Gross morphology of Aristotle's lantern and corresponding muscles in <i>Paracentrotus lividus</i>.

<p>(A) Virtual vertical section through a volume-rendered 3D model based on a μCT dataset with 27 µm isotropic voxel resolution. (B) Virtual horizontal section through a MRI dataset with 81 µm isotropic voxel resolution at the level of the retractor muscles. (C) Semi-schematic illustration of the main lantern muscles as well as the compass depressors (right-hand side) and their corresponding insertion sites on skeletal elements (left-hand side). Not to scale. (D) Virtual vertical section through a MRI dataset with 81 µm isotropic voxel resolution at the level of the central oral-aboral axis. The dotted line indicates the exterior septum that separates the peripharyngeal coelom from the perivisceral coelom. (E) Innervation of the protractor, postural, and retractor muscles. Adapted to <i>P. lividus</i> from results acquired by Boltt & Ewer <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037520#pone.0037520-Boltt1" target="_blank">[45]</a> and Cobb & Laverack <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037520#pone.0037520-Cobb1" target="_blank">[33]</a> on two closely related species, <i>Parechinus angulosus</i> and <i>Echinus esculentus</i>. Not to scale. (F) Virtual vertical section through a MRI dataset with a resolution of 50×50×200 µm at the level of the retractor muscles. In horizontal section, the protractor muscles appear as flat bands. al = Aristotle's lantern, am = ambulacrum, au = auricle, cc = central cavity, cd = compass depressor, ce = compass elevator muscle, co = compass, ec = exterior cavity, eg = esophagus, ep = epiphysis, es = exterior septum, ic = interradial cavity, im = interambulacrum, in = intestine, ip = interpyramidal muscle, nt = nerve trunk, pe = peristome, pg = perignathic girdle, ph = pharynx, pm = peristomial membrane, po = postural muscle, pp = peripharyngeal coelom, pr = protractor muscle, ps = perradial septum, pv = perivisceral coelom, py = pyramid, re = retractor muscle, ro = rotula, st = stomach, te = test, to = tooth, wv = water vessel.