CHAPTER 9 

 SOURCES OF ENERGY AND MATERIALS 



Muscular action and the other activities of an organism entail destruc- 

 tion of living substance, which must be steadily replaced. In growing 

 animals, not only are repairs necessary, but provision must be made 

 for new construction. The general source of material for growth and 

 replacement is food. How this material is utilized in single cells has 

 already been described; how it is transformed in multicellular animals is 

 now our concern. 



Since most food is not in a form that can be transported through 

 protoplasm, it must usually be converted in some way. In large part 

 the conversion consists of making it soluble. But even some soluble 

 foods are unable to pass through tissues, because of the selective action 

 of protoplasm which will receive some substances and not others. The 

 conversion is accomplished by the process of digestion which, in multi- 

 cellular animals, is carried on in some sort of digestive system. 



The Locus of Digestion. — In the protozoa digestion is an intracellular 

 process. Amoeba engulfs food by flowing around it at any part of the 

 cell. Paramecium takes the food in at a particular place, through a 

 permanent gullet. In either case the food is surrounded by a droplet 

 of liquid, which is acid in reaction at first, and presumably enzymes are 

 secreted into this fluid. The food vacuole thus formed is the digestive 

 apparatus. These features of protozoan digestion were described earlier 

 but are repeated here in the first two parts of Fig. 88 for contrast. Among 

 the multicellular animals, sponges retain the intracellular type of diges- 

 tion. Through the channels and cavities which are characteristic of 

 sponges, water flows, kept in motion by the flagella of collar-bearing 

 cells in some of the channels (Fig. 33). From the water the collared 

 cells seize organisms, after the manner of Amoeba, and digest them. 

 Products of this digestion are passed on to other cells by diffusion or 

 osmosis, so that nutrition in sponges is on a cooperative basis; but just 

 as in protozoa, digestion is done within the cells. 



In all metazoa other than sponges digestion is performed partly, even 

 chiefly, in cavities of organs — -surrounded by cells, but not in cells. The 

 process is at least bcgvm in these cavities, and in the higher animals is 

 almost completed there. The more complicated types of food are 

 rendered quite simple before they leave these cavities. Some foods are 



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