what's the niche of an enichoderm?

Answer 1

You surely mean echinoderms

like star fish and sea urchins

They are an old and diverse phylum of sea creatures with 5-fold symmetry. The sessile ones tend to feed on marine plankton or decaying debris whereas the motile ones can prey on molluscs and algae.

Some are predators. Some are more towards the saprophytic end of things.