The coelom is reduced to small spaces associated with the gonads and kidney. The functional body cavity is a hemocoel. The blood vascular system is open and a dorsal, tubular, ostiate heart pumps blood to the hemocoel. Excretory and respiratory organs vary with taxon. The nervous system consists of a dorsal, anterior brain of two or three ganglia, circumenteric connectives, and a paired ventral nerve cord with segmental ganglia.
Development is derived with centrolecithal eggs and superficial cleavage.
There is frequently a larva although development is direct in many. Crustacea Crustaceans are primarily aquatic arthropods. The crustacean head consists of five fused segments and bears five pairs of appendages including two pairs of antennae. Appendages are primitively biramous. Excretion and osmoregulation are via modified metanephridia known as end sac organs, respiration is via gills or a permeable body surface.