| Summary: | Dilute hydrochloric acid, carbonic acid, and potassium nitrate solutions dissolve the spicule calcite revealing a thin, partially contracted sheath. Corrosion by potash solution, however, produces a rigid, brittle sheath, which consists of the true sheath and an inorganic deposit laid down on its inner surface as the calcite dissolves away. The oscular rays, gastral rays, and curved monaxons corrode much more rapidly than the basal rays and slender monaxons in potash solution, and the corrosion is most noticeable on the surfaces transverse to the optic axis, particularly when dilute potash is used. Potassium nitrate solution and water, however, corrode the surfaces parallel to the optic axis, whereas in hydrochloric and carbonic acid solutions the calcite dissolves uniformly all round the rays. When spicules bearing calcite crystals are corroded, the calcite of the spicules dissolves more rapidly than the crystals, which then remain attached to the sheath and appear to have crystallized upon it, though attempts to crystallize calcite upon isolated sheaths, or sheaths supported by the inorganic deposit, have been unsuccessful. The evidence suggests that when crystals form on the spicule surface they are oriented by crystallizing on the calcite through perforations in the sheath.
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