CHAPTER XV 



METABOLISM 



I. Effects of starvation 



Apart from eventual death, the most conspicuous response of 

 protozoa to suspension of feeding is an often marked decrease in 

 size of the individual. That is to say, the animal consumes some of 

 its own substance before dying. Protozoa differ in the extent of 

 reduction w^hich is possible. Paramecia decrease little before they 

 shrivel and die, but Dileptus (Visscher, 1923) and Amoeba 

 (Hartmann, 1928) can persist and dwindle to i/iooth, and 

 Didinium (Mast, 1909) to i/6th their original volume. Bursaria 

 truncatella can diminish in length from 500 to 90 /x as the feeding 

 organelles become proportionately smaller (Lund, 19 17), and 

 Dembowska (1938) showed that under starvation Stylonychia 

 repeatedly reorganizes on a smaller scale until very tiny animals 

 are produced. When great latitude in size is permitted, the protozoa 

 do not simply become thin and emaciated like starving vertebrates ; 

 as with many invertebrates, including hydras and flat worms, they 

 become proportionately reduced in most of their parts so that they 

 may properly be called dwarfs. Minute forms are not only the 

 result of individuals consuming their own substance but may 

 possibly also involve so-called " hunger divisions ", or an initial 

 persistence of the rhythm of fission in spite of decreasing size 

 during the first days of starvation. The two factors are not easily 

 separable when dealing with large samples difficult to count. Yet 

 Maupas (1888) confirmed Gruber (1886) in reporting that large, 

 well-fed coeruleus, when isolated, continued dividing 3 or 4 times, 

 producing smaller than normal individuals. Division without 

 attaining maximum size was indicated, though on Maupas' 

 evidence the stentors must not have been totally without food 

 since division products much larger than one-eighth, say, of the 

 maximum volume were produced. In my experience, on the 

 contrary, even large stentors very seldom divide after they are 



259 



