Sponges have an internal skeleton, but it is not made of bone like ours. Their skeletons are made of stiff, mineral rods called spicules, or a matrix of strong but flexible protein called spongin, or both. Spicules may be loosely scattered in the body tissue of a sponge, gathered into little bundles, or arranged in symmetrical patterns to form a structured skeleton. You can judge a sponge by its spicules. They come in many shapes, from as simple as toothpick-straight to complex, branching stars. The end of each spicule is specialized too; it may be pointed, flat, shovel-like, or even look like a mushroom cap. By using microscopes to look at spicules (which are tiny), scientists are often able to identify the type of sponge they came from. Given that the spicules are often all that remain of a dead sponge, this is quite handy.