“Sea urchins or urchins (/ˈɜːrtʃɪnz/), archaically called sea hedgehogs, are small, spiny, globular animals that, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. About 950 species of echinoids inhabit all oceans from the intertidal to 5,000 meters (16,000 ft; 2,700 fathoms) deep. The shell, or “test”, of sea urchins, is round and spiny, typically from 3 to 10 cm (1.2 to 3.9 in) across. Common colors include black and dull shades of green, olive, brown, purple, blue, and red. Sea urchins move slowly, feeding primarily on algae. Sea otters, starfish, wolf eels, triggerfish, and other predators hunt and feed on sea urchins. The name “urchin” is an old word for hedgehog, which sea urchins resemble.
Sea urchins are members of the phylum Echinodermata, which also includes sea stars, sea cucumbers, brittle stars, and crinoids. Like other echinoderms, they have five-fold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive “tube feet”. The symmetry is not obvious in the living animal but is easily visible in the dried test.
Specifically, the term “sea urchin” refers to the “regular echinoids”, which are symmetrical and globular, and includes several different taxonomic groups, including two subclasses : Euechinoidea (“modern” sea urchins, including irregular ones) and Cidaroidea or “slate-pencil urchins”, which have very thick, blunt spines, with algae and sponges growing on it. The irregular sea urchins are an infra-class inside the Euechinoidea, called Irregularia, and include Atelostomata and Neognathostomata. “Irregular” echinoids include flattened sand dollars, sea biscuits, and heart urchins.
Together with sea cucumbers (Holothuroidea), they make up the subphylum Echinozoa, which is characterized by a globoid shape without arms or projecting rays. Sea cucumbers and the irregular echinoids have secondarily evolved diverse shapes. Although many sea cucumbers have branched tentacles surrounding their oral openings, these have originated from modified tube feet and are not homologous to the arms of the crinoids, sea stars, and brittle stars.” https://en.wikipedia.org/wiki/Sea_urchin