. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. OPHIUROID SKELETAL DEVELOPMENT 83 skeleton, but not the ophiuroid skeletal elements, have distinct and different angles of extinction under polarized light. In other words, a larval skeletal element, which grows from a tetraradiate spicule, does not act as a single calcite crystal hut is composed of irregular segments with different crystallographic orientations. This condition, unusual for echinoderm skeletons, may he an effect of a peculiarity in the coordination between the skeleton-depositing cells. Ophiuroid skeleton Be
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. OPHIUROID SKELETAL DEVELOPMENT 83 skeleton, but not the ophiuroid skeletal elements, have distinct and different angles of extinction under polarized light. In other words, a larval skeletal element, which grows from a tetraradiate spicule, does not act as a single calcite crystal hut is composed of irregular segments with different crystallographic orientations. This condition, unusual for echinoderm skeletons, may he an effect of a peculiarity in the coordination between the skeleton-depositing cells. Ophiuroid skeleton Between 30 and 35 hours, the radial, central, and terminal plates appear and regroup from a sagittal plane to form concentric rings on the ventral surface of the embryo (Fig. 2, Table I). Judging from the relative size (breadth rather than mass) of the spicules in slightly older specimens, the radial, central and terminal plates are produced in that order. Minute, granular rudiments of the proximal oral plates and adoral shields are visible by 35 hours (Fig. 2). By 42 hours the adoral shield rudiments are larger than the proximal oral-plate rudiments, and the spicule is triradiate. Some 45-hour specimens have granular rudiments of the buccal scales and the distal oral plates. By 55 hours the embryonic ophiuroid rudiment is pentaradiate, and its skeletal plates are arranged as successive stacks of concentric rings. In the dorsal-most plane are central and radial plates; terminal plates, shaped like stick figures with outstretched arms, are more ventral and on the perimeter of the disc; the branched proximal oral plates and adoral shields are, respectively, proximal and distal to the center of the disc and at a level below the terminals; and buccal scale granules and triradiate distal oral plates are near the ventral surface (Fig. 2). In some specimens, after 45-55 hours of development, a single triradiate spicule, a rudiment of the initial aboral interradial plate (interradial-1),
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
