Larval ontogenetic stages of Chaetopterus: Developmental heterochrony in the evolution of chaetopterid polychaetes

Seven post-gastrulation larval stages are described for the sedentary polyc haete Chaetopterus. Analysis of larval anatomy and morphology through ontog eny reveals significant differences in the temporal sequence of segmentatio n, and in the character of segments formed, from the typical embryological pattern described for other polychaete families, such as nereidids of spion ids, When compared in alternative phylogenetic schemes, these differences r epresent significant developmental heterochrony, among other evolutionary t ransitions, which has arisen in the chaetopterid lineage. The heterochrony is correlated with the extreme morphological regionalization along the ante rior-posterior body axis, a feature that is also characteristic of chaetopt erids.