Holasteroida is an order of irregular sea urchins.

These irregular sea urchins are characterized by a particularly marked bilateral symmetry, including for the apical system, which is highly elongated.

In some contemporary abyssal groups such as Pourtalesiidae, some species are even bottle-shaped.

The anus (periproct) has migrated towards the periphery of the test.

This article about a sea urchin is a stub.