368 GENERAL ZOOLOGY 



the organs of metazoans. Examples of such structures, not all 

 of which are present in every protozoan, are the following: one or 

 more contractile vacuoles, by means of which a rudimentary circu- 

 lation is maintained; a mouth or cytostome, where food enters; a 

 number of food vacuoles, where food is digested; an eyespot that is 

 sensitive to light; and organs of locomotion, such as pseudopodia, 

 cilia, and flagella. Protozoa occur practically everywhere — in 

 water, soil, air, and in or on the bodies of other animals. In the 

 air they are carried in an encysted form that prevents desiccation 

 and permits them to be distributed by air currents. As para- 

 sites they cause a number of diseases in man and other animals. 

 SUBPHYLUM 1. PLASMODROMA. Includes Protozoa hav- 

 ing pseudopodia, or flagella, or entirely without organs of 

 locomotion. A pseudopodium is a finger-shaped or filamen- 

 tous extrusion of the cytoplasm, usually of a temporary nature. 

 A flagellum is a permanent, threadlike, and vibratile process of 

 the cytoplasm. 

 CLASS I. MASTIGOPHORA. Protozoa whose motile organs 

 consist of one or more flagella, which by whipping movements 

 draw or propel the animal through water. 

 SUBCLASS 1. PHYTOMASTIGOPHORA. Plantlike flag- 

 ellates. 



Examples: Volvox globator, a colonial form (Fig. 203); 

 Euglena viridis, a common pond form; Ceratium, an armored 

 form, found in fresh and salt water; Noctiluca, a luminescent 

 marine form (Fig. 204). 



Euglena viridis has a fusiform body, colored green by masses 

 of chlorophyll (chromatophores) and is provided with a long 

 flagellum at its anterior end. The body is enclosed in an 

 elastic pellicle which permits contractile movements but other- 

 wise holds the body in a more or less definite shape. Near the 

 anterior end of the body is a light-sensitive organ, the eyespot, 

 or stigma, which contains red pigment. The nucleus is single, 

 oval in shape, and centrally located. The presence of chloro- 

 phyll indicates that photosynthesis is possible, but there is also 

 a gullet or cytopharynx, a small depression at the base of the 

 flagellum, through which food particles are probably taken. 

 Thus Euglena seems to utilize the nutritional methods both of 

 plants (holophytic nutrition) and of animals (holozoic nutrition). 

 It is also thought that Euglena may to a certain extent be 



