178 



THE INDIVIDUAL ORGANISM 



plexity in organization and the extent of "division of labor" among the 

 parts of the individual. We shall first briefly examine four different 

 levels of construction found among animals and then see how the common 

 tasks of life are performed at each of these levels. 



Structural Levels 



Protoplasmic level. In one large group of animals, the Protozoa, 1 

 each individual consists of a single cell. (Some are colonial, the individuals 

 being held together mechanically in a group). This one cell must perform 

 all the functions essential for animal life. It must capture its own food 



food vacuole 



pseudopodia 



endoplasm 



contractile va 



\ 



water vacuoles 

 Fig. 13.1. Amoeba, a single-celled protozoan animal. 



and digest it, carry on respiration, excrete metabolic wastes, respond 

 appropriately to environmental stimuli, and reproduce its kind. Such a 

 single-celled animal lives on what may be called the protoplasmic level 

 of construction. Performance of its various functions is provided for by 

 the organization of its protoplasm, and such organization as it shows is 

 all intracellular. There are no tissues and organs, and the Protozoa do 

 not have any very complex structures correlated with particular func- 

 tions. Nevertheless they should not be thought of as simple. No cell is 

 simple, and these separate-living cells have to do many more things than 

 the specialized cells of the human body and are correspondingly com- 

 plicated. Amoeba (Fig. 13.1) is an excellent example of these unspecialized, 



1 The animal groups mentioned in this chapter are defined and illustrated in Appen- 

 dix B. 



