Gross morphology, histology, and ultrastructure of the frilled protractor muscle found in <i>Echinometra mathaei</i>.

<p>(A) Virtual horizontal section through a MRI dataset with 81 µm isotropic voxel resolution at the level of the retractor muscles. (B) Close-up view. In horizontal section, the protractor muscles appear as frilled bands. (C) Semi-thin section through a frilled protractor muscle. (D) Semi-thin section of a fascicle (indicated by the dotted line). (E) Semi-thin section of the protractor muscle epithelium directed towards the interradial cavity. (F) Semi-thin section of the protractor muscle epithelium directed towards the exterior cavity. (G) Ultra-thin section through the bordering area of four muscle fibers. (H) Ultra-thin section of the ciliated cuboidal epithelium directed towards the interradial cavity. (I) Ultra-thin section of the ciliated epithelium directed towards the exterior cavity. (J) Ultra-thin vertical section through a cilium. All epithelia found covering the protractor muscles are ciliated. (K) Ultra-thin vertical section through a collagen fibril. The presence of collagen fibrils varies between the adaxial and the abaxial connective tissue layers. au = auricle, bl = basal lamina, cc = central cavity, cf = collagen fibril, ci = cilium, ct = connective tissue, ec = exterior cavity, ep = epithelial cell, fa = fascicle, ic = interradial cavity, ip = interpyramidal muscle, lo = lobe, mf = muscle fiber, np = nerve process, nu = nucleus, ph = pharynx, po = postural muscle, pr = protractor muscle, pv = perivisceral coelom, py = pyramid, re = retractor muscle, to = tooth.