The Orientation of the Optic Axis of Spicules of Leucosolenia complicata

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Abstract
The orientation of the optic axes of the spicules of Leucosolenia complicata has been studied by using isolated spicules and longitudinal sections of the oscular tubes. For tri- and quadri-radiates the mean angle of inclination of the optic axis to the tube wall varies with the distance of the spicule from the oscular edge, particularly over the first 200 µ. The spicule form likewise varies with position, but is not closely correlated with the orientation of the optic axis: two spicules can have the same optic angle but different shapes, or the same shape and yet different optic angles. For curved monaxons the optic axis is directed almost at right angles to the spicule axis at the distal end, and the spicule curvature is independent of the position in the tube. Slender monaxons, on the other hand, have an optic axis coincident with the spicule length. It seems a general rule, at least for spicules of L. complicata and L. coriacea, that the optic axis of the first formed calcite rudiment lies in the direction of the cell division (or cell association) that directly precedes spicule formation. A mechanism is suggested for the orientation of the optic axis. This is explained in terms of an oriented overgrowth of the calcite upon an organic precursor, which is structurally fixed and oriented within the formative cell complex. The arrangement of this complex, by mechanical factors arising during the development of the oscular tube, results in the orientation of the optic axis of the developing spicule. The hypothesis is in accordance with the observations of Minchin on spicule formation.