MONOCELLULAR ANIMALS-THE PROTOZOA 121 



ing them in a group larger than the phylum, chlorophyll. Because it is able to live on the 



that is, a subkingdom. simple elements that plants utilize, and at 



The Protozoa are divided into four the same time possesses certain animal char- 

 classes, based on their means of locomotion, acteristics of behavior, euglena is thought 

 In the probable chronological order of their to be intermediate between the plant and 

 appearance in the evolution of life on earth, the animal world, and is frequently re- 

 they are: class Mastigophora — those that f erred to as a plant-animal type, 

 move by means of flagella; class Sarcodina The anterior end of euglena is usually 

 or Rhizopoda — those that employ pseudo- more blunt than the posterior end, in fact, 

 pods; class Sporozoa — those that have no the latter is pointed in some species. There 

 clearly defined method of locomotion; fi- is a gullet at the anterior end, the walls of 

 nally, class Ciliophora — those that move by which give ri-se to the fiagellum, and lying 

 means of cilia. The student is already famil- near the gullet is the contractile vacuole. 

 iar with representatives of two of these In the immediate region of the gullet is the 

 classes, the Sarcodina (amoeba) and the stigma or eye-spot, a conspicuous red dot 

 Ciliophora (paramecium). Undoubtedly which apparently functions in aiding the 

 there was some overlapping, and there is no cell to find the proper light intensity for 

 assurance as to which types actually pre- photosynthesis. In order to manufacture its 

 ceded which others, except in a general way. own food, euglena must receive the proper 

 For our purposes the order above will be amount of light, hence the significance of 

 followed, that is, we shall consider the Mas- the stigma. Euglena can, however, live in 

 tigophora as the most primitive and the the dark providing nutrient materials are 

 Ciliophora as the most complex. present in the surrounding medium, in 



which case it absorbs its food directly. 



Class Mastigophora Under favorable conditions euglenas are fre- 



This is a widely diverse group of Proto- quently found in such numbers as to pro- 

 zoa in which some members are colored duce a green scum at the surface of the 

 and live independently like plants, whereas water. As some species grow old they pro- 

 others are colorless and require food from duce a red pigment called haematochrome 

 the outside, such as parasites living in the in their cytoplasm. If this happens when 

 intestinal tract of termites. they exist in great numbers, a visible red 



Colored flagellates. Although there are layer appears on the surface of the water, 



wide divergences in structure and habitat This has sometimes given rise to the name 



of flagellates, a brief description of Euglena "bloody pools." 



(Fig. 7-14) will help in understanding the Euglena divides longitudinally, that is, in 



group as a whole. an anterio-posterior direction, splitting the 



Anatomically, euglena is quite different cell into two equal parts (Fig. 7-14). In a 

 from amoeba. It has a rather definite gen- rapidly growing culture, cells can be ob- 

 eral shape, which is something like a spin- served in all stages of division. Because this 

 die, although it is sufficiently elastic to protozoan can be grown in sterile cultures, 

 be able to undergo animal-like movements that is, free from all other microorganisms, 

 when confined to a small space (Fig. 7-14). it has been used in experimental work in 

 It moves by means of a single, hair-like, an effort to determine its basic nutritional 

 vibratile fiagellum ( plural, ftagella ) which, needs. It is an example of the increasing use 

 when active, pulls the organism through the of Protozoa as experimental animals in fun- 

 water in a spiral path. In its cytoplasm damental biological research, 

 euglena bears bodies known as chloroplasts, Other colored flagellates. All kinds of 

 which contain the green plant pigment, fresh water as well as the oceans are teem- 



