Sea urchin (Echinoidea)
Like a hedgehog, they have globular bodied and many spines.
But, CT-scannd data indicates their pentagonal body structure, the main feature of Echinodermata.
Their mouths, also known as "Aristotle's lantern" , has pentagonal shapes too.
Sea star (Asteroidea)
They speak nothing and just sit down at the summer sandy beaches.
But their bodies star-shaped, straightly indicates the Echinodermata body plan.
Brittle star (Ophiuroidea)
The group has long arms to crawl the sea floor.
Almost genus don't live in the shallow sea, so maybe you think they are uncommon, but this is a misunderstanding.