The dichotomy between smooth and striated myocytes is fundamental for bilaterian musculature, but its evolutionary origin remains unsolved. Given their absence in Drosophila and Caenorhabditis, smooth muscles have so far been considered a vertebrate innovation. Here, we characterize expression profile, ultrastructure, contractility and innervation of the musculature in the marine annelid Platynereis dumerilii and identify smooth muscles around the midgut, hindgut and heart that resemble their vertebrate counterparts in molecular fingerprint, contraction speed, and nervous control. Our data suggest that both visceral smooth and somatic striated myocytes were present in the protostome-deuterostome ancestor, and that smooth myocytes later co-opted the striated contractile module repeatedly - for example in vertebrate heart evolution. During these smooth-to-striated myocyte conversions the core regulatory complex of transcription factors conveying myocyte identity remained unchanged, reflecting a general principle in cell type evolution.