. Bulletin. Natural history; Natuurlijke historie. 40 PEABODY MUSEUM BULLETIN 40. Fig. 20. Reconstruction of the possible contents of a single pore-cavity mEctosteorhachis nitidus, showing a sensory neuromast consisting of sense cells and supporting cells, be = basal chamber; bv = blood vessel; cc = cross-canals; k = kinocilium; mc = mesh-canal; n = nerve; s = stereocilia; sc = supporting cell; sy = sensory cell. pore-cavity. Next the mesh-canal system was reduced to a single canal (apparently the upper canal) and a large basal chamber formed where the former lower mesh-canals met under the ba
. Bulletin. Natural history; Natuurlijke historie. 40 PEABODY MUSEUM BULLETIN 40. Fig. 20. Reconstruction of the possible contents of a single pore-cavity mEctosteorhachis nitidus, showing a sensory neuromast consisting of sense cells and supporting cells, be = basal chamber; bv = blood vessel; cc = cross-canals; k = kinocilium; mc = mesh-canal; n = nerve; s = stereocilia; sc = supporting cell; sy = sensory cell. pore-cavity. Next the mesh-canal system was reduced to a single canal (apparently the upper canal) and a large basal chamber formed where the former lower mesh-canals met under the base of the pore-cavity. The sensory neuromasts were now presumably restricted to this basal chamber and their kinocilia extended upwards into the base of the pore-cavity through the foramen between the two spaces (Fig. 20). The occasional cross-connections between the cross-canal system and the mesh-canal system (like the remnants of the lower mesh-canals sometimes found in Megalichthys specimens) presumably existed for the passage of vascular elements to the mesh-canals, these vascular elements having previously been carried in the lower mesh-canals. In all the latter forms the (upper) mesh-canal presumably continued to contain either water or mucus. In this evolutionary sequence (which is, no doubt, only the most crude approximation of the actual situation) the striking features are: the concentration and reduction of the neuromast organs; the enlargement of the pore-cavity; the migration of sensory elements to a position immediately underneath the pore opening; and a change of the potentiality for directional response, as the neuromasts move from a linear to a point configuration. At the same time, the essential features of the pore-canal system—namely the functional integration via the mechanism of the mesh-canals and the enclosure of the system in hard tissue—are preserved. All this suggests that the pore-canal system in Ectosteorhachis represents the last stage in the
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