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. The brain is tripartite. Compound eyes are frequently present. Most crustaceans are gonochoristic. Branchiopoda Branchiopoda is an ancient group of primitive crustaceans found today primarily in inland, often temporary, waters such as snow-melt pools, wet weather ponds, and saline lakes.