Ostia are holes in heart wall that let blood in Small coelom around some organs. Segmentation allows specialization of body regions. Human muscles and nerves have segmented arrangement. 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.