This book is the second volume of two books about sea urchins (echinoids) since 1953 (Th. Mortensen). The detailed and coloured illustrated descriptions enable the reader to identify all species living today in our oceans. Recent scientific searchings are included. Volume 2 describes the Irregularia with bilateral symmetry, including the well known "sand dollars" and "heart urchins", living mostly more or less deeply burrowed in the substrate.