The Echinoidea are a class of the phylum Echinodermata. Present-day echinoids include the sea urchins. They are exclusively marine creatures, mostly burrowing, and have bilateral symmetry imposed on a more basic five way symmetry. The shell, or 'test' is made up of numerous calcite plates. Regular echinoids are described as 'epi-faunal'; moving about the sea floor on tube feet. Some echinoids developed a stationary or 'infaunal' lifestyle, a dominant bilateral symmetry as shown in the picture, and are called 'irregular'.

Echinoids have existed from the upper Ordovician to the present day and are used as index fossils for the Cretaceous.

The picture shows Micraster cortestudinarium, a common sea urchin found in the Cretaceous Chalk of Kent, England.