. The aboral nervous system of Marsupiocrinus Morris. Crinoidea, Fossil; Nervous system -- Echinodermata; Paleontology -- Decatur County, Tennessee; Paleontology -- Silurian. 466 FIELDIANA: GEOLOGY, VOLUME 16. Fig. 6. Diagrammatic representation of aboral nerve system in Antedon as seen in plan view (after Cuenot, 1948, fig. 53). C, chiasmata; N, nerve strands; R, radial plates. both the fossil structure and the surrounding plates "flash" when held at a suitable angle to a light source. Undoubtedly, the structure is part of the crinoid. Among recent crinoids the only organ system dev
. The aboral nervous system of Marsupiocrinus Morris. Crinoidea, Fossil; Nervous system -- Echinodermata; Paleontology -- Decatur County, Tennessee; Paleontology -- Silurian. 466 FIELDIANA: GEOLOGY, VOLUME 16. Fig. 6. Diagrammatic representation of aboral nerve system in Antedon as seen in plan view (after Cuenot, 1948, fig. 53). C, chiasmata; N, nerve strands; R, radial plates. both the fossil structure and the surrounding plates "flash" when held at a suitable angle to a light source. Undoubtedly, the structure is part of the crinoid. Among recent crinoids the only organ system developed within the plates of the calyx is the aboral nerve system. Crinoids have three more or less separate nerve systems of which the aboral is cen- tered on the base of the calycal cavity at the top of the stem and is the main motor system concerned with orientation and movement. Most knowledge of the aboral nerve system in recent crinoids is based on studies of Antedon, a free-swimming comatulid crinoid. This is very unfortunate from a paleontological point of view since Antedon is a highly specialized crinoid with an atypical calycal structure. The aboral nerve system of Antedon was described by Hamann (1889) and has been refigured recently by Cuenot (1948), Hyman (1955), and Nichols (1962). It consists of an aboral center which gives off branches to the cirri aborally. Laterally, five main branches pass outward, immediately bifurcate, and join a pentagonal ring of nerve tissue within the radial plates (fig. 6). The five main branches are radial. Distal to the pentagonal ring the main branches pass into the arm plates and divide in a chiasma with each division of the arms. This structure is very much simpler than the fossil structure. There is nothing to correspond to the curved processes nor to the cross. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrati
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