Biramous appendages - appendages with 2 jointed branches from base. Trilobites and crustaceans Uniramous appendages are unbranched. Most biologists think monophyletic, have common ancestor with annelids. Similar embryo segments, nervous systems. If paraphyletic, marine ancestor may have given rise to chelicerates and crustaceans.
Velvet worms have characteristics of both arthropods and annelids and live in tropical rain forests.
Lived on sea bottom. Hard segmented exoskeleton with head, thorax, abdomen. Each segment had pair of biramous appendages of an inner walking leg and outer branch with gills Probably evolved into marine chelicerates like horseshoe crabs. Other marine chelicerates moved to fresh water, became arachnids, then arachnids moved onto land. Long tail used in locomotion.