UNICELLULAR FORMS 55 



constant shape and move by means of fine fibre-like structures ex- 

 tending from the cell wall called cilia. The class has been named 

 INFUSORIA (Fig. 30). 



The third class also has a constant shape, but the animals move 

 by means of one or more whip-Uke flagell^, attached either to the 

 forward or rear end of the animal, that by a whirling motion 

 pull or drive the organism through the water. These are the mas- 

 TiGOPHORA (Fig. 29). 



The fourth class, the sporozoa (Fig. 31), are not active but may 

 at times move by pseudopodia, or in other periods of their lives 

 by means of flagellae. They are never found free in Nature but al- 

 ways living within the bodies of other animals. They increase in 

 number by multiple division, a single cell thus giving origin to 

 many daughter organisms, or spores, simultaneously. 



Amoeba. The common types of amoebae are free living animals 

 found in partially decaying infusions of vegetable material. They 

 may be obtained in numbers by collecting water plants, together 

 with some mud from a stagnant pool, and allowing the mass to 

 decay in water. Although tending toward a spherical or flattened 

 disk shape, the outline of the amoeba is not fixed but depends on 

 the state of motor activity of the animal (Fig. 12). Under the 

 microscope an active amoeba appears to be flowing across the field 

 of vision; numerous pseudopodia may be seen. The protoplasmic 

 contents appear to be flowing into these extrusions, extending them 

 and bringing about a shift in the position of the animal. The sur- 

 face of the amoeba is covered by a thin, non-living pellicle or layer, 

 under which is a region of protoplasm slightly denser and more 

 refractive to light than the deeper portion, known as the ectosarc 

 (Fig. 12), made up of ectoplasm. The more fluid inner cytoplasm 

 is known as the endos.\rc, made up of endoplasm. In the endoplasm 

 may be seen considerable numbers of cavities, the food or gastric 

 vacuoles, each containing a bit of food. A large liquid-filled vacuole 



