The pluteus larva grows over the period of 4-6 weeks, till it is big enough and developed enough to metamorphose. Metamorphosis itself takes place in only a few hours time. The "skeleton" of the pluteus becomes the spines of the new sea urchin. The gut transforms into the test (shell) of the sea urchin.

Many thanks to Margaret and Greg for this contribution. If you use this animation, please give them credit.