Different hypotheses on the evolutionary transformation of annelid and capitellid segmental chaetae (A) into echiuran ventral (B) and caudal anal chaetae (C).

<p>Annelid chaetae are formed on the ventral edge of the notopodial chaetal sac and on the dorsal edge of the neuropodial chaetal sac. Developing chaetae are <i>red</i>, completed chaetae are <i>black</i>, chaetal sac is <i>grey</i>. All structures that are reduced according to the different hypothesis are <i>paler</i>; presumably reduced chaetal sacs are marked by a <i>dotted line</i>. <i>Arrows</i> mark expansion or shifting of chaetal sac. <i>H1</i> hypothesis one: notopodial chaetae have been reduced and the neuropodial chaetal sac shifted ventrally. <i>H2</i> hypothesis two: notopodial chaetae plus one neuropodial group of chaetae have been reduced and the remaining neuropodial sac shifted ventrally. <i>H3</i> hypothesis three: all chaetal sacs expand dorsally and ventrally, respectively. <i>H4</i> hypothesis four: neuropodia are reduced and the notopodial chaetae expand dorsally and ventrally.