Holasteroida

Holasteroida is an order of irregular sea urchins.

== Characteristics == thumb|upright=1.1|Ceratophysa ceratopyga by A. Agassiz (1881) 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 mouth (peristome) does not contain an Aristotle's lantern. The anus (periproct) has migrated towards the periphery of the test. The plastron is never amphisternous.