Morphology of the organic matrix of the spicule of the sea urchin larva. The morphology of the organic matrix of the skeletal spicule of the sea urchin pluteus larva has been analyzed by light and electron microscopy. Purified isolated spicules can be demineralized, and they reveal lamellae of an irregular fibrillar nature with overall outlines similar to the shape of the intact spicule. Sections through the spicule show the fibrous lamella is probably composed of interconnected concentric sleeves.