The sea cucumber doesn’t look like much until you really look at it. They’re complicated little creatures. They have complicated little parts. In this case, they are labeled as: a – Tentacles, b – Cloaca, c – Ambulacral feet on the ventral side, d – Papillae on the back.
This diagram came from the classic encyclopedia Nordisk Familjebok, which you can peruse in this subsection here, on Wikimedia Commons.