THE LARGE GROUP OF THE SMALLEST ()IU;\NISMS m 



case containing 26 of his microscopes, a gift which was later lost ( )„,. 

 of the few remaniing of the 419 lenses put up at auction after van 

 Leeuwenhoek's death was recently examined by an expert who 

 reported that the biconcave lens that he inspected "was very good 

 indeed" and proved that its maker had attained "a very high degree 

 of proficiency in grinding extremely small glasses." 



With the modern microscope of the college laboratory, infinitely 

 better work can be done than with this old pioneer. The best of \an 

 Leeuwenhoek's lenses are said to have magnified not more than 270 

 diameters, while the " high dry " power of the average modern micro- 

 scope gives a magnification of about 440 diameters, so that the college 

 freshman today has a far better physical equipment than did this 

 famous Dutchman. He also has much more. In the years that have 

 intervened between the time of van Leeuwenhoek and the present, 

 patient observations of minute forms of life ha\-c been made by 

 hundreds of scientists whose results may be found in these pages and 

 in other books suggested for collateral reading. With this intro- 

 duction the student might begin the study of simple organisms in 

 some such way as Antony van Leeuwenhoek did, by examining a 

 drop of pond water. 



Some Forms Found in a Drop of Fresh Water 



The pages that follow^ will serve to give us a slight acquaintance 

 with some of the simplest plant and animal forms that are likely to be 

 met in the examination of a drop of pond water or water from a 

 laboratory aquarium. Li addition to the unicellular organisms, 

 scores of other higher forms are likely to be seen. Countless protozoa, 

 including the many tiny species of monads, dart across the field of the 

 microscope ; others many times larger, with their highly specialized 

 cell parts, as Euplotcs or Stylonychia, may be found browsing on tiny 

 plants. Frequently one also encounters threads of the filamentous 

 algae, Zygneyna or Sjpirogyra, while debris, consisting of tiny bits of 

 wood, sand grains, and the glasslikc cases of diatoms and desmids. 

 may abound. 



Many tiny crustaceans, water fleas, and cojx'pods are usually 

 present, and in addition one finds the easily recognizable rotifers, 

 with their whirling wheels of cilia, their prominent grinding organ 

 or mastax, and their slender toelike posterior foot by means of which 

 they often become attached to .solid objects. Sometimes a small 

 roundworm may be found working its way through the dt^bris. while 



