The main body cavity is an open circulatory system: blood is pumped into the haemocoel by a heart. Malacostraca have haemocyanin as the oxygen-carrying pigment, while copepods, ostracods, barnacles and branchiopods have haemoglobins. This indicates something very unusual: a phylum (or subphylum) with such a fundamental difference between the physiology of its member groups. 