Homology and Evolution of the Chaetae in Echiura (Annelida)
Fig 1
Different hypotheses on the evolutionary transformation of annelid and capitellid segmental chaetae (A) into echiuran ventral (B) and caudal anal chaetae (C).
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 red, completed chaetae are black, chaetal sac is grey. All structures that are reduced according to the different hypothesis are paler; presumably reduced chaetal sacs are marked by a dotted line. Arrows mark expansion or shifting of chaetal sac. H1 hypothesis one: notopodial chaetae have been reduced and the neuropodial chaetal sac shifted ventrally. H2 hypothesis two: notopodial chaetae plus one neuropodial group of chaetae have been reduced and the remaining neuropodial sac shifted ventrally. H3 hypothesis three: all chaetal sacs expand dorsally and ventrally, respectively. H4 hypothesis four: neuropodia are reduced and the notopodial chaetae expand dorsally and ventrally.