Identify structure A in the image. 
Definitions:
Silica Crystals
Rigid, crystalline structures made of silicon dioxide (SiO2), found in nature as quartz and used industrially and scientifically for various applications.
Spicules
Small, spike-shaped structures found in certain animals like sponges, serving as a skeletal element.
Radial Symmetry
Body plan in which similar parts are arranged around a central axis, like spokes of a wheel.
Tissue Layers
Various layers of cells in an organism that have specific functions and form organs and other body structures.
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