Cell migration into the myosepta. In late larval stages, fibroblast-like cells are rare but present along both sides of the myosepta, and these become abundant at adult stages. (A) Overview of metamorphic stage myoseptum, with no non-muscle cells present. Rarely, fibroblast-like non-muscle cells are observed within the myosepta in metamorphic (B) and late larval (C) stages, where they are most commonly found near the ends of the myosepta and continuous with the mesothelium derived from either the sclerotome or external cell layer. Nuclei of myoseptal cells are marked by asterisks. (D) In a subadult, fibroblast-like myoseptal cells are present along the length of each myoseptum (arrowheads indicate nuclei). (E) Detail of one myoseptal cell (nucleus marked by an asterisk). Panels (A), (B), and (D) are sagittal sections; (C) and (E) are transverse sections. Stage abbreviations: GS, gill slit; met, early metamorphosis; adult, subadult.