In the following chapters the most common fresh-water forms, which are visible without a hand lens, are described. Not all the questions you may wish to ask about them are answered. You will find answers to some of the questions in other reference books. You will not find answers to all the questions you may wish to ask in any book. Many authors, the present writers included, leave out all reference to matters about which there is considerable doubt. Others will tell you frankly that little or nothing is known about the subject they have omitted, and that information on this point is yet to be collected and recorded.

This lack of complete information makes the study of fresh-water life doubly interesting, for while the student is learning facts which scientists have already discovered, he may, if he keeps a careful record of his observations, make a contribution to the sum of the knowledge of this particular subject.