Alexander Ziegler; Leif Schröder; Malte Ogurreck; Cornelius Faber; Thomas Stach

“ Our attempt to correlate the presence of frilled muscles with data on sea urchin size and shape failed, as we were unable to find any parameter, including peristome size or shape, test diameter or height, lantern size or shape, or test eccentricity that would permit us to infer the type of protractor muscle. Because the lantern is used predominantly in mastication, frilled muscles might constitute an adaptation to certain types of food or feeding. ”

“ The evolutionary origin of lantern muscle innervation is certainly of a more general interest because postural and protractor muscles are derived from the retractor muscle, and the protractor in turn constitutes an adapical extension of the postural muscle [2] . ”

“ A semi-thin section of a frilled protractor muscle of E. mathaei reveals that the fascicles are, on average, evenly distributed throughout the muscle, although they are slightly more concentrated towards the muscle's adaxial side and within the lobes than they are in the central part of the muscle (Fig. 2C) . The fascicles are contained within the connective tissue layer between the two epithelia, and the fascicles found at the center of the muscle are composed of about a dozen muscle fibers of varying cross-sectional shapes and with diameters ranging from about 2–10 μm (Fig. 2D) . ”

“ Interestingly, another muscle of the lantern is also characterized by considerable folding of its surface area, i.e., the interpyramidal muscle [15] , [19] , [20] . However, the characteristic folding of this muscle is present in all sea urchin taxa that possess a lantern [2] , and this morphology is likely to be an adaptation to the continuous growth of the skeletal element onto which this muscle is inserted, i.e. the pyramid [20] . ”

“ However, our analyses reveal that the lobes may occur at different time points during ontogenesis (Toxopneustidae and Echinometridae as opposed to Strongylocentrotidae, see Table 2) . The lobes first appear as small bumps, but continue to grow as the animal ages [27] , [29] . We can only hypothesize that either an increase in the rate of muscle fiber production along the bumps in the epithelial layer, an inhibition of muscle fiber production in the interjacent furrows, or a combination of both leads to the growth of the lobes. ”

“ 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. ”

“ These animals exhibit a considerable degree of extension of the muscle surface area in the longitudinal muscles of the trunk. Form and arrangement of these specialized muscles are used as taxonomic characters [69] , [70] . The smooth phoronid trunk musculature shares with the echinoid lantern musculature the absence of a vascular system as well as the presence of individual myoepithelial lobes immersed in coelomic liquid. As in sea urchins, coelomic fluid acts as the main medium for nutrient, gas, and waste transport in horseshoe worms [71] . ”

“ Although we experienced considerable difficulties with conventional histological analyses using museum material, the sections nonetheless permitted us to identify the presence or absence of lobes and to measure other parameters (Table 1) . The process of lobe formation has not yet been fully described and based on our data we are not able to provide further clues. ”

“ Acquisition times per sample varied between 13–18 min, depending on the number of averages used (3–5) . In addition, whole fixed adult sea urchin specimens measuring about 1–3.5 cm in test diameter were imaged using 7 T and 9.4 T small animal MRI scanners and a 17.6 T NMR scanner equipped for imaging (Bruker Biospin GmbH, Germany) . The protocols employed were either FLASH 3D or RARE 3D with 31–81 μm isotropic resolution. Scanning was performed during overnight measurements. ”

“ Observations of skeletal elements were performed on the same six large sea urchin species as for MRI. After dissection along the midline of the test, a single pyramid as well as part of the perignathic girdle were cut out and immersed in 5% NaOCl for 3 h. ”

“ Scanning parameters were 100 kV source voltage, 160 μA source current, 0.2 mm copper filter, 750 ms exposure time, 1440 angular steps over 360° with 2 averaged images per rotation position, 2304×2304 pixel detector size, and about 1 h 20 min scan time. ”

“ 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 P. lividus from results acquired by Boltt & Ewer [17] and Cobb & Laverack [18] on two closely related species, Parechinus angulosus and Echinus esculentus. 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. ”