(A–C) Contractile epithelial muscles (MEC, myoepithelial, predominantly circular) and antagonistic mesoglea exemplified by olivooiids and free-swimming jellyfish; idealized relaxed (A) and contracted (B) states and simplified transverse section (C). (D–F) Grid-like network of circular and longitudinal subepidermal muscle fibers (HMS) around a cylindrical body filled with antagonistic internal fluid (hydroskeleton) exemplified by scalidophoran worms: idealized relaxed state (D); peristaltic contractions along body (E) and transverse section (F). (G–H) Longitudinal, circular muscles and extrinsic retractor muscles in lobopodians: idealized relaxed state (G); transverse section (H). Images (from top to bottom) represent an olivooid cnidarian, an extant jellyfish, the scalidophoran worm Ottoia (see Vannier, 2012) and the lobopodian Hallucigenia (see Chen and Zhou, 1997). Drawings and images not to scale. Abbreviations: cm, circular muscle; cu, cuticle; ec, ectoderm; en, endoderm; ep, epidermal layer; gu, gut; lc, lobopod claw; lim, limb muscle; lm, longitudinal muscle; lo, lobopod (soft leg); mb, manubrium; me, mesoglea; om, oblique muscle; oo, oral opening (bell margin); pc, primary cavity filled with fluid; rc, radial canal; se, sediment; tc, terminal claw.