10 



Comparative Animal Physiology 



OSMOTIC ADJUSTMENT TO THE MEDIUM; 

 POIKILOSMOTIC ANIMALS 



The statement is sometimes made in textbooks that all marine invertebrates 

 are isotonic with the ocean; this was found true for a great variety of 

 species.^*^' ^''' '^^' Some crustaceans and a few annelids show deviation in osmo- 

 concentrations, but most marine invertebrates are isotonic and adapt their 



VOL 

 100 



4 000 



3500 



2000 - 



OBSERVED 



60 



■To 



T^ 



CONCENTRATION OF SEA WATER 

 IN PER CENT 



Fig. 3. Volume changes in cubic micra (XlO") in fertilized Arhacia eggs as a function 

 of dilution of sea water as per cent normal sea water. • , Lower curve, observed volumes; 

 Oi upper curve, volumes calculated on basis of gas laws. From Lucke and McCutcheon.''^ 



internal concentration to external changes. The limitation on the distribution 

 of all osmolabile animals is probably the dilution at which their enzyme 

 systems can still carry out sufficient metabolism for active life. The simplest 

 and most economical osmotic adjustment is found in marine and parasitic 

 organisms, which are highly permeable to water and gain or lose water accord- 

 ing to the concentration of the medium. No osmotic work is done, and body 

 volume is not regulated. 



Osmotic Change without Volume Regulation : Unicellular Animals. Much 

 evidence indicates that marine Protozoa are isotonic with sea water. The 

 flagellate, Noctiluca, is in osmotic equilibrium with sea water but may have a 

 lower specific gravity owing to the presence of considerable amounts of a salt 

 of lower specific gravity than sodium chloride (Goethard and Hensius, quoted 

 by Krogh ^■*'0- This salt is probably ammonium chloride, which is ionic at 

 the remarkably low pH of 3.0 in the cytoplasm. "- When Noctihica is put 

 into diluted sea water it swells and finally bursts at concentrations correspond- 

 ing to a specific gravity between 1.007 and 1.012. ^*^ Thus the only adapta- 



