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. Notostraca The notostracans are the tadpole shrimps, of which there are fewer than 15 described species worldwide. They inhabit quiet, fishless, usually temporary, freshwaters where they crawl over the bottom or swim in the water. They are often common in rice fields.