Sea anemones are among the most ancient of living metazoans. Long a paradigm of the "elementary nervous system" and constructional simplicity, sea anemones have been favourite experimental subjects in behavioural neurophysiology and in biomechanics. Their unique nematocyst toxins are used to study the sodium and calcium channels in excitable membranes, and their agonistic interactions provide insights into the evolution of allororecognition systems. Incorporation of algal endosymbionts by some of these animals combines in a single unit both primary producer and consumer, and provides a tight recycling of scarce nutrients between host and symbiont. Many species include vegetative proliferation or parthogenesis in their life cycle and are important organisms in the study of the evolutionary potential and adaptive demography of co-ordinated clonal growth and bisexual reproductive strategies. The ability of clonal anemones to recognize members of different clones upon contact provides insight into the evolution of immune systems. This book provides an introduction and synthesis of the biology of sea anemones. The text is cross-referenced and integrated, and together with a bibliography, should be valuable to those interested in the physiology, ecology, biochemistry, behaviour, population biology and evolution of this group of marine invertebrates. This book should be of interest to marine biologists, ecologists and zoologists with a general interest in themes such as symbiosis and life cycles.