22. Annelida

Abstract

Annelida, or segmented worms, is a morphologically diverse group of animals, with a large number of species (around 22,000) that inhabit practically all marine habitats, as well as many terrestrial and freshwater environments. The placement of annelids in the tree of life is still unclear, although most recent hypotheses relate them to molluscs, nemerteans, phoronids, and brachiopods. The phylogeny within this group also presents majar challenges. Annelida has traditionally been split into three classes: "Polychaeta" (bristleworms), "Oligochaeta" (earthworms), and Hirudinea (leeches); however, recent phylogenetic analyses have shown that Hirudinea is nested within a paraphyletic "Oiigochaeta" forming a clade now referred to as Clitellata. Furthermore, Clitellata also appears to be nested within a paraphyletic "Polychaeta." Additionally, other groups displaying a morphology far from the typical annelid and considered as separate phyla in the past, such as Echiura (spoon worms), Sipuncula (peanut worms), Siboglinidae (pogonophores and vestimentiferans), and Myzostomida are now also considered to be annelids, though there have been some controversies about their specific position. This chapter offers an updated synthesis of the evolution of segmented worms and particular characteristics that were key to their evolution since the Cambrian.