Fig. 1

a Diagram modified from [46] showing morphology of adult sea urchin test from adoral view. b SEM image showing aboral view of test with juvenile and primary spines Scale bar is 100 μm. b’ Close-up of b showing juvenile spines, ambulacral spine, and sphaeridia. b’’ Close-up of b showing the details of the interambulacra. Primary interambulacral spines articulate with the bosses of primary tubercles on interambulacral plates. b’’ Isolated primary spine. Scale bars in (b’–b’’) 10 μm. c Staging scheme showing example drawings and descriptions of each stage and the addition of morphological structures through the first 4 weeks of development. Drawings in c are based off of traces of fixed and stained individuals. Structures are color coded as in the figure legend. IA, interambulacral. Scale bars in c are 100 μm