The polychaete Boccardia proboscidea has poecilogonous development that includes the production of both planktotrophic and adelphophagic young. In this study, we use scanning electron microscopy to analyse external morphogenesis of planktotrophic offspring with emphasis on early embryos, morphogenesis during metamorphosis, and the dynamic nature of larval structures during early ontogeny. Larval growth involves addition of terminal chaetigers and formation of segment-specific structures such as cilia and chaetae. Our observations reveal that most ciliary bands are reduced or incomplete relative to those found in larvae of other polychaete families. We describe a small metatroch (consisting of only a few trochoblasts) in early embryos, which has not previously been reported in the Spionidae. The presence of the metatroch does not imply a function in opposed-band feeding, as a food groove intermediate between the prototroch and metatroch is lacking. A neurotroch, inconsistently reported in the Spionidae, is also present and terminates in a ciliated pit. Many larval structures (e.g., presumptive sensory organs) are short-lived, implying a shift towards early functionality of adult traits in larvae. Metamorphosis is gradual and occurs over the latter half of the larval life. The reduction of larval structures, and early development of adult traits, suggests an overall shift in morphology facilitating settlement and juvenile development.