The larval stages of the sea urchin, Strongylocentrotus purpuratus.

The adult body plan of Strongylocentrotus purpuratus is established within the imaginal rudiment during the larval stages. To facilitate the study of these stages, we have defined a larval staging scheme, which consists of seven stages: Stage I, four-arm stage; Stage II, eight-arm stage; Stage III, vestibular invagination stage; Stage IV, rudiment initiation stage; Stage V, pentagonal disc stage; Stage VI, advanced rudiment stage; and Stage VI, tube-foot protrusion stage. Each stage is characterized by significant morphological features observed for the first time at that stage. This scheme is intended as a guide for determining the degree of larval development, and for identifying larval and adult structures. Larval anatomy was visualized using light and confocal microscopy as required on living material, whole mount fixed specimens, and serial sections. Antibody staining to localize specific gene products was also used. Detailed analysis of these data has furthered our understanding of the morphogenesis of the rudiment, and has suggested provocative questions regarding the molecular basis for these events. We intend this work to be of use to investigators studying gene expression and morphogenesis in postembryonic larvae.