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Jellyfish is an informal term used for members of the subphylum Medusozoa and similar animals such as those in the phylum Ctenophora. Jellyfish are mainly free-swimming marine animals with umbrella-shaped bells and trailing tentacles, although a few are not mobile. They move by pulsating the bell. Their tentacles are armed with stinging cells and may be used to capture prey and defend against predators. Jellyfish are found all over the world, from surface waters to the deep sea.
This image shows a labelled cross-section of the flower hat jelly (Olindias formosus).Illustration: Mariana Ruiz Villarreal.