At least 40,000 species. Most are aquatic, found in both fresh water and in the oceans. Includes crayfish, lobsters, barnacles, crabs, shrimp. Body segments grouped into head , thorax , and abdomen . Gas exchange through a tracheal system.
Nitrogenous waste is uric acid thus conserving water.
Dominate all habitats except for the oceans. Some 13,000 species of centipedes and millipedes Chelicerata Anterior segments fused into a cephalothorax . There are no antennae. 8-legged scorpions, mites, ticks, spiders, daddy longlegs. Evolutionary relationships of the arthropods They are uncertain.