58 ORGANISMS OF ONE CELL 



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 flagellated 

 protozoa, however, the cells after division do not separate com- 

 pletely but remain attached to each other in one way or another 

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

 ing 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 forming relatively 

 large spherical masses (Fig. 24). Again they are limited to a 

 certain number of cells and this number always reappears upon 

 reproduction so that the multicellular individual is much more 

 specific in nature, as in Gonium pectorale where the individual 

 always consists of 16 cells (Fig. 25). 



These colony forms are of peculiar interest in that they have 

 many features in common with the higher animals and plants, 

 but the cells are not differentiated for the performance of differ- 

 ent functions each one acting for itself rather than for the aggre- 

 gate as a whole. They represent therefore, a phase in the 

 complexity of form and function intermediate between the uni- 

 cellular organisms (protozoa, protophyta) and the multicellular 

 (metazoa and metaphyta). 



C. A CILIATED PROTOZOAN, PARAMECIUM CAUDATUM 



An infusion of vegetable or animal matter becomes the feeding 

 ground not only of bacteria and flagellated protozoa, but also 

 after some considerable time of ciliated protozoa as well. From 

 the fact that all of these organisms appear in such infusions, 

 the term Infusoria was formerly employed to designate all of 

 them indiscriminately. The bacteria were first recognized as 

 having no systematic relation to the other forms and were sepa- 

 rated in classification from the protozoa found in infusions. 

 Later the flagellated forms were recognized as entirely different 



