60 ZOOLOGY. 



structure which constitutes an adaptation to the special work 

 to be done, and is of great advantage. We have described 

 these structure-groups as tissues (see Chapter V.). 



87. Organs. The tissues which have been described are 

 never independent, but are associated with each other in the per- 

 formance of a common function, to form an organ. In each 

 organ there is usually a principal tissue which determines its 

 function (as muscular tissue in muscle, or the glandular tissue 

 in glands), and one or more accessory tissues for support or 

 control (as connective or nervous tissue in the organs men- 

 tioned). To accomplish some of the activities, in the higher 

 animals especially, several organs of a similar kind must work 

 together. These are sometimes spoken of collectively as sys- 

 tems of organs, e. g., digestive system, circulatory system, 

 and the like. 



88. Classification of the Systems of Organs and Func- 

 tions. The work that needs to be done by an organism may 

 be considered under the following heads: (i) metabolism 

 including digestion, circulation, assimilation, respiration, and 

 excretion; (2) protection and physical support; (3) growth; 

 (4) reproduction; (5) movement; (6) sensation. Eight sys- 

 tems of organs can be distinguished by which this work is 

 done. They are (i) the digestive; (2) circulatory; (3) 

 respiratory; (4) excretory; (5) skeletal and integumentary; 

 (6) reproductive; (7) muscular, and (8) nervous. 



89. Metabolism (Nutrition). Metabolism embraces two 

 sets of processes, (i) constructive or anabolic, known as 

 assimilation, and (2) destructive or katabolic. By construc- 

 tive we mean all the. processes in the organism which result in 

 the storing of food and energy, in growth, repair, and repro- 

 duction. We class as destructive all those processes by which 

 the complex cell substances are broken down and energy set 

 free, leading to change of temperature, to nervous or muscular 

 action, to secretion and excretion. In the higher animals the 

 nutritive process is a very complicated one and demands the 



