sea urchin

noun

: any of numerous echinoderms (class Echinoidea) that are usually enclosed in thin brittle globular tests covered with movable spines

Examples of sea urchin in a Sentence

Recent Examples on the Web

Perhaps the most intriguing fossils are the ones that potentially represent deuterostomes, the animal group that includes vertebrates, as well as starfish and sea urchins. —Mindy Weisberger, CNN Money, 4 Apr. 2026

And there’s one additional example of what appears to be a fatter worm, and another four fossils of a creature that appears to be near the base of the group that includes echinoderms (think sea urchins and their relatives). —ArsTechnica, 2 Apr. 2026

Silver Sea will offer $200 sea urchin or $160 wagyu. —Ella Gonzales april 1, Fort Worth Star-Telegram, 1 Apr. 2026

Echinus Geyser is about 66 feet wide, with a spiky rim made of silica, and is surrounded by rocks that resemble sea urchins. —Munis Raza, Interesting Engineering, 4 Mar. 2026

Word History

First Known Use

1591, in the meaning defined above

Kids Definition

sea urchin

noun

: any of a class of small marine invertebrate animals that are echinoderms, usually live on or burrow in the sea bottom, and are enclosed in roundish shells covered with spines that can move