Mooi, Rich Department of Invertebrate Zoology and Geology, California Academy of Sciences, San Francisco, California.
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The name originally given by Louis Agassiz in 1840 to a natural (monophyletic) group of echinoids (sea urchins) containing the heart urchins. Members of the order Spatangoida (spatangoids) constitute one of the most diverse sea urchin groups, with about 250 species in a dozen families. With a fossil record dating back to the Jurassic, the Spatangoida also contains more than 80 extinct genera. The most commonly encountered modern groups include the families Spatangidae, Loveniidae, Schizasteridae, and Brissidae. Like all sea urchins, the body, or test, of heart urchins (see illustration) is made up of a radiating series of columns of adjoined plates. The test of spatangoids is unlike that of “regular” sea urchins in being strongly bilaterally symmetric, with the anus at the posterior end. On the top, or aboral surface, of the test is a radiating set of four petaloids, with one in each of the paired ambulacral (tube-foot-bearing) regions. Petaloids are made of small, closely spaced, leaflike tube feet and function as gas-exchange organs (similar to gills). A small, star-shaped plate called the madreporite (or sieve plate) is located at the aboral convergence of the petaloids (the apical region); this structure is homologous with the madreporite found in other echinoderms. In between the petaloids, and very close to the madreporite, are the genital openings (gonopores). The configuration of the madreporite as well as the number and placement of the gonopores are important in the taxonomy of the heart urchins. See also: Echinodermata; Echinoidea; Regularia
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