Body Plans

Pancrustacea is a misnomer; the overwhelming number of species are not crustaceans—they are insects (Hexapoda). But most of the anatomical disparity in this subphylum occurs among the crustaceans. The preferred name would have been Tetraconata, based on an actual diagnostic feature—the presence of four cone cells in their eye ommatidia. The “crustacean” paraphyletic components of the subphylum form the main focus in this book. These form three monophyletic clades. Members of one clade have their gonopores located mid-body, somewhere on the trunk segments 6–8: Malacostraca, Copepoda, Thecostraca, and the extinct Cyclida. Members of a second clade, the oligostracans, have gonopores closer to the posterior aspect of the head on trunk segment 4. These include the parasitic branchiurans and their sister group, the tongue worms (pentastomids); the mystacocarids along with the Cambrian skaracarids; and Ostracoda. A third clade contains three groups: the freshwater branchiopods, the xenocaridans (remipedes and cephalocarids), and Hexapoda (insects). None of the classic so-called diagnostic features apply to all pancrustacean clades. Nevertheless, a great disparity of bauplan is evident in the subphylum.