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The body of hear urchins is shaped a bit like a Valentine heart lying on its back, with the indented end pointing forward. The mouth is toward that end, on the bottom of the urchin below the indentation, so that the urchin has a definite front end, with the anus at the back. Because these displacements destroy the nice five-part symmetry seen in “regular” urchins, these are logically known as “irregular” echinoids.
Lives buried in the sand. At night, a species in the genus Maretia emerges from the sand where it buries itself during the day, and marches in comical herds across the bottom in search of food. With the long trailing spines and amazingly rapid motion, it’s not difficult to see how these became known as sea mice.
Info extracted from: http://scientistatwork.blogs.nytimes.com...