Though the larval development of bivalves has been extensively studied for commercial purposes, the dynamic development of larval muscle system remains largely unknown. In this study, we characterized the larval muscle system at different developmental stages (D-shaped veligers, umbo veligers and spats) in the bay scallop ( Argopecten irradians) by phalloidin staining and under a confocal microscopy. The functional muscles are initially established at the early stage of veligers, which have four pairs of velar retractors and one anterior adductor. At the veliger stage, the velum and posterior retractor muscles are functionally important for velar contractility but undergo an irreversible shrink until they disappear at the end of the larval stage. During metamorphosis, three crucial modifications take place in the larval muscle system. The metamorphosis process involves the gradual degeneration of velum retractors, mantle margin development from an unfolded to a three-fold state, and remodeling of the adductor muscle system from dimyarian (two adductors) to monomyarian condition (one adductor) as in juveniles/adults. All retractor muscles are composed of striated muscle, but both anterior and posterior adductors have smooth and striated components. These findings highlight that the morphological changes at different stages are typical features of myogenesis in scallops. The present knowledge on the developmental dynamics of myogenesis in the bay scallop will not only improve our understanding of phenotypic diversity of larval myoanatomy in bivalves, but also provide useful information on the larval culture in hatcheries.