86 THE ANATOMY OF THE HONEY BEE. 



first supposed from the study of plants, but is a little protoplasmic 

 body or corpuscle, visible only under the microscope, surrounded by 

 a membranous cell wall and containing a small internal body called 

 the nucleus. The different cells of the body are specialized in groups 

 to do some one particular thing— the salivary cells secrete saliva, the 

 muscle cells contract, the excretory cells pick out waste substances 

 from the blood, and 39- on. Btit this specialization does not signify 

 that each cell does not perform its^own vital processes in addition to 

 its specialty. The fact that it remains alive and works means that 

 the complex chemical components of its body substance or protoplasm 

 are constantly being reduced to simpler compounds which are ex- 

 pelled', while new protoplasm is built up from the supply of food 

 material brought by the blood. This double process of destruction 

 and reconstruction is known as metaiolism, while its two phases, the 

 breaking-down process and the building-up process, are known as 

 katabolism and anaSoKsm, respectively. 



Now, while all the cells of the body must have nourishment, none 

 of them, except those of the alimentary canal, is capable of utiliz- 

 ing the raw food materials that an animal obtains in a state of nature. 

 These materials must therefore be changed into some other form in 

 order that they may be assimilated by the cells. This change is called 

 digestion. ^ 



The single cell composing the body of a Protozoan, living free in 

 nature, digests its own food and then assimilates the products of its 

 own digestion. But, of the cells constituting the body of any mul- 

 ticellular animal, only those of the alimentary canal are capable of 

 digesting raw foodstuffs, and, moreover, as digestion is the specialty 

 of these cells, they have also to digest the food for all the other ^pUs 

 of the body. 



The two most important changes that must be brought about in 

 the natural food by digestion are those which make it soluble in the 

 blood and which render it capable of passing through animal tissues. 

 In the first place, the food must diffuse through the walls of the 

 alimentary canal as a liquid which mixes with the blood, for there 

 are no pores or openings of any sort from the alimentary canal into 

 the body cavity; and in the second place, it must pass through the 

 walls of the cells themselves. The digestive changes result chiefly in 

 a breaking down of the complex molecules of the raw food materials 

 into more simple chemical substances. These are taken up by the 

 cells and reconstructed into complex protoplasmic molecules which 

 can not escape through the cell membrane until they are again broken 

 down into simpler forms. 



The waste products of the cells consist principally of carbon, hy- 

 drogen, and nitrogen. These are converted by the oxygen supplied 

 by the respiratory system into carbon dioxid, water, and compounds of 



