130 General Biology 



the manufacture of food without light, the animal must be able to feed 

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

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

 in the dark, such method of obtaining food is said to be saprophytic 

 ( ), 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 is 

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

 the anterior end. The old flagellum is retained by one-half of the body, 

 while a new flagellum is developed by the other half. Division often 

 takes place while the animals are in an encysted condition. One cyst 

 usually produces two Euglenae, although these may divide while still 

 within the old cyst wall so as to make four in all. Recent observers 

 have recorded as many as thirty-one young, flagellated Euglenae which 

 escaped from a single cyst. 



BEHAVIOR 



Euglena swims in a spiral manner as does Paramoecium. Like 

 Paramoecium, too, it has only two reactions to the stimuli of touch. 

 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 which the forepart moves. 



Euglena is positively phototropic, but direct sunlight will kill it. 

 All plants and animals thrive in certain quantities of oxygen, moisture, 

 and heat, but are injuriously affected if too much of these is applied. 

 This explains phototropic action as well as the killing by an over- 

 abundance of light. The environmental condition in which an organism 

 thrives best, is called the optimum ( ) for such 



organism. 



VOLVOX 



All unicellular animals which have whip-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 animals 

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

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

 found as many as 22,000 cells in a single colony. There is a division 

 of labor in the colonies, for the various cells are not all alike, though 



