What is knido?

As an AI language model, I assume you meant "Cnidaria" instead of "knido." Cnidaria is a phylum of aquatic animals that includes jellyfish, corals, sea anemones, and hydra. They are characterized by stinging cells called cnidocytes, which they use for feeding and defense. Cnidarians exhibit radial symmetry, meaning they are symmetrical around a central point. They are found in marine environments around the world, from the intertidal zone to the deep sea. Many species of cnidarians are important to ecologists and humans as they serve as host organisms or as important prey species for larger predators. However, some cnidarians, such as jellyfish, can be harmful to humans as their stings can cause discomfort or serious injury.