Smith, Andrew B. Department of Paleontology, Natural History Museum, London, United Kingdom.
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An order of irregular echinoids (sea urchins) of the superorder Atelostomata (class Echinoidea), with a strongly bilaterally symmetrical test. Members of the Holasteroida have a small, oval mouth (lacking buccal notches) that lies close to the anterior on the lower surface; a jaw apparatus is never present. Ambulacral plating is simple. The anterior ambulacrum is often differentiated, and petals are developed adapically (see illustration), except in deep-sea forms. The anus lies on the posterior face. The elongate apical disk, in which ocular plates II and IV abut along the midline and separate the anterior and posterior pairs of genital plates, is the most distinctive characteristic. In a few genera, the apical disk is split, with anterior and posterior parts being separated by intervening interambulacral plates. Phyllodes (petal-shaped ambulacra near the mouth) of penicillate feeding tube feet, with their associated characteristically large pores, are typically developed around the mouth. See also: Echinodermata; Echinoidea
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