Lion's mane jellyfish (Cyanea capillata) with sheltering butterfish.
A jellyfish is a marine cnidarian belonging to the Scyphozoan class. The body of an adult jellyfish is composed of a bell-shaped, jellylike substance enclosing its internal structure, from which the creature's tentacles suspend. Each tentacle is covered with stinging cells (cnidocytes) that can sting or kill other animals: most jellyfish use them to secure prey or as a defense mechanism. Others, such as Rhizostomae, do not have tentacles at all.