PHYLUM PROTOZOA 31 



away from the direction of movement; that is, Ameba flows 

 away, leaving the undigested solids behind. 



ASSIMILATION. The peptones, derived from the digestion 

 of proteid substances, together with the water and mineral 

 matter taken in when the food vacuole was formed, are absorbed 

 by the surrounding protoplasm, and pass into the body substance 

 of the animal, no circulatory system being present, so far as we 

 know. These particles of organic and inorganic matter are then 

 assimilated; that is, they are rearranged to form new particles 

 of living protoplasm, which are deposited among the previously 

 existing particles. The ability to thus manufacture protoplasm 

 from unorganized matter, it will be remembered, is one of the 

 fundamental properties of living substance (p. 10). 



KATABOLISM. The energy for the work done by Ameba 

 comes from the breaking down of complex molecules of proto- 

 plasm by oxidation or " physiological burning." This is known 

 as katabolism or dissimilation. The products of this slow com- 

 bustion are the energy of movement, heat, and residual matter. 

 This residual matter ordinarily consists of solids and fluids, 

 mainly water, some mineral substances, urea and carbon dioxide. 

 Secretions, excretions, and the products of respiration are in- 

 cluded in this list. 



SECRETION. We have already noted that an acid is poured 

 into the gastric vacuole by the surrounding protoplasm. Such 

 a product of dissimilation, which is of use in the economy of the 

 animal, is known as a secretion. 



EXCRETION. Materials representing the final reduction of 

 substances in the process of katabolism are called excretions. 

 These are deposited either within or outside of the body. A large 

 part of the excretory matter, including urea and carbon dioxide, 

 passes through the general surface of the body. The fluid con- 

 tents of the contractile vacuole are known to contain urea, there- 

 fore this organ is excretory in function. 



RESPIRATION. The contractile vacuole is also respiratory, 

 since carbon dioxide probably makes its way to the exterior by 



