86 



PROTOZOA. 



External 

 Features. 



If the outline of an Amoeba be sketched every few 

 seconds and the series of drawings examined, it 

 will be seen that the shape has undergone con- 

 siderable change, and it will also probably be 

 found that the whole animal has moved somewhat from 

 its original position. 



Further, this peculiar change of shape is evidently due 



to the pushing outwards of processes, which gradually 



„ grow larger and larger. If a drop of treacle 



be placed on a flat surface and the surface 



gently inclined, the drop will progress in a similar fashion 



Fig. 28. — Amceba Proteus (Magnified). 



A large diatom is enveloped on the left. The largest sphere 

 is the contractile vacuole ; the smaller is the nucleus. 



by processes thrust out in the direction of least resistance ; 

 but the treacle processes differ from those of Amceba^ 

 because in the former the energy for such movement is 

 derived from without (the acceleration due to gravity), 

 and they are hence only in a downward direction, 

 whereas in the latter the energy is provided by the 

 chemical decomposition of the protoplasm itself, hence 

 their direction is determined by other factors and the 



