Cidaroida

Article By:

Pawson, David L Department of Invertebrate Zoology, Smithsonian Institution, Washington, DC.

Last reviewed:October 2019

DOI:https://doi.org/10.1036/1097-8542.900225

An order of sea urchins (phylum Echinodermata, class Echinoidea) in which the 20 columns of plates in the essentially spherical test consist of five pairs of narrow and numerous ambulacral plates and five pairs of broad and few interambulacral plates. In Cidaroida (Fig. 1), the ambulacrals are simple, with a single pore-pair to each plate, and these plates continue across the peristomial membrane, which is fully plated. The interambulacrals are large; in most cases, each interambulacral carries a prominent primary tubercle. In most cidaroids, the tubercle carries a central perforation; a ligament runs from this perforation to a corresponding hole in the base of a primary spine. The tubercle sits near the center of a shallow depression—the areole. Surrounding the areole is a ring of small scrobicular tubercles, which support scrobicular spines. A primary spine consists of a large shaft, which is usually more or less straight; it can be cylindrical or flattened, or sometimes paddle-shaped, and it may carry small to large thorny spinelets. Only two types of pedicellariae (pincerlike structures)—globiferous and tridentate—are present. The Aristotle's lantern (a five-sided feeding and locomotor apparatus surrounding the esophagus) is of the aulodont type, with teeth that are longitudinally grooved, not keeled, and having a very shallow foramen magnum and with the epiphyses not in contact with each other. The apical system surrounding the anus in the middle of the upper surface consists of five genital plates alternating with five ocular plates. A typical cidaroid (for example, Goniocidaris) may have a skeleton consisting of more than 3000 pieces. See also: Echinodermata; Echinoidea

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