Annelids offer a diversity of experimentally accessible features making them a rich experimental subject across the biological sciences, including evolutionary development, neurosciences and stem cell research. This volume introduces the Annelids and their utility in evolutionary developmental biology, neurobiology, and environmental/ecological studies, including extreme environments. The book demonstrates the variety of fields in which Annelids are already proving to be a useful experimental system. Describing the utility of Annelids as a research model, this book is an invaluable resource for all researchers in the field.

Annelida is a diverse group of animals, commonly referred to as segmented worms and currently comprising around 14000 described species. Found in most marine and freshwater areas, annelids have also successfully occupied many subterranean habitats. This volume documents annelid reproduction in the context of their phylogenetic relationships. It presents an introduction and overview to the current systematics of annelids and provides reviews to broad aspects of reproduction across Annelida. The chapters cover oogenesis, sperm, mating, early development, larval development and larval ecology. The book also covers some of the major clades (or purported clades) of annelids and addresses similar issues. The final chapter covers some of the more problematic annelid groups in terms of their phylogenetic placement.

In this Special Issue, we address the state of the art of the systematics of the main annelid groups and the improvements in the diversity they hold, with special emphasis on the latest discoveries in well-studied areas, expeditions to unsurveyed areas or environments, or the use of novel techniques that allow for the improvement of biodiversity knowledge. We are hoping that this Special Issue will provide a platform facilitating a review of current knowledge on the subject, identifying current research problems, as well as indicating directions and research trends for the future.

Annelida, mainly consisting of marine Polychaeta and in faunal and partly parasitic Clitellata, is one the most significant metazoan taxa. Its more than 20.000 described species invade nearly all habitats and play a central role in marine benthic systems as well as in terrestrial soil communities. Annelids include all soft-bodied segmented worm-like organisms and have been recognized as a separate "phylum" for almost 200 years. 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.

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.