166 REPRESENTATIVE SINGLE-CELLED ANIMALS 



flagellated stage in their life cycles. The Mastigophora are more 

 primitive in their metabolic activities, since they may combine 

 the modes of nutrition of both animals and plants even within the 

 same individual. Nevertheless, the amoebas and their relatives 

 are fairly called the simplest of all organisms that exhibit the 

 typical animal functions. 



The Mastigophora 



Structure of Euglena. — The Mastigophora include types that 

 are distinctly plant-like in their appearance and manner of nutri- 

 tion, and others that resemble animals. The genus Euglena 

 (Fig. 91) is representative, because in a measure it combines the 

 characteristics of the plant-like and the animal-like mastigo- 

 phorans. What follows applies to Euglena viridis and E. gracilis, 

 two of the commonest forms, and also to the other species of this 

 genus that are abundant in fresh water. The cell in typical 

 euglenoids, as members of the Family Eugenoidina are often called 

 (Fig. 91 A), is covered with an outer layer of ectosarc, suflaciently 

 stiff to preserve the contours of the organism as it swims through 

 the water, but flexible enough to allow the extensions and con- 

 tractions that are termed "euglenoid movements" (Fig. 91 B). 

 It is quite characteristic of the euglenoids to have spiral markings 

 of various sorts upon the outer surface of the ectosarc. In some 

 species these are highly developed. When seen in what might be 

 termed the "lateral" view (Fig. 91 C), the anterior end of the 

 euglenoid shows a mouth-like opening, the cytostome, with one 

 side suggesting a projecting upper lip. However, the cytostome 

 is not so much a slit between the lips as it is an oval opening leading 

 into the gullet. The flagellum, a whip-like structure used in 

 locomotion, extends from the gullet into the water. At one side of 

 the gullet is a vacuole system consisting of a reservoir which drains 

 into the gullet and is surrounded by contractile vacuoles (Fig. 91 A). 

 An excretory function can perhaps be ascribed to these vacuoles, 

 as with the vacuoles of the Sarcodina. Also near the anterior end, 

 is a mass of red pigment (Fig. 91 C and C), called the eye-spot 

 because it is probably the part that is most sensitive to light; 

 although it would be diflficult to prove this beyond question. 

 Euglenoids that regularly ingest food particles and pass them into 

 the cell in food' vacuoles have a more highly developed gullet region 

 than Euglena viridis. 



