Fig. 1

Owenia fusiformis, phylogenetic position, sampling location, and embryonic development. a O. fusiformis (blue box) is a member of Palaeoannelida, the sister clade to all remaining annelids. Annelid phylogeny according to [11]; b, c as adult, O. fusiformis dwells inside a self-built sand tube. The adult body is divided into a head with tentacles, three thoracic segments, abdominal segments and a pygidium (c); d embryological studies on O. fusiformis have focused on specimens collected from the English Channel, with the most recent ones studying a population near the Marine Biological Station of Roscoff, France. e Time course of developmental from oocyte to late mitraria larvae. Top row: Owenia fusiformis undergoes stereotypical equal spiral cleavage, formation of a coeloblastula and gastrulation by invagination. Bottom row: organogenesis starts with the formation of the mesodermal precursors and chaetoblasts, followed by the apical tuft and prototroch. By 13 hpf the blastopore elongates and a neurotroch forms posterior to the blastopore. Myogenesis starts when different larval muscles extend from the chaetal sac to the apical organ and gut. As the larva continues to grow until competence, several other muscles form. Red in e marks the first appearance of the most relevant cells or tissues at that stage. an: anus; ao: apical organ; at: apical tuft; as: abdominal segments; at: apical tuft; bp: blastopore; cb: chaetoblasts; cht: chaetae; dl: dorsal levator muscles; dlh: dorsolateral hyposphere muscles; em: esophageal muscle; en: endomesoderm; fgm: foregut muscle; ho: head opening; jr: juvenile rudiment; mo: mouth; ms: mesentoblasts; mt: metatroch; np: nephridia; nt: neurotroch; p: pygidium; pb: polar bodies; pt: protrotroch; rm: retractor muscles; st: sand tube; tc: tentacles; to: tail opening; ts: thoracic segments; vlh: ventrolateral hyposphere muscles. Scale bar is 2 cm