CHAPTER 8 



GENERAL PROBLEMS RELATED TO SINGLE-CELLED 



ANIMALS 



It is evident from the special accounts in the preceding chapter 

 that many important biological problems are intimately related 

 to single-celled organisms. The existence of colonial species among 

 unicellular animals and plants suggests the transitional steps that 

 probably occurred in the development of various organisms from 

 the primitive unicellular forms of life that are supposed to have 

 existed at a very remote period and to have been the ancestors of 

 many-celled animals. The manner in which individuals come into 

 being at the present time, whether by reproductive processes, 

 biogefiesis, or by processes of spontaneous generation, abiogenesis, 

 may be considered here, since unicellular organisms were the last 

 stronghold of the advocates of spontaneous generation. Again, 

 many diseases are caused by protozoa living as parasites in the 

 bodies of men and animals, thus Hnking the protozoa with medical 

 problems. 



Colonial Protozoa and the Comparison of Unicellular with Multi- 

 cellular Organisms 



The Colonial Organization. — Although the typical protozoan 

 is an independent and self-su.staining cell, there are manj^ colonial 

 species. These protozoan colonies are produced when the cells 

 arising from the single cells, which occur at one or more phases of 

 the life cycle, remain together, instead of separating after division as 

 in non-colonial forms. During this colonial phase the group of cells 

 constitutes an individual of a shghtly higher order, which is the 

 colony. Such colonies are called gregaloid, if their cells are arranged 

 irregularly (Fig. 103 A) ; linear, if in a line (Fig. 103 B) ; arhoroid, 

 if branchinj; (Fig. 103 G); or spheroid, if in a spherical or globular 

 mass (Fig. 103 E). The cells of a colony are associated but in- 

 dependent, since each cell can reproduce a new colony by division 



19.5 



