Cell lineage and gene expression in the development of polychaetes -- Trochophora larvae and adult body regions in annelids: some conclusions -- Comparative structure of the epidermis in polychaetes (Annelida) -- Chaetae and chaetogenesis in polychaetes (Annelida) -- Sense organs in polychaetes (Annelida) -- Morphology of the nervous system of Polychaeta (Annelida) -- Muscular system in polychaetes (Annelida) -- The coelom and the origin of the annelid body plan -- Structure and development of nephridia in Annelida and related taxa -- Annelid sperm and fertilization biology -- Oogenesis and oocytes -- Pharynx and intestine -- Pogonophora (Annelida): form and function -- Myzostomida: A review of the phylogeny and ultrastructure -- Reconstructing the phylogeny of the Sipuncula -- Molecular phylogeny of siboglinid annelids (a.k.a. pogonophorans): a review -- Molecular systematics of polychaetes (Annelida) -- Evolution of interstitial Polychaeta (Annelida) -- Polychaete phylogeny based on morphological data — a comparison of current attempts -- Phylogeny of oligochaetous Clitellata.Recently, evidence has been accumulated which shows that some of the groups formerly regarded as independent "phyla" such as Pogonophora (now recognized as Siboglinidae), Echiura, Myzostomida and perhaps Sipuncula, are most probably nothing else than greatly modified Annelida. The extreme morphological diversity found especially in Polychaeta displays the plasticity of a simple segmented organisation that basically is nothing else but a serial repetition of identical units. Thus, annelids are highly important to our understanding of fundamental questions about morphological and adaptive diversity, as well as clarifying evolutionary changes and phylogenetic relationships. The book aims to summarize our knowledge on Polychaetes polychaetes and their allies and gives an overview of recent advances gained by studies that employed conventional and modern methods plus, increasingly and importantly, the use of molecular markers and computer-assisted kinship analyses. It also reflects the state of art in polychaete sciences and presents new questions and controversies. As such it will significantly influence the direction of research on Polychaeta and their related taxa.