Smith, Andrew B. Department of Paleontology, Natural History Museum, London, United Kingdom.
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An extinct order of primitive irregular sea urchins of the class Echinoidea, which retain a functioning jaw apparatus throughout life. Although the periproct opens on the oral surface in the posterior interambulacrum of members of the Holectypoida, the test still shows considerable radial symmetry. The mouth is large, circular, and centrally positioned, and is indented by sharp buccal notches. The jaws have stout teeth that are wedge-shaped in cross section. Ambulacral plating is compound adorally (near or toward the mouth), but usually simple adapically (near or toward the apex or tip); the tube feet and their pore pairs are undifferentiated. Tubercles are arranged radially on the oral surface, and tuberculation is distinctly finer adapically. The holectypoids are distinguished from other primitive irregular echinoid groups by the presence of a fifth genital plate in the apical disk, which is perforated by a gonopore in more advanced forms. See also: Echinodermata; Echinoidea
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