Echinoids, or sea urchins (oursins [French], Seeigel [German], erizos de mar [Spanish]) constitute a group of exclusively marine invertebrates inhabiting the intertidal down to the deep-sea trenches. They are characterized by a globose or flattened skeleton known as a test. It is important to note that this is not a shell as in mollusks, because the test is mesodermally derived, and actually internal to the ectoderm, which in the form of epithelium covers the entire test and in fact the spines and other external appendages mounted upon it. The test is in turn made of stereom, the specialized manifestation of calcium carbonate endoskeleton unique to the echinoderms. The test comprises a corona, plus a peristomial region around the mouth, and a periproctal region around the anus. The corona also supports a variety of mobile appendages, including spines.
Sea urchins come in a variety of different shapes representing adaptations to specific habitats and feeding strategies. Most readers will be familiar with the globose forms covered with longish spines usually several centimeters long. Often referred to as "regular" urchins because they exhibit five-part radial symmetry in which the anus is located at the summit of the body, these forms can be extremely common along rocky shores all over the world. They are also ubiquitious members of the benthos even at the greatest ocean depths. Although once used taxonomically, the term "regular" is now regarded as an informal, functional description because it does not describe a natural grouping. In contrast, the irregular urchins, denizens of sand and mud bottoms, form a natural (monophyletic) group. The irregular echinoids, although still exhibiting a basic five-part radiality, have secondarily acquired bilateral symmetry superimposed upon the radial symmetry when the anus evolved from the summit of the body towards what is now functionally defined as the posterior end of the animal. The spines of irregular echinoids are extremely miniaturized to form a dense felt which facilitates locomotion and burrowing, as well as feeding.
Today two large groups of irregular sea urchins exist, the heart-shaped Spatangoida (heart urchins) and the disc-shaped Clypeasteroida (sand dollars and sea biscuits). Less well known are the Holasteroida, which largely migrated to the deep sea during the Upper Cretaceous (about 70 million years ago) and which include the most bizarre of extant echinoids — the amphora- and vase-shaped pourtalesiids. Other minor types of irregular urchins include the holectypoids and "cassiduloids" (also not a natural group), which comprise a handful of species that essentially represent living fossil remnants of groups once dominant in echinoid faunas of the Mesozoic.
Currently there are a slightly more than 1000 valid extant species known from the World Oceans, including Arctic and Antarctic seas. Taxonomic coverage of the database includes all currently known extant echinoid genera and species, as well as most of the valid fossil echinoid genera. Extinct species, which surpass modern echinoid diversity by about an order of magnitude, formerly included in the database only when they have erroneously been reported from modern faunas, are now part of a major effort to integrate them with the data on extant taxa.
The information contained in the World Echinoidea Database (WED) derives largely from Mortensen's monumental Monograph of the Echinoidea (1928-1951), updated by the data contained in the Index of Living and Fossil Echinoids by Kier & Lawson (1978) covering the years 1925-70 and Kroh (2010) covering the years 1971-2008. Information on the distribution, ecology, and physiology are still largely missing from WED, but are included progressively as the database is complemented with data from additional, detailed studies. An effort has been made to cross-reference the data contained in the World Echinoidea Database with Andrew Smith's Echinoid Directory, an indispensible resource for echinoid taxonomy and identification. The classification used in the WoRMS and the World Echinoidea Database is that of Kroh & Smith (2010).
Please inform the editors, Andreas Kroh (NHM Vienna) and Rich Mooi (CAS), of any omissions or errors you may come across and thus help us improving the quality of the database.
Version History

1. Version 1.0 (July 2010): first version published including all accepted extant species of echinoids published before 2010 (~ 1000 species) and all echinoid genera (extant & fossil)
2. Version 2.0 (April 2011): revised version now including synonyms and unaccepted combinations of extant echinoid taxa (> 1800 additional names)
3. Version 3.0 (April 2014): revised version now including names of fossil echinoid taxa between 1925 and 1970 and their original description references (~ 2500 additional names & 650 sources from Kier & Lawson (1978))