Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. There are c. 950 species of echinoids inhabiting all oceans from the intertidal to 5000 meters deep. Their shell, or 'test', 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, and red. They move slowly, feeding mostly on algae. Sea otters, wolf eels, triggerfish, and other predators feed on them. Their 'roe' (actually the gonads) is a delicacy in many cuisines. The name 'urchin' is an old name for the round spiny hedgehogs that 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 fivefold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive 'tube feet'.
Contributions by Dpbsmith, Debivort, and 18.104.22.168.
That would be considered sea cucumbers.
A sea urchin uses its many spines to protect itself, true.