The Protozoa not only claim the interest of the professional naturalist, but also that of a wider circle of nature students who, with the aid of the microscope, have always found here a fastcinating field for observations and research. In writing the present volume, embodying a summary of the more recent discoveries concering these minute animals, I have aimed to keep in mind the needs of the latter class of naturalists, as well as those who search more deeply in the unicellular organisms for the solutions of many morphological problems which remain unsolved in the higher animals, or for vital processes which afford a transition from the manifestations of life in its simplest expression to life as seen in the lower forms if invertebrates.