Class Maxillopoda Dahl, 1956
Members of the class Maxillopoda are distinguished by having 5 cephalic, 6 thoracic, and usually 4 abdominal somites and a telson; thoracomeres variously fused with cephalon; usually with caudal rami; thoracic segments with biramous (sometimes uniramous) limbs, lacking epipods (except in many ostracods); abdominal segments lack typical appendages; (reductions of this basic 5-6-4 body plan are common); carapace present or reduced; eyes variable, but naupliar eyes (when present) unique, with three cups, each with tapetal cells (= maxillopodan eye).
Although the class Maxillopoda is accepted by most specialists as a valid monophyletic taxon, there is still some controversy about its diagnosis and component groups. Included are the taxa Mystacocarida, Ostracoda, Copepoda, Branchiura, Tantulocarida, and Cirripedia.
A warning is given here that different specialists sometimes interpret the nature of maxillopodan tagmata in different ways, leading to some confusion. Inclusion of members of the subclass Tantulocarida is controversial because of the six- or seven-segmented abdomen in the juveniles of some species.
Maxillopodans are mostly small crustaceans, barnacles being a notable exception. They are recognizable by their shortened bodies, especially the reduced abdomen, and absence of a full complement of legs. The reductions in body size and leg number, emphasis on the naupliar eye, minimal appendage specialization, and certain other features have led biologists to hypothesize that paedogenesis may have played a large role in maxillopodan origin. That is, in many ways they resemble early postlarval forms that evolved sexual maturity before attaining all the adult features.
Brusca and Brusca, 1990