Class Echinoidea, the sea urchins, have a rounded calcareous 'test' made up of five interlocking plates. The mouth apparatus, called aristotle's lantern, is found on the bottom towards the substrate. Urchins are opportunistic feeders preying on anything from organic particles, seaweed, to fish. Some species have spines, which can be poisonous, and they walk around using their hydraulic tube feet. Echinoids live in primarily marine habitats with some species known to burrow under sand or hide within rock crevices.
Durham, J. W. et al., 1966. Echinoids in Moore, R. C., ed., Treatise on Invertebrate Paleontology, Part U, Echinodermata 3(1), Volume 1. The University of Kansas and Geological Society of America. 366 pp.