CHAPTER III 

 AN INTRODUCTION TO THE METAZOA 



The Metazoa (Gr. meta, beyond; zoon, animal) are animals 

 consisting of many cells. These cells are not all alike, as in the 

 colonial Protozoa, but are separated into groups according to 

 their structure and functions. Although every Metazoon be- 

 gins its existence as a single cell, in the adult stage there are many 

 cells, and one kind of cell cannot exist without the presence of 

 the other kinds of cells; that is, the cells are not independent as 

 in the Protozoa, but are dependent upon one another. This 

 is the result of the division of labor among the cells. 



There is no sharp line between the Metazoa and the Protozoa. 

 The colonial Protozoa are many-celled animals, and, as we have 

 seen (p. 46), Volvox (Fig. 27) consists of cells which are made 

 interdependent by protoplasmic connections. There are a con- 

 siderable number of animals which are intermediate between the 

 Protozoa and the Metazoa, but, on the whole, the two groups 

 are fairly well defined. 



1. Germ-cells and Somatic Cells 



There are two chief kinds of cells in all the Metazoa, germ- 

 cells (Fig. 46, A, B) and somatic cells (Fig. 46, C-G). The germ- 

 cells, like those in Volvox (Fig. 27, $ , $ ), are set aside for reproduc- 

 tive purposes only ; the somatic cells form a distinct body, which 

 carries on all the functions characteristic of animals except re- 

 production. The detailed study of these two kinds of cells in 

 all groups of the Metazoa has led to the idea that the somatic 

 cells constitute a sort of vehicle for the transportation of the germ- 



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