GENERAL ZOOLOGY 



important in the metazoan is the fact that the various kinds of somatic cells, 

 as well as the germ cells, are dependent on the collective activities of the 

 metazoan individual. The cell of a metazoan is a unit subordinated to the 

 activities of the multicellular whole, which is the organism. 



By contrast, the protozoan cell is an independent, self-sustaining individual. 

 In most colonial protozoans, each cell of the colony is likewise an inde- 

 pendent individual, so far as the fundamental capacities of metabolism, 

 responsiveness, and reproduction are concerned. In such a species the colony 

 eventually disintegrates by separation of its units, so that each cell goes 

 its way, encysting, dividing, conjugating, or uniting with another cell in 

 syngamy, until a new colony is formed again from a single cell by repeated 

 divisions and by the remaining together of the daughter cells. Every cell 

 of the colony is, therefore, as independent functionally as though there were 

 no colonial stage in the life cycle. Such species are manifestly no more 

 than aggregations of independent protozoan cells, each of which is sufficient 

 unto itself, or physiologically balanced. 



In a few species of colonial Protozoa, however, the distinction can be made 

 between somatic cells, which are destined to die, and germ cells, which can 

 in a sense continue to live by contributing their substance to the next gen- 

 eration, if they become gametes and unite in syngamy. A comparison of these 

 species with other Protozoa, on the one hand, and with the Metazoa, on the 

 other, makes clear the basic continuity in reproductive processes from one 

 end of the Animal Kingdom to the other. Such a comparison also indicates 

 that this primitive step in differentiation may have been fundamental in lead- 

 ing to the development of truly multicellular types, with their characteristic 

 wide additional specializations within the somatic cell line. 



A series of forms in the flagellate family Volvocidae will be used for this 

 discussion, although this family belongs among the Phytomastigina or plant- 

 like flagellates. Of course, no inference is intended that this series of forms 

 marks the actual pathway of the development of multicellularity in animals. 

 Rather, this alignment represents merely an arrangement of existing organ- 

 isms into a series showing an increase in mutual interdependence between the 

 individuals of the colony. The many-celled condition in animals may well 

 have arisen through .some analogous process. 



The members of the family Volvocidae are colonial forms of varying size 

 and complexity, the colonies in each species consisting of a specific number 

 of component individuals. These individuals strongly resemble a simple, non- 

 colonial flagellate of the genus Chlamydomonas (Fig. 9.1), which is covered by 

 a cell wall of cellulose, bears two flagella, and contains a prominent chloro- 

 plast, a sensitive pigmented eye spot, and two contractile vacuoles. A simple 

 colonial form of the genus Gonium may be composed of 4 of these units, or of 

 16, depending on the species. In Gomum, the individuals are arranged in a 

 flat plate, and as in all the Volvocidae the colony is held together by a 

 mucilaginous matrix secreted by the individuals. Aside from the fact that 

 the colony moves as a unit, propelled by the flagella which all lie on one 



266 



