CHAPTER XV 



STRUCTURES OF ALIMENTATION 



POTENTIAL energy is acquired by animals in two ways (a) as oxygen 

 taken unchanged from the atmosphere, and (b) as matter taken as food 

 and transformed. The acquiring of energy and the accumulation of 

 material for growth through the transformation of food materials is 

 what we call alimentation. This process takes place in every living 

 animal. The process is a double one, involving the dissolving or digest- 

 ing of the food and the absorbing of the food after it is digested. The 

 structures concerned with alimentation must, therefore, provide cavities 

 for the retention of the food while it is being digested, and surfaces which 

 can absorb the food when once digested. Food vacuoles and food 

 cavities retain the food until digested. The surfaces of such vacuoles 

 and cavities discharge the necessary digestive juices and afterward 

 absorb the digested food. In the Protozoa, where an individual con- 

 sists of but one or a few cells, there can be no inter-cellular food cavity or 

 enteron ; hence digestion must take place within a cell in food vacuoles. 

 These unicellular animals secure their food by means of protoplasmic 

 processes, called pseudopods, flagella, cilia, and membranelles. 



By these structural devices food, suspended in some water, is carried 

 within the cell where the cytoplasm surrounds it on all sides, forming 

 a rounded, digestive vacuole. This vacuole is not a permanent struc- 

 ture of the cell. 



The size of the vacuole is not constant. Its place of origin may be 

 no fixed region of the cell, as in Amoeba, Vampyrella, and other naked 

 Rhizopoda; or the food vacuole may arise at a definite region of the 

 cell, as is the case in the Infusoria of which Paramcecium is a represen- 

 tative; nor is the position of the food vacuole once formed stationary; 

 it is carried with the cyclosis of the cell. The vacuole receives the digest- 

 ing fluids secreted by the cell, and retains the food until digested. Its 

 surface absorbs the digested food. At the completion of this double 

 process the vacuole moves to the surface of the cell to discharge the 

 waste matter brought in with the food and to disappear as a feature 



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