220 PRACTICAL ZOOLOGY 



The digested food, 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. 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 is one of the fundamental properties of living substance. 



The energy for the work done by Paramecium comes from the 

 breaking down of complex molecules of protoplasm by oxidation 

 or " physiological burning." This is known as katabolism or 

 dissimilation. The products of this slow combustion are the 

 energy of movement, heat, secretions, excretions, and the prod- 

 ucts of respiration. 



The acid that is poured into the gastric vacuole by the sur- 

 rounding protoplasm is of use to the animal and is known as a 

 secretion. 



Materials representing the final reduction of substances in 

 the process of katabolism are called excretions. These are de- 

 posited either within or outside of the body. A large part of the 

 excretory matter passes through the general surface of the body, 

 but the two contractile vacuoles are also excretory in function. 



A contractile vacuole is present near either end of the body. 

 Each communicates with a large portion of the body by means of 

 a system of radiating canals, six to ten in number. These canals 

 collect fluid from the surrounding protoplasm and pour it into 

 the vacuole. The vacuoles contract alternately at intervals of 

 about ten to twenty seconds and their fluid contents are dis- 

 charged to the outside. The contractile vacuoles are also res- 

 piratory, since carbon dioxide is probably also discharged from 

 them. Oxygen dissolved in water is taken in through the sur- 

 face of the body. As in higher animals this gas is necessary for 

 life. 



THE NUCLEUS. In stained specimens of Paramecium a 



