GENERAL SKETCH 33 



Thus, in sewage, one finds occasional ciliates and flagellates, but no 

 such numbers as are sometimes found even in good drinking waters, 

 where, obtaining their food as do the green plants through the agency 

 of their green or yellow coloring matter (chlorophyl), the Protozoa 

 sometimes become a source of annoyance. They thrive in standing 

 waters where the accumulation of bacteria gives food for numerous 

 ciliates and rhizopods ; the decomposing organic matter dissolved in 

 the water is taken in by saprophytic forms of flagellates, which 

 multiply to prodigious numbers, and these in turn may form the food- 

 supply for predaceous Infusoria and Sarcodina. Rotifers, Crustacea, 

 molluscs, and worms prey upon all forms, and when the cycle is 

 passed, the water becomes cleared of animal life. In nature, the 

 pools rarely if ever become thus cleared, because new food is con- 

 stantly brought in from fresh sources, and the cycle becomes continu- 

 ous. In such places the superficial slime upon the bottom contains 

 naked and shelled rhizopods, although the latter are more often found 

 alive upon the leaves and stems of water-plants ; here, too, are colo- 

 nial Infusoria or single forms attached by their stalks. Suspended in 

 the water are to be found the majority of species of Flagellidia, a few 

 Dinoflagellidia, the majority of Heliozoa, many predatory ciliates, and 

 a few rhizopods, especially certain shelled forms which secrete a 

 bubble of gas to buoy them up. 



Many forms of Protozoa are capable of sustaining life either as 

 terrestrial or as parasitic organisms. The former, allied to the Myceto- 

 zoa, grow over damp wood, while a number of rhizopods are almost 

 able to withstand dryness, for as Dujardin, Ehrenberg, Greeff, and 

 others early pointed out, they live in damp moss and leaves of the 

 woods. Forms which have become adapted to a parasitic mode of 

 life may be found in all classes. Among the Rhizopoda, various 

 species of intestinal Amcebce may be found in all sorts of vertebrate, 

 and invertebrate hosts ; about twenty species of Flagellidia and many 

 more of Ciliata live as parasites, some in the blood, some in the intes- 

 tinal fluids, and others in the cavities of various organs in man and 

 other hosts. These forms, however, are only occasional parasites, 

 and are more like commensals than parasites, having little signifi- 

 cance when compared with the Sporozoa, a class of Protozoa which, 

 without any exceptions, are parasitic. These infest all animal 

 forms from Protozoa to man : one group lives in the digestive tract 

 and the cavities of the body (Gregarinida) ; another in the cells of the 

 digestive organs (Coccidiida) ; another in the muscle-cells and lymph 

 surrounding them (Myxosporidiida, Sarcosporidiida); and still another 

 in the blood-corpuscles and in the blood-plasm (Haemosporidiida). Of 

 all Protozoa these are the only forms which are known to menace 

 the life of man. 



