Fossil records of cephalocaridans have been found in the Ordovician Castle Bank site.

[3] These are hermaphroditic and pigmentless crustaceans with an elongated and translucent body that measures 2 to 4 mm (0.079 to 0.157 in) in length.

The first pair of maxillae is very small, and the second pair has the same structure as the following thoracic legs: a large basal part, equipped with outgrowths on the inner side, used in locomotion, a forked inner branch and two outer lobes - referred to as the "pseudoepipod" and the "exopod".

[8] Cephalocaridans are found from the intertidal zone down to a depth of 1,500 m (4,900 ft), in all kinds of sediments.

To bring in food particles, they generate currents with the thoracic appendages like the branchiopods and the malacostracans.