168 



ANALYSIS OF THE ENVIRONMENT 



sure that results from this osmotic process 

 is called osmotic pressvire; it may be defined 

 as the difference in pressure on solution and 

 solvent that establishes an equihbrium such 

 that there is no longer a tendency for the 

 solvent to flow in either direction. Organ- 

 isms with a higher concentration of solutes 

 within the body than is found outside tend 

 strongly to take in water and exhibit turgor. 

 Some of the physiological adaptations to 

 counteract such processes will be outhned 

 on page 169. 



The outer covering of Ascaris mega- 

 locephala, a nematode intestinal parasite, is 

 practically impermeable to digestive fluids 

 of the host and even to 10 per cent forma- 

 lin. The vitelline membrane of trout eggs 

 becomes impermeable to water when placed 

 in contact with fresh water. Such egg mem- 

 branes are peiTaeable to oxygen and to 

 carbon dioxide, and respiratory exchanges 

 can take place between the developing egg 

 and the surrounding fresh water. 



Membrane permeabiUty for water is 

 measured by a "minute number" which is 

 the time taken for 1 cc. of water to pass 

 through 1 square cm. of membrane under 

 a pressure of 1 atmosphere. It approximates 

 one minute for certain filters, but for many 

 animal membranes the "minute number'" 

 varies from a few thousand minutes (a few 

 days) to several years. In general, mem- 

 branes with relatively low permeability for 

 water are only shghtly permeable for some 

 associated ions. For those substances for 

 which the membrane is readily permeable, 

 osmosis follows the rule apphcable to sim- 

 ple diffusion. The passage of electrically 

 charged particles through a membrane is 

 more compficated, and more specialized 

 accounts should be consulted for details; see 

 especially Krogh (1939), who gives a use- 

 ful guide to the compHcated hterature of 

 this whole phase of physiological ecology. 



Living organisms are not simple osmotic 

 machines. They can maintain differences in 

 salt concentration across a membrane de- 

 spite its semipermeabihty. Animals and 

 plants in fresh water, even when water- 

 permeable, keep a much higher total con- 

 centration within the body than exists in the 

 surrounding water. To do so requires a con- 

 tinuous expenditure of energy. With such 

 organisms, there is no true osmotic equi- 

 librium between the internal fluids and 

 environment. Rather, they are said to main- 



tain a "steady state," and steady states may 

 apply either to the distribution of water or 

 of ions, or both. 



Animals may be either poikilosmotic or 

 homoiosmotic. A poikilosmotic animal is in 

 osmotic equihbrium with its environment; 

 the equihbrium shifts through rather wide 

 degrees, depending on the dilution or con- 

 centration of the environment. Marine 

 invertebrates are frequently poikilosmotic. 

 A homoiosmotic animal steadily maintains 

 a total internal concentration of body fluids 

 unhke that of the environment. Fresh water 

 animals and most marine fishes belong in 

 this category. This division on the basis of 

 osmotic characteristics suggests the better- 

 known division into poikilothermy and 

 homoiothermy. All homoiothermal animals 

 are also homoiosmotic, but poikilothermal 

 animals may be either poikilosmotic or 

 homoiosmotic. 



The body fluids of many poikilosmotic 

 invertebrates of the sea have an ionic com- 

 position closely approximating that in sea 

 water. In this they are unhke the poikilos- 

 motic marine vertebrates whose body fluids 

 may differ decidedly in ionic constitution 

 from their environment. Homoiosmotic ani- 

 mals, especially those of the fresh water, 

 have ionic concentrations that differ widely 

 from those found in the surrounding 

 medium. 



The poikilosmotic character of marine in- 

 vertebrates is shown by the almost isotonic 

 relation between their body fluids and the 

 sea water in which they hve. The inverte- 

 brates of the Mediterranean, in keeping 

 with its higher sahnity, have a higher con- 

 centration of salts than those of the North 

 Sea or the open Atlantic. These inverte- 

 brates do not require osmotically protective 

 structures or processes in skin, gills or gut 

 or other specialized devices for maintaining 

 osmotic balance or keeping the internal os- 

 motic pressure within the bounds of tolera- 

 tion. 



The invertebrates in fresh water maintain 

 much higher salt concentrations in body 

 fluids than those in the water around them. 

 Some large groups are excluded from the 

 fresh waters because they have not evolved 

 effective mechanisms to control or counter- 

 act osmotic exchanges of water and ions, 

 among others, these include the entire phy- 

 lum of Echinodermata. On the other hand, 

 successful groups in fresh water, notably 



