The many small bumps (clearly visible in the bottom picture) cover the juvenile rudiment and develop into adult spines. The larger bumps of the juvenile rudiment are the developing rays of the seastar. Once the juvenile rudiment and the brachiolar arms are formed (which may be as early as 9-10 days) the larva is capable of metamorphosing. M. aequalis will only settle and metamorphose on a suitable substratum, such as a tube of the annelid Phyllochaetopterus. If no suitable substratum is available, larvae can delay metamorphosis for as long as 14 months (Birkeland et al. 1971).
Birkeland, C., Chia, F-S., Strathmann, R.R. 1971. Development, substratum selection, delay of metamorphosis and growth in the seastar, Mediaster aequalis stimpson. Biological Bulletin. 141:1, 99-108.