CHAPTER XIX 



METAZOA 



All animals which are not protozoans are included in one subkingdom 

 known as Metazoa. All such animals are similar in that they have a 

 many-celled body in which the cells are not all alike but are varied in 

 structure* and function, the activities of the whole being the result of their 

 cooperative efforts. 



115. Differentiation. — The modification of certain parts for the 

 performance of corresponding functions is known as differentiation. In 

 the Protozoa has been seen intracellular differentiation (Sec. Ill), as a 

 result of which one particular structure within the cell comes to perform 

 one function and the other structures other functions. In the highest 

 of the Protozoa this results in an exceedingly complex cell. In the 

 Metazoa, however, complexity does not result from the complexity of 

 the individual cells which make up the animal but from differences 

 between them. This intercellular differentiation results in a great variety 

 of cells within one body. Differentiation which is concerned with struc- 

 ture alone is morphological differentiation. Accompanying this, however, 

 is differentiation in function, which results in cells of different structures 

 having different functions, appropriate in each case to the structure. 

 This may be termed 'physiological differentiation. All differentiation is 

 based upon modification in the metabolic activities of the cells. As a 

 result of differentiation the cells in the metazoan body become inter- 

 dependent, in contrast to the independence which exists between 

 protozoan cells, even in colonies. This interdependence, however, is fore- 

 casted in the case of certain of the colonial Protozoa such as the volvox, 

 in which protoplasmic bridges extend from cell to cell and in which certain 

 reproductive cells are differentiated. It thus appears that the line of 

 demarcation between the Metazoa and the Protozoa is not so sharp as 

 might be supposed. It may, however, be drawn on the basis of the 

 differentiation in the non-reproductive, or somatic, cells, which does not 

 occur in the Protozoa but is characteristic of the Metazoa. 



116. Division of Labor. — Another term for physiological differ- 

 entiation is that of division of labor. The idea conveyed by this expres- 

 sion involves an analogy between the development of the animal body 

 and that of human society. Among very primitive peoples each indi- 

 vidual is largely independent of his fellows, doing for himself all that he 

 needs to have done. As society develops, certain individuals become 



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