12 AN INTRODUCTION TO ZOOLOGY 



The elements constituting living substance, form complex 

 organic compounds which exhibit three striking peculiarities: 

 (i) they are in most cases labile substances, i.e. they are un- 

 stable and maybe broken up into simple compounds, or synthe- 

 sized into more complex molecules, (2) carbon is always an im- 

 portant constituent, and (3) their important elements are all 

 non-metallic. 



The chemical composition of living matter is highly variable. 

 A complete analysis of representatives of any two species would 

 never give identical results; and even young and old individuals 

 of the same species would not agree. Nevertheless, all animals 

 show a remarkable uniformity in chemical composition. Some 

 animals might be considered exceptions to this general rule; 

 for example, a clam upon analysis would show a large percentage 

 of calcium carbonate owing to the presence of that salt in its 

 shell. The shell, however, is really not a part of the living sub- 

 stance, but represents an inorganic accretion that has formed 

 around the body. 



(4) Organization. Although living organisms exhibit great 

 diversity in structure, a careful examination of all the parts 

 proves that the fundamental elements are essentially the same 

 in every individual. All plants and animals are composed of 

 similar microscopic units, called cells, which are arranged in a 

 characteristic manner for each species. Thus the body of a very 

 simple organism may consist of only a single cell, whereas that 

 of a complex organism may contain billions. Cells may be 

 compared with bricks which are used to build houses. Bricks 

 may show variations in form, color, and composition, but have 

 certain characteristics which make them easily recognizable 

 whether they are separate or built into walls. In a similar way, 

 though individual cells show minor differences, they agree in 

 fundamental characteristics, and may readily be identified, even 

 when constituents of a many-celled animal. 



Whether an organism is unicellular or multicellular, it always 

 consists of a number of unlike parts which are incapable of inde- 



