Spicule form in calcareous sponges (Porifera: Calcarea). The principle of uniplanar curvature



A study of the shapes of secondary sagittal tri- and quadri-radiate spicules of the calcareous sponge Leuconia fistulosa has revealed that the paired rays appear kinked, with straight sections in between the kinks, when the spicules are viewed along the optic axis of the constituent calcite. The secondary sagittal spicules thus conform with monaxons and with per-regular and primary sagittal tri- and quadri-radiates, in general, in that whenever a ray is curved, the curvature is always in a single plane that includes the optic axis. The kinks are not necessarily distributed in symmetrical fashion along the paired rays. A mechanism is proposed whereby uniplanar curvature could be achieved.