Differences between sea urchins and sea cucumbers?

Echinoderms are a category of sea invertebrates; there are 5 groups of echinoderms, Asteroidea, Crinoidea, Echinoidea, Holothuroidea and Ophiuroidea. A sea cucumber is a holothuroidea; their lengthy body surrounded by leathery skin identifies holothuroideas. A sea lily is a crinoidea; crinoidea feed off the seabed with a mouth at the top of their body.