Stony corals are the most familiar corals to many people because of the reefs they build. They secrete calcium carbonate to form a cup-shaped, outer skeleton. The living coral (polyp) sticks itself out of the top of the cup to feed. Over time, the polyp secretes more calcium carbonate at the base, raising it higher, and leaving layers of cup-shaped chambers below. While some stony corals live alone (solitary), many live in groups (colonial). Often, their skeletal material cements them together into a mass, with the polyps inside. Their skeletons form reefs made of layers like a condominium with living corals at the top. The reef-building stony corals live mostly in warm, tropical waters. Solitary stony corals, less well-known, are more evenly distributed in the oceans of the world, even in the total darkness of ocean depths. Some are tiny like grains of rice, while others are fist-sized.

Anthozoans (anemones and corals) have stem-like bodies that anchor to the ocean body or other surface. Tentacles radiate like petals around the top of the stem (anthozoa flower animals). Both their bodies and tentacles are hollow, opening into a mouth in the center of the flower. Anemones and corals capture food with their tentacles and push it into their mouths. While they mostly eat small, floating food (plankton), stingers (nematocytes) on their tentacles allow some anthozoans to consume larger prey (even fish or crabs) by stunning them first. Food travels from the mouth to the hollow space inside their bodies, where it is digested. Since they have only one opening, waste products are expelled back out their mouths. Some anthozoans benefit from a relationship with unicellular algae (mutualism). While the coral provides a safe place for the algae to live, the algae makes food by photosynthesizing.