Phylum Cnidaria is a group of sea animals including jellyfish, sea anemones, hydra, and coral. There are two forms a Cnidaria can take; A polyp (like sea anemones), and medusa (like jellyfiish). Jellyfish inhabit every ocean in the world; Most live in salt water, but a few species are able to survive in fresh water. Jellyfish aren't really supposed to be called “fish” because technically they are not fish. Some people just refer to them as Jellies (Which is more appropriate). When jellies are grouped together, they are called a swarm. As mentioned earlier, jellyfish are medusa, which means they have an umbrella shaped head.
Corals are invertebrates that live in the sea. They are made up of many individual, identical polyps. Corals are mostly found in reefs, the biggest being The Great Reef, off the coast of Australia. Many different species of fish live in and around coral to protect themselves from predators.
Hydra are cnidaria that take the form medusa. They are found mostly in lakes, unpolluted ponds, rivers, and streams. Hydra have little tentacles at the opening of their mouth; when a prey swims by these tentacles, little dart-like threads shoot into the prey, paralyzing it. When they are attacked, hydra retract their tentacles until they look like little buds. Another interesting fact about hydra is that they are immortal. Biologically, they never age. This has made them a popular topic amongst biologists.
Overall, cnidaria are very cool, and interesting little organisms.