a firm continuous limiting membrane, it is 

 obvious that they must have possessed a very small osmotic 

 pressure. For either they developed in fresh water — in which 

 case their pressure could not have greatly exceeded 0*25 atmo- 

 sphere, that of moderately hard water freezing at —0*02° C. — 

 or they developed in the sea-water as it was then constituted. 

 The ocean has, however, been growing continually richer in salts 

 owing to denudation of igneous and plutonic rocks. Indeed, 

 the rate of increase of its sodium content has been employed 

 by Joly to estimate the age of the earth reckoned from the time 

 at which water condensed. And although for various reasons 

 all the constituent solutes have not increased at the same rate, 

 and some may even perhaps have decreased, yet the movement 

 of the osmotic pressure has been steadily upwards. The same 

 is true of all water-dwelling animal organisms, so far as the 

 author is aware ; their development always involves a rise in 

 osmotic pressure, unless they have a large diminution in the 

 pressure of the external medium superimposed upon their normal 

 condition. Accordingly the naked ova, plant or animal, which 

 develop in fresh water must increase in osmotic pressure from 

 that of their primitive ancestors, namelj'- very approximately 

 the pressure of fresh water, up to that of the mature organism. 



Osmotic Pressure in Animals 



(a) The Lower Water-Dwelling Phyla. — In all these groups 

 there is contact either complete or partial between the proto- 

 plasm and the external medium. Even though a coslom has 

 been developed in the higher groups, contact is still maintained 

 through the external surfaces, and the nephric system serves to 

 prevent the too great accumulation of solutes. It has been 

 shown by the extended researches of Fredericq, Bottazzi, 

 Quinton and others that all these organisms possess an osmotic 

 pressure equal to, or very slightly higher than, that of the water 

 in which they live. Thus the same species will have a higher 

 pressure when in the Bay of Naples than when in the less saline 

 waters of the Atlantic. The slight excess of pressure inside the 

 organism serves to maintain its turgidity. Since abrupt 

 changes in salinity are often fatal to such organisms, the dis- 



