PROTOPLASM OF PROTOZOA 77 



presence of a viscid secretion, or by active suction of the hollow tentacles. 

 Hov^ever, the character of the outer surface of the prey is an important 

 factor in its capture (Root, 1914). 



Specific Gravity or Density 



Whole organisms. — Since their specific gravity is slightly greater than 

 that of water, most unicellular organisms can remain suspended in fresh 

 or salt water only by the use of special locomotor organs such as cilia 

 or flagella. Among the advantages of this slightly greater specific gravity 

 is that the organisms are not caught in the surface film, nor in the con- 

 gealing water on the surface when their locomotor activities are depressed 

 because of low temperature; then, too, the simple methods of locomotion 

 of Protozoa would hardly suffice to move bodies of very great density or 

 to keep them suspended against the pull of gravity. In this connection the 

 work of Jensen (1893) should be mentioned. In spite of the inaccuracy 

 of his absolute measurements, it is clear that a 'Paramecium can lift 9 

 times its own weight in water. Probably a Paramecium with a specific 

 gravity above 1.35 could not keep itself in suspension because a very in- 

 significant amount of energy 1 1 of the total is available for 



* 100 ~ 1000 



locomotion in this form (Ludwig, 1928a, 1930). 



Certain Protozoa of floating habit frequently have hydrostatic devices 

 which aid in flotation, such as the gas bubbles secreted in the protoplasm 

 of Arcella and Difflugia; special layers of vacuolated protoplasm, such 

 as the calymma of Radiolaria; or very highly vacuolated protoplasm, as 

 in Noctiluca and in Heliozoa. 



According to Bles (1929) the gas bubbles of Arcella are formed in 

 the marginal protoplasm and are filled with oxygen. They are secreted 

 when the oxygen tension is reduced experimentally and are adaptive, in 

 that they reduce the specific gravity of the organism so that when oxy- 

 gen tension is low the organism may float to the surface, where the oxy- 

 gen tension is always somewhat higher. When Arcella is turned upside 

 down so that the external pseudopods cannot adhere to the substratum, 

 gas bubbles appear in from three to six minutes, before the animal be- 

 gins to right itself and aid in this process by lowering the specific gravity. 

 The bubbles disappear rapidly after the righting process is completed. 

 When there is more than one bubble present all grow at the same time 



