They are believed to represent the most primitive members of their class, the Malacostraca, and first appear in the fossil record during the Cambrian period.

It covers the head and the thorax, including most of the thoracic appendages, and serves as a brood pouch for the developing embryos.

Also unique among malacostracans is their eight pairs of thoracic appendages which have been specialized into leaf-like filter feeding organs, and are not used for locomotion.

The eggs hatch as a postlarval, or "manca" stage, which lacks a fully developed carapace, but otherwise resembles the adult.

[13] The order Leptostraca is divided into three families, with ten genera containing a total of around 40 validly described extant species:[14]