Cidaroids, one of the two major sister clades of sea urchins, first appeared during the lower Permian (ca. 270 mya) and are considered to represent the primitive form of all living echinoids. This study of Cidaris blakei, a deep-sea cidaroid urchin with planktotrophic larvae, provides a description of development from fertilization through early juvenile stages and is the first report of a deep-sea urchin reared through metamorphosis. C. blakei resembles other cidaroids in its lack of a cohesive hyaline layer, the absence of an amniotic invagination for juvenile rudiment formation, and the presence of spines with a single morphotype at metamorphosis. C. blakei differed from other cidaroids in the presence of an apical tuft, the extent of fenestration of postoral skeletal rods, the shape of juvenile spines, and an extended (14-day) lecithotrophic stage prior to development of a complete gut. The development of C. blakei, 120 days from fertilization to metamorphosis, was protracted relative to that of shallow-water cidaroids. Preliminary work on temperature tolerances suggests that C. blakei larvae would be unable to survive the warmer temperatures higher in the water column and are therefore unable to vertically migrate.