122 AN INTRODUCTION TO ZOOLOGY 



time to get rid of the indigestible residue the Amceba simply flows 

 on and leaves the useless particles, termed the faeces, behind, a 

 process known as egestion or defaecation. Digestion results in the 

 breaking down of the food into comparatively simple substances, 

 and in order that they may be utilised by the cell they have to be 

 assimilated. This assimilation consists of a series of building up 

 processes whereby the simpler compounds are once again trans- 

 formed into complex protoplasm, and these constructive changes 

 are collectively termed anabolism. Interesting light has been 

 thrown on these processes by a number of experiments in which the 

 Amceba was cut into two pieces, one with and one without a nucleus. 

 The part without a nucleus, although capable of living for some 

 days, loses all power of digesting or assimilating food ; therefore, 

 we look to the nucleus as the centre producing the enzymes required 

 for digestion, and as the origin of the chemical activities concerned 

 in assimilation. The digestion of food in Amceba takes place in a 

 way markedly different from that in Rana. In the latter case the 

 whole of the process is gone through in the alimentary canal, and 

 the food does not pass into the cells of the mucosa until it has been 

 rendered soluble : the digestion takes place in a cavity lined by 

 cells, and is termed intercellular. In Amceba the raw food is taken 

 straight into the cell within which the digestion occurs, and this in 

 consequence is described as intracellular. 



There is no special organ in Amceba which corresponds 

 functionally to the lungs, but the exchange of Oxygen for Carbon 

 dioxide takes place by diffusion over the general surface of the 

 body. In this way the animal obtains the oxygen required to oxidise 

 its protoplasm and gets rid of the carbon dioxide, one of the waste 

 products resulting from this process. 



In order to perform its vital functions it is necessary for Amceba 

 to obtain kinetic energy, and this it does by the oxidation of its 

 tissues in the way just indicated. Again, arguing- from analogy 

 with the frog, we should expect certain nitrogenous waste matters 

 such as urea to be produced, and these need to be eliminated from 

 the body. It is practically certain that these substances are removed 

 through the agency of the contractile vacuole, and although the 

 amount of fluid it discharges at a time is very small, the presence of 

 uric acid crystals in it has been demonstrated. The vacuole is, 

 therefore, an organ of excretion corresponding in function with the 

 frog's kidney, and, like it, may be concerned with other functions 

 such as the removal of superfluous water, etc. 



The phenomenon of growth is clearly exhibited by Amceba, and it 

 affords us an interesting example of the way this occurs in living 

 matter. As anabolism proceeds new matter is added throughout 



