ORGANIZATION 137 



digestion, etc., may be taken up by the different cells or groups 

 of cells and performed more efficiently. 



The various kinds of differentiation ' may be grouped under 

 the following heads : 



324. Integument. An extremely important set of organs 

 are the various protective structures which cover the entire 

 surface of the body of all but the very lowest animals. The 

 most important of these structures are the hair, feathers, scales, 

 bony plates, cuticular secretions, shells, glands and the un- 

 modified skin itself. These together comprise the integument. 



325. The Nerve-Muscle Mechanism. The nervous and the 

 muscular tissues are the most highly differentiated tissues of 

 the body. For efficiency the sense organs must be very numer- 

 ous, so that on the surface of the body there is scarcely a point 

 large enough to be visible which is not occupied. The muscles 

 for strength must be massive. The central nervous system, 

 through which all the various organs of the body are brought 

 into harmonious relations, especially the sense organs and the 

 organs of response, is the most complicated organ of the body 

 and is also of considerable size. The weight of a large body 

 requires special supporting structures, and for the most efficient 

 application of muscular energy for locomotion, a system of 

 levers is necessary. These structures comprise the skeleton 

 and the connective tissues, which together constitute the largest 

 set of organs in the body. 



326. Digestion. The highly differentiated tissues just de- 

 scribed have surrendered the function of digestion to other cells 

 of the body, and these are connected with the central cavity, 

 in which digestion takes place. The various phases of the 

 digestive process are separated and distributed to distinct 

 groups of cells, composing as many organs. These together 

 constitute the digestive system. 



327. Circulation. The digested food is absorbed by the 

 digestive tract in much larger quantities than is necessary to 



