Phylum Protozoa 361 



served in some free-living forms but have never been demonstrated in 

 laboratory cultures of A. proteus. 



The amoeba shows various responses to changes within its en- 

 vironment. For example, it demonstrates avoiding reactions to sudden 

 bright light, touching with a hard object, or various chemicals. Posi- 

 tive responses are shown by the movement toward food and contact 

 with a solid object by a floating amoeba. 



Amoeba are usually found crawling about on the undersurfaces 

 of the floating leaves of aquatic plants in fresh water. In these fresh- 

 water habitats, other types of Sarcodina are also found. Some of these 

 are other species and genera of the naked forms, while still others have 

 shells or tests. 



Other Sarcodina. — Among the shelled forms, some of the com- 

 moner ones are the members of the genus Arcella (Fig. 115, C, D), 

 which look like miniature doughnuts when seen from the underside, and 

 Difflugia, which cement grains of sand together to build their domed 

 houses. In these same fresh waters may be found the interesting group, 

 the Heliozoa, or sun animalcules (Fig. \\S,A). These latter have 



Fig. 115. — Some members of the class Sarcodina. A, A heliozoan, Actino- 

 sphaerium sp.; B, a foraminiferan, Discorbis; C, top view of Arcella; D, side view 

 of Arcella; E, the radiolarian Thalassicola sp.; F, cyst of Endamoeba histolytica; G, 

 active form of Endamoeba histolytica. 



