Echinoidea

Echinoidea is a class of echinoderms that includes the familiar sea urchins, sand dollars, and heart urchins. They are characterized by a calcareous endoskeleton called a test, formed from fused plates, fivefold radial symmetry, and a system of movable spines and tube feet used for locomotion and feeding. Most adult echinoids lack arms and have their mouths oriented on the underside.

The test bears ambulacral and interambulacral zones with pores for tube feet. On the oral side is the mouth equipped with Aristotle’s lantern in many regular echinoids, a complex jaw apparatus for chewing algae and detritus. The anus is on the aboral side near the top. Pedicellariae, tiny pincer-like appendages, defend the surface and help keep the skin clean.

Echinoidea includes two major evolutionary lineages: regular echinoids (sea urchins) with globose or elliptical tests and a pronounced oral surface, and irregular echinoids (sand dollars and heart urchins) with a more bilateral body plan and often a burrowing habit. Within these, groups such as Cidaroida (primitive pencil urchins) and other euechinoids are recognized. Spines vary in length and composition and can serve defense, locomotion, or camouflage.

They are marine organisms found from shallow coastal areas to deep oceans, typically herbivorous or detritivorous, grazing on algae or consuming sediment. Their life cycle is often slow but flexible, with a free-swimming larval stage (commonly echinopluteus) that metamorphoses into a benthic juvenile.

The Echinoidea have a rich fossil record dating back to the Ordovician, with tests and spines preserving well. They are important for biostratigraphy and marine paleoecology, and they occur worldwide in diverse habitats.