This volume is the first of two books on recent sea urchins (class Echinoidea). The descriptions with illustrations enable the reader to identify most of the species living today in our oceans. Recent scientific results are included. Vol. 1 treats the sea urchins with pentameral symmetry and includes the cidarids and the regular Acroechinoidea. Living on the sea floor their habitat reaches from the intertidal zone with heavy surf to the abyssal.