As the rudiment develops, adult features, such as tube-feet, begin to appear while larval structures, including the arms and even the larval gut and mouth, are resorbed and eventually lost. The newly metamorphosed juvenile sinks to the sea floor to begin life as a benthic adult. Sea urchins with a planktonic stage in their life cycle are termed planktotrophs . A schematic life cycle is shown below. These lecithotrophs produce a smaller number of much larger yolky eggs which are released into special holding chambers or brood pouches externally.
Fertilization is still external but the eggs and developing larvae remain with the mother.
These eggs develop directly into small adults without passing through a pluteus stage and without needing to feed in the surface waters. Lecithotrophic species are today largely confined to polar waters. Because female lecithotrophs produce large eggs and have specialized brood chambers they are often morphologically distinguishable from males of the same species. Sea urchins are primitively planktotrophic and only secondarily do they adopt a lecithotrophic life cycle.