Jellyfish are invertebrates composed of gelatinous substances and water. The creatures are similar to sea anemones and coral, which all belong to the phylum Cnidaria, as stated by the National Oceanic and Atmospheric Association.
Jellyfish do not contain essential organs like the brain and heart, but are wired with an intricate nervous system that allows the creatures to respond to stimuli. They also contain a digestive gut, which is an opening where food passes and is also used for reproductive purposes, according to the National Oceanic and Atmospheric Association. Although there are many types of jellyfish, it is common for the creatures to have venomous tentacles that they use to sting and stun prey.