THE PROTOZOA 129 



in some other way also When organic substance in solution is taken 

 in through the body wall, as probably happens in the case of Euglena 

 when in the dark, such method of obtaining food is said to be sapro- 

 phytic ( ), while those animals which ingest solid 



particles of food like the frog are said to be holozoic ( ). 



ENCYSTMENT 



Euglena, like Amoeba, when food becomes scarce, as well as for 

 unknown reasons, may encyst. It does this by becoming spherical, 

 secreting a rather thick gelatinous covering, and throwing off the 

 flagellum. 



REPRODUCTION 



This takes place by binary longitudinal division. "The nucleus 

 divides by a primitive sort of mitosis. The body begins to divide at 

 the anterior end. The old flagellum is retained by one-half, while a 

 new flagellum is developed by the other. Often division takes place 

 while the animals are in an encysted condition. One cyst usually pro- 

 duces two Euglenae, although these may divide while still within the 

 old cyst wall, making four in all, while recent observers have recorded 

 as many as thirty-one young flagellated Euglenae which escaped from a 

 single cyst." 



BEHAVIOR ' : i 



Euglena swims in a spiral manner as does the paramoecium. Like 

 the paramoecium, too, it has only two reactions to stimuli. But in the 

 case of Euglena, the forepart of the animal swings about in a circle 

 while the posterior part remains more or less stationary, thus forming 

 a sort of pivot around w r hich the forepart moves. 



It is positively phototropic though direct sunlight will kill it. As 

 all plants and animals need certain quantities of air, moisture and heat, 

 in which they thrive best, being injuriously affected if too much of these 

 substances is applied, the phototropic. action, as well as the killing by an 

 overabundance of light, is understood. The environmental conditions 

 in which an organism thrives best is called the optimum ( ) 



for such organism. 



VOLVOX 



All the unicellular animals having w r hip-like flagella come under a 

 sub-grouping known as Mastigophora. This group is particularly inter- 

 esting in that it furnishes us with our first example of unicellular ani- 

 mals forming colonies. The best known and studied of this group of 

 colonial flagellata is Volvox (Fig. 49) found in fresh water ponds. 

 Doflei found as many as twenty-two thousand cells in a single colony. 

 There is a division of labor in the colonies, for the various cells are not 

 all a 1 ike. though each is a separate and distinct animal. Some of these 



