Invertebrate of Your Choice: Jellyfish

Jellyfish are invertebrates, and belong to the phylum Cnidaria. They are the oldest existing animals that have specialized tissue. Jellyfish are medusas (non-polyps). They are part of the scyphozoans class of Cnidaria. Jellyfish are found in every ocean, and are about 500 to 700 million years old. Jellyfish are the oldest animal to have multiple organs. They cannot see, but some do have ocelli, which allows them to detect light. Jellyfish come in a very wide variety of sizes, which can range from 1 mm in height to about 2 meters in height. They don't have a central nervous system, a digestive system, a respiratory or a circulatory system. They obtain their oxygen instead by diffusion through their skin.

Source: Wikipedia

Moon jellyfish