Tentacles encircle the mouth and are used in part for food capture.
Specialized stinging structures, called nematocysts, are a characteristic of the phylum and are borne in the tentacles and often in other body parts. These contain a coiled fiber that can be extruded suddenly. Some nematocysts contain toxic substances and are defense mechanisms, while others are adhesive, helping to anchor the animal or to entangle prey. The motile jellyfish, or medusoid form, is flattened, with the tentacles usually located at the body margin. The medusoid's convex aboral surface is oriented upward, and the concave oral surface is oriented downward.