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Holasteroida

Holasteroida is an order of irregular sea urchins.

Characteristics

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.

This order seems to have appeared at the lower Cretaceous.

List of families

According to World Register of Marine Species :

References