Morphology and the classification of the so-called Cladocera

The validity as a monophyletic taxon of the group of branchiopod crustaceans long regarded as constituting the Cladocera is questioned. This seems in fact to be a heterogeneous assemblage whose members probably merit assignment to four separate orders. The Onychopoda and Haplopoda (the so-called Gymnomera) clearly stand apart from the Ctenopoda and Anomopoda (the so-called Calyptomera) and differ in important respects from each other. While sharing several characters, some doubtless indicative of distant ancestral similarities, others probably convergent, the Ctenopoda and Anomopoda differ in many respects, have clearly evolved along different lines, and are probably much less closely related than is generally assumed.