Cnidarians are a group of simple, aquatic animals that include creatures like jellyfish, coral, and sea anemones. They are known for their unique stinging cells called cnidocytes, which they use for capturing prey and defending themselves. Cnidarians typically have a soft, gelatinous body and can exist in two main forms: the stationary polyp and the free-swimming medusa. These animals are primarily found in ocean environments, but some species can also live in freshwater. Cnidarians play a crucial role in marine ecosystems, providing habitat for other organisms and contributing to the health of coral reefs. Their fascinating life cycles and diverse forms make them an important subject of study in biology.