CHAPTER IV 



METAZOAN ORGANIZATION 



All animals whose bodies consist of few or many cells functioning 

 as a unit are called metazoans. 



The cells of their bodies are definitely organized and classified mor- 

 phologically as well as physiologically. There is a well-regulated 

 division of labor. Among the single-celled animals each cell is largely 

 independent of its fellows, doing for itself all that is necessary to 

 carry on living processes. In the many-celled animal, as in a highly 

 developed society of men, certain individual cells become more pro- 

 ficient in doing certain kinds of work, and as a result, a special 

 'group is able to care for a particular function necessary to the life 

 of the entire organism. In return, other special groups care for 

 other functions. In this way each exchanges the products of its 

 labor for the products of the labors, of the other groups. In human 

 society this becomes more and more complicated as civilization 

 advances; so it is with development of complexity in metazoans. 

 Another characteristic of Metazoa is the presence of a definite 

 center of control localized in a particular group of cells which be- 

 comes the nervous system in higher forms. 



Cellular Differentiation 



In Protozoa (single-celled animals) there is seen fair development 

 of intracellular differentiation, making it possible for one part of a 

 cell to perform a particular function, and for other parts to perform 

 other functions. The complexity of Metazoa is not the result of great 

 complexity of the individual cells, but it is due to the special differ- 

 ences between them. The presence of a variety of cells within one 

 body is spoken of as intercellular differentiation. The modification^ 

 of metabolic activity is the basic factor in the development of all 

 differentiation. Certain groups of cells become specialists in a par- 

 ticular phase of the metabolic activity. Some become protective sur- 

 face cells, others secrete special enzymes, still others specialize in ex- 

 cretion, and so on. 



The entire metazoan body is usually divided into gerjn cells, which 

 are specialized for reproduction, and somatic cells or body cells, 

 which compose the remainder of the body and are grouped in layers. 



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