Organization of the hood musculature in Phoronopsis harmeri larvae. longitudinal semithin (A) and thin (B-D) sections. A. Anterior part of the hood. B. Annular muscle. C. The projection of the radial muscle cell. D. Organization of the muscle cell. Abbreviations: am – annular muscle; bc – blastocoel; cm – circular muscle; m – mitochondrion; mf – myofilaments; n – nucleus; nf – nerve fibers; rm – radial muscle; s – sarcomere; v – vestibulum; Z – Z-line. bb – basal body; c – cilium; G – Golgi apparatus.