The ancestry of arthropods is uncertain. From one common ancestor or from several ancestors, arose the arthropod subphyla, the chelicerates, the crustaceans, and the uniramians. They have in common the exoskeleton and jointed appendages. They also share the phenomenon called tagmatization, fusion of segments. As arthropods evolved from a segmented ancestor, they too fused parts. The chelicerates, represented by spiders, scorpions, etc., are distinct from other arthropods in the possession of pinchers for mouthparts instead of jaws. The crustaceans are represented by lobsters, crabs, shrimp, and barnacles. They are predominantly marine or aquatic. The crustaceans share with the chelicerates the feature of legs on the abdomen as well as on the thorax. Their two paired antennae are unique as is the biramous nature of most appendages. The Uniramia are represented by centipedes, millipedes and insects. The millipedes and centipedes are largely inhabitants of soil and detritus. The centipedes are mostly predatory, and millipedes are mostly herbivores. The insects are probably the most evolved. At some point the deuterostomes diverged from the protostomate ancestry. The deuterostomes were still coelomate but the mode of coelom formation was different. The most striking difference between deuterostomates and protostomates is the reversal of the fate of the blastopore. The protostomate blastopore became the mouth but the deuterostomate blastopore became the anus, and the mouth developed elsewhere. As the deuterostomates were different from the protostomates, so are the two phyla of the deuterostomates different from each other. The echinoderms differ greatly from the chordates. Echinoderms, though bilaterally symmetrical as larvae, are all radially symmetrical as adults. The echinoderms are entirely marine, filling their coelom with seawater.