360 The Animal Kingdom 



consists chiefly of small protozoans or bits of algae, thus its nutrition 

 is holozoic. The pseudopodia engulf the smaller protozoans or algae 

 with some water and the food vacuoles are thus formed. As this food 

 vacuole circulates through the cytoplasm, digestion takes place. Indi- 

 gestible particles are thrown out at the surface. The digested material 

 becomes part of the cytoplasm. 



FOOOVACUOLC 

 K70PLASM. 



PSCUOOPOOIUM 

 NUCLCUS 



CNOOPLASM 



^CONTRACTILE 

 VACUOLE 



CX^- 



e^ 



Fig. 114. — Amoeba proteus. A, View of entire animal; B, movements. 



Respiration takes place directly through the cell surface, and meta- 

 bolic wastes produced by cell activity are likewise excreted through 

 the cell surface. 



The contractile vacuole functions to regulate the osmotic balance 

 of the animal. This organelle is present only in the free-living forms 

 dwelling in fresh water. Parasitic and salt water forms usually lack 

 them. Due to the higher concentration of salts within the amoeba, 

 water is continuously moving into the cell body. Unless there was 

 some special structure to remove the water, the animal would swell up 

 and burst. The contractile vacuole performs this important function 

 of maintaining proper osmotic balance. If salt water forms are trans- 

 ferred into fresh water, contractile vacuoles appear, thus indicating 

 that the action of these vacuoles is strictly a physical one. These vacuoles 

 can best be shown in the laboratory by allowing the slide containing 

 the animals to dry slightly. 



Amoeba ordinarily reproduces by means of binary fission. Before 

 division, the amoeba rounds up and produces many short pseudopodia. 

 Division occurs by means of mitosis. Resistant cysts have been ob- 



