Abstract
The modified setae in the larvae and the adults of two chaetopterid species (Annelida, Polychaeta) are compared: Spiochaetopterus costarum and Phyllochaetopterus socialis. The morphological variation of these setae is comparatively small all through ontogenesis. The morphology of the adult stage can already be recognized in the larvae. If this feature is general, it greatly facilitates identification of planktonic larval stages.