the.com/sea urchin

means A small, round, spine-covered sea creature that crawls the ocean floor and is, under all those needles, mostly prized for the edible roe inside.

from The 'urchin' part is the old story here — in Middle English, 'urchin' meant hedgehog, borrowed from Old French 'herichon' (ultimately from Latin 'ericius,' hedgehog). Sailors and naturalists looked at this round, spiny sea animal and thought: hedgehog of the ocean. Only later did 'urchin' also come to mean a scruffy street kid, likely because hedgehogs were seen as small, bristly, and a bit disreputable — the sea creature kept the original hedgehog sense long after the word moved on elsewhere.