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And isn't a phrase for a street kid, or homeless child, like a theater- term?

Sea urchins or urchins, sometimes called sea hedgehogs, are small, spiny, globular animals which, 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 5000 m deep.^[1] 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 mostly on algae. Sea otters, sea stars, wolf eels,triggerfish, and other predators feed on sea urchins. Their roe is a delicacy in many cuisines. The name "urchin" is an old name for the round spinyhedgehogs which sea urchins resemble.

Source > http://en.wikipedia.org/wiki/Sea_urchin