Irregularia

Irregularia is an extant infraclass of sea urchins that first appeared in the Lower Jurassic.

== Description and characteristics == These particular sea urchins are distinguished from other sea urchins by their irregular shape: the anus and often even the mouth are at the two poles of the test, creating a bilateral symmetry instead of the classical 5-fold symmetry of echinoderms. The group includes the well known heart urchins, as well as flattened sand dollars, sea biscuits and some other forms. Most of them live inside the sediment, moving in thanks to their particular spines, and feed on its organic fraction.