Fig. 9.1. Solitary and colonial plant-like flagellates. A, Chlamydomonas, a solitan- individual. 

 B, Gonium pectorale, a plate-like colony of 16 individuals. C, Pandonna morum, a solid, mul- 

 berrv-like colony. (Adapted from G. M. Smith, Fresh-water Algae of the United Slates, first 

 edition, copyright 1933 by McGraw-Hill Book Co., Inc., printed by permission.) 



(see Fig. 7.4, p. 221). The simplicity of their organization, however, appears 

 to present features which may have been characteristic of early metazoans. 



Colonial Protozoa 



Although the typical protozoan is a single cell, as we have seen, there are 

 many species of Protozoa, particularly ciliates and flagellates, in which many 

 similar cells live together in groups, or colonies, during a considerable part 

 of the life cycle. The diflference between these colonial protozoans and a 

 metazoan lies in the relationship of the individual cell to the other cells 

 with which it is associated. In the adult metazoan the cells can be classified 

 as somatic cells and germ cells, depending on their relation to the reproductive 

 process. This classification is not hard and fast; but in general the somatic 

 cells are specialized for the various functions of metabolism and responsive- 

 ness, whereas the germ cells are specialized for sexual reproduction. During 

 asexual reproduction in a metazoan, there may be formative cells which can be 

 called reproductive insofar as they become an important source for the cells 

 of the new parts. In asexual phases no cells that are comparable to the 

 germ cells, with their strictly reproductive functions, may be present. Most 



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