. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. LARVAL SKELETONS AND ORIENTATION 129. Figure -4. Echinoplutei of Strongylocentrolus species: (A) ventral and (B) lateral views of four-armed 5. purpuratitx. (C) and (Dl four-armed S. frandxcanux, (E) eight-armed S Irandscaints without pedicel- lariae, (F) and (G) eighl-armed S. franascanus with three pedicellariae, and (H) eight-armed 5. pallidiis with juvenile plates and spines. Calcite skeleton shown with polarizing niters partly crossed in (C, E, F) fully crossed in (A, B, D, G. H). Because of their optical axis the preor
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. LARVAL SKELETONS AND ORIENTATION 129. Figure -4. Echinoplutei of Strongylocentrolus species: (A) ventral and (B) lateral views of four-armed 5. purpuratitx. (C) and (Dl four-armed S. frandxcanux, (E) eight-armed S Irandscaints without pedicel- lariae, (F) and (G) eighl-armed S. franascanus with three pedicellariae, and (H) eight-armed 5. pallidiis with juvenile plates and spines. Calcite skeleton shown with polarizing niters partly crossed in (C, E, F) fully crossed in (A, B, D, G. H). Because of their optical axis the preoral arm rods are not shown. Scale bar is 400 ^im for all photos. rior end upward when in seawater or in low-density mix- tures of seawater and isosmotic NaCl and Percoll or in seawater and isosmotic sucrose. However, echinoplutei of C. atratus, E. mathei, T. gratilla, , and S. pallidus reoriented with their arms downward as they sank into the denser fluid in the density gradients and approached the depth at which they were neutrally buoy- ant (Tables I, II). The four-armed plutei of S. purpuratus and the plutei of D. exccntricitx and O. aculeuta did not reorient at neutral buoyancy, nor did auriculariae and bipinnariae. Stability of orientations at neutral buoyancy was demonstrated by changing the positions of larvae with a needle and observing their return to their undis- turbed positions. The reorientation of some plutei in denser fluids was caused by a different distribution of excess mass in dense media. The center of buoyancy was anterior to the center of gravity when the surrounding fluid was seawater, but as these plutei sank into denser fluid, the excess density of the skeleton relative to the fluid decreased, and the excess density of soft tissues and body cavities ap- proached zero and then became negative. The center of buoyancy did not move because the larvae did not change shape, but the center of gravity (defined in terms of excess mass) moved as the
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
