428 EVOLUTION OF ANIMALS Part V 



can resist drying while in cysts or spores, but only for a time. This limitation 

 has not hindered their success. 



In spite of their remarkably long history of life in watery environments, 

 they are the most widely distributed of all animals, both geographically and 

 ecologically; they have found the greatest number and variety of homes. They 

 live in the upper soil along with hordes of bacteria, worms, and rotifers. They 

 swarm through the surface waters of the seas, both polar and tropical. The 

 luminescence of Noctiluca lights the surfaces of temperate as well as tropical 

 seas. Protozoans live in hot springs and in the snow and ice of the Rocky 

 Mountains, at times covering the glaciers with pinkish films. 



One of the largest protozoans 



Fig. 21.2. One of the largest protozoans, Spirostomiiin ambiguum, easily visible 

 to the naked eye. They look like white flecks against the dark bottoms of fresh- 

 water pools where they are occasionally abundant. Contractile vacuole (cv) con- 

 nected with a canal; (fv) food vacuoles; the macronucleus (M) is shaped like a 

 string of beads. (Courtesy, Jahn and Jahn: The Protozoa. Dubuque, Iowa, Wm. 

 C. Brown and Co., 1949.) 



Many live in the wet surroundings within the bodies of land or water ani- 

 mals, usually as parasites, sometimes only as passengers. Within flies, bees, 

 horses, cattle, and man protozoans can travel far and wide in the safety of a 

 fluid environment. 



Ways of Living. Protozoans live more or less independently. They are free 

 Hving or in loose association with plants and animals. 



Free-living ones, paramecia and others, ingest solid food — bacteria, diatoms 

 and other protozoans; some of them absorb food in solution through the body 

 covering. Those that contain chlorophyll — Euglena, Volvox, and others — 

 make their own food from inorganic material elaborated by photosynthesis 

 (Fig. 21.3). 



Associations. Colonies of Vorticella and Epistylis are attached to sub- 

 merged objects in ponds; to the naked eye they may seem to be patches of 

 mold, but through a lens they are like miniature gardens of nodding flowers. 

 Kerona creeps louse-like over hydra (Fig. 21.1). Green paramecia {Parame- 

 cium biirsaria) and green stentors {Stentor polymorphum) are colored by uni- 

 cellular algae (Chlorella vulgaris) that live within them. There are mutual 

 benefits in such associations; the protozoans receive food and oxygen from the 

 algae, and the algae secure protection from the protozoans. Wood-eating 



