This book provides a comprehensive synthesis of the range of biological information that can be retrieved from the fossilised skeletons of echinoids. The first half consists of a detailed explanation of the functional significance of skeletal structures and the inferences to be drawn from them about the living animals. This is followed by a discussion of the important evolutionary changes that have taken place and why they may have occurred. Finally, there is a section on how echinoids relate to other echinoderm groups.