English: (A) Photograph of the oral part of the interambulacrum showing the location of the paired buccal sacs. (B) Volume-rendered model of a μCT dataset with 27 µm isotropic voxel resolution showing the same view as in (A), but soft tissues are inapparent due to the type of analysis employed (i.e., X-ray). The dotted line indicates the location of a single buccal sac. (C) Virtual vertical section through a MRI dataset with 42 µm isotropic voxel resolution. The lumen of the buccal sacs is continuous with the interradial cavity. The labels marked (D–I) indicate the location of the horizontal sections shown hereafter. (D–I) Virtual horizontal sections through a MRI dataset with 78×78×500 µm resolution. The protractor muscles are located directly above the buccal notches. am = ambulacrum, bn = buccal notch, bp = buccal plate, bs = buccal sac, ce = compass elevator muscle, es = exterior septum, ic = interradial cavity, im = interambulacrum, in = intestine, lo = lobe, pe = peristome, pm = peristomial membrane, pr = protractor muscle, pv = perivisceral coelom, py = pyramid, sp = spine, st = stomach, tf = tube foot, to = tooth.