The ancestry of arthropods is uncertain. From one common ancestor or from several ancestors, arose the arthropod sub phyla, 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 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.