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.

Sponges achieve reproduction in a whole variety of ways. All sponges can reproduce sexually, meaning that egg and sperms cells get together for fertilization of the egg. How that happens varies from sponge to sponge. Usually, fertilization occurs outside of the female sponge in a meet-up of egg and sperm cells in the ocean (good luck!). Some sponges have internal fertilization, where the sperm cell swims to fertilize the egg inside the female. She may then lay eggs (oviparity) or release live larvae that have developed inside her body (viviparity). Most sponges are hermaphrodites, able to be both males and females. They might act as both sexes at the same time, or more likely one sex first and then the other. Many sponges sidestep fertilization at times by reproducing asexually. They either pinch off buds of new cells or break off clumps from their body. In either case, the cells reattach attach to a substrate and grow into adult sponges.