Arthropods are the largest most successful invertebrate group having now perhaps a million species. Arthropods have exoskeletons containing chitin, and these rigid exoskeletons must shed as their bodies grow. Arthropods are segmented into a head, thorax, and abdomen, but segments may be fused together as are the head and thorax, the cephalothorax, of crawfish and lobsters. Arthropods have jointed appendages. Gas exchange occurs direct through body surfaces in the smaller forms, as well as through gills, tracheae or book lungs that maximize respiratory surfaces. Blood that may or may not transport oxygen circulates within the open circulations of arthropods.

Modern Arthropod groups are three: Chelicerates, Crustaceans and the Uniramia. Chelicerates are spiders, ticks, mites, scorpions, horseshoe crabs and sea spiders. Many of these have book lungs, and most are terrestrial. Crustaceans are mostly marine, but some inhabit freshwater and a few are terrestrial: the fairy shrimps, water fleas, isopods, krill, crabs, shrimps, lobsters, copepods, barnacles as well as a small newly discovered group, the Remipedia. Uniramia include the centipedes, millipedes and insects. Uniramians have one pair of antennae and one or two maxilla, as well as a pair of mandibles. Uniramians breathe through their body surface, gills or tracheae.

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