32 



THE BIOLOGY OF MARINE ANIMALS 



salt concentration, osmotic equilibrium is attained by transfer of water, 

 and swelling or shrinkage of the body ensues. Probably in all poikilos- 

 motic species some diffusion of salts takes place in addition to water 

 transfer; when salt transfer is rapid the internal body fluids soon approach 

 the external medium in salt concentration, and there is little or no altera- 

 tion in body volume. 



Marine Protozoa are probably isotonic with sea water and the plasma 

 membrane is permeable to water. The outer covering usually possesses 



Fig. 2.1. Depression of the Freezing Point of Sea Water 

 as a Function of Salinity and Chlorinity 



little power of resisting hydrostatic pressure and even in skeletogenous 

 forms (Radiolaria and Foraminifera) there are openings through which 

 the protoplasm stands fully exposed to the external medium. Marine 

 protozoans, as a rule, are slightly heavier than sea water. The flagellate 

 Noctiluca miliaris, on the contrary, is demonstrably lighter: when taken 

 from sea water with a specific gravity of 1 -024 and tested in more dilute 

 media, Noctiluca will just float in sea water of specific gravity 1-014. The 

 animal appears to be in osmotic equilibrium with sea water. The cell sap 

 of Noctiluca displays several peculiar chemical features (low SO^, high 



