CHAPTER II 



GENERAL SKETCH 



IT is a widely accepted opinion among men of science that life 

 originated in the sea, and here to-day are found the great majority of 

 species of Protozoa. In the littoral regions, particularly in the super- 

 ficial slime and upon submerged water-plants, are found a profusion of 

 Rhizopoda. Farther out, Radiolaria and shelled Rhizopoda belong- 

 ing to the order Reticulariida float upon the surface or at varying 

 depths below it, while their empty shells, settling slowly to the bottom, 

 have added little by little to the accumulations of the past, until to-day, 

 under the names Radiolarian ooze and Globigerina ooze, they form 

 vast areas, miles in extent, and often attaining a depth of many feet. 

 By the agency of earthquakes or slow upheavals, these beds have 

 become exposed from time to time, and we recognize the Barbadoes as 

 composed in large part of the skeletons of Radiolaria, or the chalk 

 cliffs of England as built up of the lime shells of reticulate Rhizopoda. 



Apart from the Sarcodina, the majority of Protozoa leave no memo- 

 rial in stone of their past existence. Pelagic forms such as Dinofla- 

 gellidia and Cystoflagellidia, living near the shores, and often drawn 

 together into great aggregates by currents, winds, etc., become the 

 food of whales, fishes, and other marine animals. Many Rhizopoda, 

 Ciliata, and Suctoria are attached by mineral secretions, or by stalks, 

 to rocks, submarine plants, etc. Others are parasites upon the out- 

 sides of fish and other animals, while still others are parasites within. 



The fresh-water Protozoa, while less rich in species, are much 

 better known than the marine forms, for their modes of life, habitats, 

 and life histories are more easily observed and controlled. Many 

 kinds of Rhizopoda, Heliozoa, and Ciliata are found both in fresh 

 water and in salt ; and numerous experiments by Verworn, Gruber, 

 and others have shown that some forms can live either in salt water 

 or in fresh ; the change from one to the other usually results, how- 

 ever, in modifications of structure. In 'general, the Protozoa abound 

 in fresh water which contains enough food material for their growth 

 and reproduction, but the widespread belief that each drop of drink- 

 ing water contains countless myriads of microscopic forms has 

 absolutely no foundation. Protozoa cannot live in chemically pure 

 water, and on the other hand, many of them cannot live in foul waters. 



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