Temporal progression of skeletal development in S. purpuratus larvae. A) Mean stage length as a function of skeletal stage. Error bars are one standard error of the mean. B) Cumulative time as a function of skeletal stage, starting at the onset of Stage 1 (first appearance of any skeleton in rudiment; see Table 2). Boxes indicate the approximate cumulative time interval during which a larva is in a given stage; error bars indicate 95% confidence interval.