Pleurobrachia pileus (ctenophore) muscle cells. (A-D) Transmission electron microscopy (TEM) views of Pleurobrachia pileus muscle cells. (A-B) Sections through the external epithelium showing a parietal muscle cell (Mu) in transverse section (A) and in longitudinal section (B). (C) Section showing a portion of mesoglea with a mesogleal muscle fibre (Me f) cut obliquely. (D) Tentacle muscle cells (Mu) in transverse section. (E-G) Pictures showing the aspect of the main muscle cell types on phalloidin-stained preparations. (E) Parietal muscle fibres. (F) Close-up of a mesogleal muscle fibre showing its ramified extremity and its multinucleate organisation. (G) Multiple parallel muscle fibres in a tentacle (Tcl) and two tentillae (Tt). BL: Basal Lamina; Me: Mesoglea; Me f: Mesogleal muscle fibre; Mu: Muscle fibre; Nu: Nucleus; Tcl: Tentacle; Tt: Tentilla. Scale bars: A, B: 1 μm; C: 2 μm; D: 5 μm; E, F, G: 20 μm.