Jellyfish are marine invertebrates belonging to the Scyphozoan class. The body of an adult jellyfish is composed of a bell-shaped, jelly producing substance enclosing its internal structure, from which the creature's tentacles are suspended. 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. To compensate for a lack of basic sensory organs and a brain, the jellyfish exploits its nervous system and rhopalia to perceive stimuli, such as light or odor, and orchestrate expedient responses. In its adult form, it is composed of 94--98% water and can be found in every ocean in the world. Some jelly fish do have these body parts such as the box jelly fish.
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