THE LIVING ORGANISM 17 



their duties. Each organ therefore occupies a particular 

 place in an organic environment, so to speak. Thus the 

 principle of adaptation holds true for the organs which 

 constitute an organism, as well as for organisms them- 

 selves in their relations to their surroundings. 



The various organs of living things are grouped so 



I as to form the several organic systems. There are 



^' eight of these, and each performs a group of related tasks 

 I which are necessary for complete life. The alimentary 



""1 system concerns itself with three things : it gets food 

 into the body, or ingests ; it transforms the insoluble 

 foods by the intricate chemical processes of digestion; 

 and it absorbs or takes into itself the transformed food 

 substances, which are then passed on to the other parts 

 of the body. It is hardly necessary to point out that 

 the ingestive stru'ctures for taking food and preparing 

 it mechanically lie at and near the mouth, while the 

 digesting parts, hke the stomach, come next, because 

 chemical transformation is the next thing to be done ; 

 while finally the absorbing portions of the tract, or. the 

 intestines, come last. The second group of organs, like 

 gills and lungs, supplies the oxygen, which is as 



.f necessary for life as food itself ; this respiratory system 

 also provides for the passage from the body of certain 

 of the waste gases, like carbonic acid gas and water 

 vapor. The excretory system of kidneys and similar 

 structures collects the ash-waste produced by the burn- 

 ing tissues, and discharges this from the whole mechan- 



c. ism, like the ash hoist of a steamship. The circulatory 

 system, made up of smaller and larger vessels, with or 

 without a heart, transports and propels the blood 

 through the body, carrying the absorbed foods, the sup- 



