OTHER PROTOZOA 97 



Finally in Volvox globator (Fig. 46), the specialization of the 

 body cells as well as the reproductive cells reaches a stage in 

 which certain cells are set apart for reproductive purposes while 

 others carry on the vegetative functions of the colony. 



Vofoox is a colonial organism found in fresh-water ponds. It 

 can easily be seen with the naked eye, being from .2 to .7 mm. in 

 diameter. It may be compared with a hollow rubber ball, since 

 it is a hollow sphere consisting of a single peripheral layer of cells 

 embedded in a gelatinous matrix. Each of the twelve thousand 

 cells which may be present has a thick envelope separating it 

 from the surrounding cells; mutual pressure gives these a hexa- 

 gonal shape (Fig. 46 A). Strands of protoplasm connect each 

 cell with the six cells that surround it (Fig. 46 C) ; physiological 

 continuity is thus established between the cells, a condition not 

 found in the colonies previously described. Most of the cells 

 contain an eye spot, chlorophyll, a contractile vacuole, and two 

 flagella. These are called "body" or somatic cells. They serve 

 to propel the colony through the water and to carry on nutritive 

 processes, but are incapable of reproducing new colonies, although 

 in young colonies they divide, giving rise to daughter cells like 

 themselves. The production of daughter colonies is accomplished 

 by special reproductive cells which are set aside for this purpose. 



Reproductive cells give rise to new colonies in two ways: (i) 

 asexually, and (2) sexually. The asexual method is as follows. 

 Certain cells of the colony are larger than others and lack flagella 

 (Fig. 46 A, i) ; a number of these in one colony increase in size, 

 and divide by simple fission into a great number of cells, produc- 

 ing new colonies without being fertilized (A, 1-7). The cells 

 that act in this way are named parthenogonidia. The colonies 

 derived from them drop into the central cavity of the mother 

 colony, where they swim about for a short time, finally escaping 

 through a chance opening in the wall. 



After colonies have been produced in this manner for several 

 generations the sexual method of reproduction may be observed 

 (Fig. 46 B). Colonies are found which contain as many as fifty 



