The Cladocera are small, mostly microscopic animals fomiing an Order in one of the sub-classes of lower Crustacea to which the general name of Entomostraca was formerly applied. The keys found in the first edition of this book have been replaced by a series of drawings showing representative genera from each family, with explanatory notes in which attention is drawn to important features. The third edition is substantially unchanged to the second. A few corrections have, however, been made and a few new records and references added.