This book is the first of four volumes in the Handbook of Zoology series, focusing on the morphology, anatomy, reproduction, development, ecology, phylogeny, and systematics of Annelida. It addresses members of the basal radiation and the initial part of Sedentaria, including chapters on the history of annelid research, their fossil record, and an introduction to their phylogeny within the tree of life. The phylogeny chapter reviews the history of their systematics, highlighting significant recent advancements that have reshaped our understanding of annelid evolution. The most basal annelids, now classified as Palaeoannelida, consist of two families of atypical polychaetes, previously thought to be highly derived, offering new insights into the ancestral annelid's appearance. These basal forms lack key features like nuchal organs and possess simpler nervous systems, indicating primitive traits. Within this basal radiation, the Sipuncula, a taxon of unsegmented and achaetigerous animals, is included, along with Lobatocerebridae, another group of platyhelminth-like annelids. The Sedentaria section begins with Orbiniida, characterized by elongated, thread-like worms without anterior appendages, often found in marine sands and believed to have evolved through progenesis. The second higher taxon, Cirratuliformia, includes nine families of sedentary polychaetes, showcasing significant variation in body plans while often lacking typi
