Echinoida

Article By:

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

Last reviewed:December 2019

DOI:https://doi.org/10.1036/1097-8542.210800

An order of sea urchins (phylum Echinodermata, class Echinoidea). Members of the Echinoida (Fig. 1) are characterized by the following features: a camarodont lantern that has keeled teeth (Fig. 2), which are T-shaped in cross section; epiphyses enlarged and connected to each other in a continuous arc; a smooth test with relatively small tubercles, which are imperforate and noncrenulate; and ambulacral plates of the echinoid type, with the lowest element being the largest (Fig. 3). Five families are included in the Echinoida, distinguished mainly on the structure of the pedicellariae (pincerlike structures scattered over their surface). Members of the Parechinidae, with three genera, have globiferous pedicellariae with several lateral teeth and polyporous ambulacra (the radial series of plates along which the tube feet are arranged). Echinidae, with six genera, have a pair of lateral teeth on the globiferous pedicellariae. Echinometridae usually have a single asymmetrical lateral tooth; in six of the 13 genera, the test is elongate oval. Strongylocentrotidae, with five genera, have globiferous pedicellariae with or without teeth, but the ambulacral plates are polyporous in all genera, with the pore zones in arcs. In the nine genera of Toxopneustidae, the globiferous pedicellariae lack teeth and the ambulacral plates are usually trigeminate, but with up to five pore pairs in a few species. Approximately 95 of the 800 species of the class Echinoidea belong to the order Echinoida. See also: Echinodermata; Echinoidea

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