58 ORGANISMS OF ONE CELL 



The color is due to a substance termed chlorophyll which, with 

 the aid of sunlight, is able to manufacture starch; this, in turn, 

 is built up into protein matter which serves as food (see Chapter 

 V). Another colored structure is also found in Euglena, 

 although it is not so conspicuous as the green chloroplastids. 

 This is the red-colored spot or stigma which is more sensitive 

 to light than other parts of the protoplasm, and is often spoken 

 of as a rudimentary " eye-spot." In many cases it is accom- 

 panied by a lens-like body which may concentrate light rays 





FIG. 24. Uroglena americana, a colony of flagellated protozoa in which the indi- 

 viduals are embedded in a common gelatinous matrix. 



on a particular spot and so act as a directive agent; at any 

 rate this spot is usually turned toward the source of light 

 and it serves therefore as a rudimentary sense organ. 



Colonies. Both Peranema and Euglena reproduce by longi- 

 tudinal division which is not different in any way from the divi- 

 sion of Chilomonas. The daughter cells separate after division, 

 and lead an independent existence. In some forms of flagel- 

 lated protozoa, however, the cells after division do not separate 

 completely but remain attached to each other in one way or 

 another (e.g., by the basal ends as in Synura uvella (Fig. 23), thus 

 forming aggregates of cells or individuals of a second order to 

 which the term colony is given. Sometimes the cells thus 

 formed are embedded in a common jelly, the aggregate form- 

 ing relatively large spherical masses (Fig. 24). Again they are 



