CELL AGGREGATION AND DIVISION OF LABOR 85 



reproductive processes, when these are asexual, as by budding or fission, 

 do not require the cooperation of another member of the species. It is 

 only in sexual reproduction, which is the least common method in most 

 Protozoa, that the cooperation of another cell is required. This state 

 of independence is exemplified in such Protozoa as Paramecium, Euglena, 

 Amoeba, Chlamydomonas, Stentor, and Vorticella. In a large number 

 of species of this type the cells are motile. If they are sessile (attached) 

 they may occur singly or in groups, but even in the latter case the cells 

 have no organic union with each other. 



In a much smaller number of protozoan species, on the contrary, 

 the cells are associated in aggregates or colonies in which there may be 

 an organic union between the cells. The aggregate may involve as few 

 as two cells or as many as thousands. The organic union is usually due 

 to the fact that when cell division occurs in such species the resulting 

 cells remain in contact with one another. The cells may, however, 

 become separated but still be retained within the jelly-like envelope of 

 the original cell; or there may be a coming together and rearrangement 

 of the cells after they have been separated. If the cells adhere after cell 

 division a more or less complex aggregation is built up whose morpholog- 

 ical characters are dependent upon the plane of division, the subsequent 

 growth of the cells or of certain of their parts, the development of new 

 parts of cells, or the rearrangement of the cells. In accordance with 

 these differences, different kinds of aggregations or colonies are produced. 



The most common forms of protozoan colonies are linear, dendritic 

 (with tree-like branchings), and spheroid. There is also a much less 

 common form of colony, the gregaloid, in which cells are scattered irregu- 

 larly through the mass. The three most common types of colony are 

 discussed and illustrated below. 



The linear colony is the result of repeated cell divisions in one plane 

 followed by incomplete separation of the resulting cells or, as among the 

 Gregarines, the linear series may be produced by the end to end union of 

 previously separate cells. The latter method is obviously one of rear- 

 rangement. In the parasitic mesozoon, Haplozoon lineare (Fig. 52), a 

 linear series is formed by repeated fission in one plane. The anterior 

 cell is the oldest. Through it nutritive solutions pass to the other 

 members of the series and by means of it they are attached to the host. 

 The other cells perform the function of reproduction. 



Dendritic colonies have a branching form like trees. Good examples 

 are Dinobryon and Codosiga. DinobryoD (Fig. 53) is a free-swimming 

 colony each cell of which is enclosed in a cup-like sheath of cellulose. 

 After reproduction by simple longitudinal fission of the cell, one of the 

 daughter cells occupies the old sheath while the other migrates to the 

 edge of the sheath where it attaches itself and there proceeds to grow 

 and to produce a new sheath. Repetitions of these processes produce a 



