ii6 MANUAL OF ELEMENTARY ZOOLOGY 



formation of a pseudopodium in a region of the body other 

 than that which has been stimulated shows the presence of 

 conductivity. Again, it does not swallow every particle it 

 comes across, but chooses those that either contain 

 nourishing substances or, being in motion, are probably 

 alive and therefore fit for food. By an unkind deception of 

 this " sporting instinct," it may be induced to capture and 

 swallow moving particles of glass. Its mode of seizing food 

 is not fixed, but adjusted with an uncanny appearance of 

 intelligence to the nature and behaviour of the prey of the 

 moment, which it dogs with perseverance and resourceful 

 changes of method. How much it can see is doubtful. 

 It will move towards light, but does not appear to perceive 

 a particle of food better in the light than in the dark. Yet 

 it seems to become aware at a short distance of a particle 

 of food even when the latter is insoluble, and therefore 

 incapable of being smelt. All this shows that it receives 

 from foreign bodies stimuli of different kinds, and dis- 

 criminates between them. In contrast to these instances, 

 many of the actions of the animal cannot be traced to any 

 stimulus, and must therefore be classed as automatic in the 

 sense in which we have used that word. 



The contractile vacuole is probably an excretory organ. 

 Waste products are shed into its cavity and 

 lSJJration" d tnus removed from the body when its contents 

 are discharged. 1 Water probably enters all 

 over the surface of the animal and is collected into the 

 contractile vacuole together with the water produced 

 during the metabolism of the animal. The water must 

 bring with it dissolved oxygen, and thus the contractile 

 vacuole aids respiration. At the same time it seems likely 

 that the whole surface of the body may serve to some 

 extent both for respiration and for excretion. 



Unfavourable conditions of life may bring about a 

 Depression disease known as depression, in which the 

 nucleus of the Ammba is enlarged and the various 

 functions become deranged. This disease, however, is more 

 familiar and has been more closely investigated in some 

 other minute animals, as, for instance, in Paramecium (p. 137). 



'The nitrogenous waste product uric acid has been found in the 

 contractile vacuole of organisms related to Amaba. 



