ZOOLOGY. 



openings in the skeleton (Fig. 73). Reproduction is usually by division, 

 or by the formation of many spores. Encystment frequently occurs. 



Class II. Infusoria (in infusions}. Types: Paramecium, Stentor, Vor- 

 ticella. Predominantly active protozoa, usually without shell, but with 

 distinct cortical portion from which project permanent vibratile threads 

 of protoplasm (cilia, flagella, or tentacles), from the possession of which 

 the sub-classes are named. There is usually a permanent mouth. The 

 nucleus is always present and assumes a great variety of shapes. The 

 infusoria are typically free-swimming, but many are capable of attach- 

 ment by a contractile stalk, to foreign objects (V orticella) . Reproduc- 

 tion is normally by equal division, but budding and spore formation occur. 

 Conjugation is common, and may be either temporary or permanent. 



FIG. 73- 



FIG. 73. Actinomma, a radiolarian with a shell and no mouth. A, whole animal 

 with a portion of two spheres of shell removed. B, section, showing relation of proto- 

 plasm to the skeleton, c., central capsule; n, nucleus; p, protoplasm; o, openings 

 through which the pseudopodia extend. (From Parker and Haswell.) 



Class III. Sporozoa (spore animals}. Protozoa predominantly passive 

 in habit, parasitic, with no pseudopodia, and no cilia in the adult. Re- 

 markable for encysted resting stages and spore formation. Conjugation 

 often precedes the formation of the cyst. 



197. Place in Nature. Protozoa are an important element 

 in the food of many aquatic animals. Despite their minute 

 size, their immense numbers make them important. Together 

 with bacteria they serve to save for the organic world much 

 decaying material which no other animals could utilize. Rhi- 

 zopod shells dropping to the bottom of the ocean form the 

 " ooze," the chalk of later geological epochs. Other forms 

 of limestone also are produced by the accumulations of these 



