Jellyfish are an invertebrate species of sea dwelling animal that are so named due to their gooey gelatin-like bodies. Despite the name, they are not fish. They belong to the Cnidaria phylum, which includes other simple-bodied marine invertebrates such as sea anemones and corals. The jellyfish is considered simple-bodied because, like its Cnidarian cousins, it has no head, brain, heart, eyes, or ears. Thus it is also lacking in the sensory systems that correspond to these organs.