CHAPTER II 

 THE ANIMAL ORGANISM 



The fundamental biological functions of animal organisms 

 are the same in all forms of animal life. The principal differences 

 between different kinds of animals lie in the manner in which 

 these functions are carried out and in the degree of morpho- 

 logical differentiation that accompanies functional differentiation. 

 Thus the Protozoa, the lowest phylum in the animal kingdom, 

 consist of animals in which the body is a single cell, free-living, or 

 united with other similar cells to form a colony. Protozoa are 

 capable of maintaining and reproducing themselves, in a favor- 

 able environment, with as much success as any higher form of 

 life. In the ascending scale of animal life one finds that the 

 organism takes on an ever-increasing structural and functional 

 differentiation of its parts that is absent or at best only very 

 feebly expressed in Protozoa. The elaboration of structural and 

 functional features found in higher forms is an inevitable accom- 

 paniment of their greater size. It is inconceivable that a frog 

 could exist as a frog in the form of a single cell. 



The varying degree of structural differentiation found in 

 different kinds of animals lends itself to an analysis in terms of 

 cells, the common though variable structural units of which their 

 bodies are composed. In forms above the Protozoa — Metazoa 

 or multicellular animals — larger morphological units composed 

 of groups of cells constitute what are called tissues and organs. A 

 tissue is a group of histologically similar cells, such as the cells 

 forming the outer layer of the frog's skin. An organ is composed 

 of different kinds of tissues, characterized by a distinctive 

 histological structure, and capable under proper conditions of 

 performing one or more distinct functions in the animal body. 

 Tissues and organs are the visible evidence of differentiation in 

 the bodies of metazoan animals. 



The Frog. — A study of the biology of an individual animal 

 might begin with any animal as the subject matter. Logically, 



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