The group of invertebrates that includes hydras, jellyfish, sea anemones, and corals is called Cnidaria.
Cnidarians are known for their unique body structure and the presence of specialized cells called cnidocytes, which contain stinging organelles known as nematocysts. These cells allow them to capture prey and defend against predators. This phylum is divided into several classes, such as Hydrozoa (which includes hydras), Scyphozoa (true jellyfish), Anthozoa (which includes sea anemones and corals), and Cubozoa (box jellyfish). Cnidarians exhibit two main body forms: the medusa, which is typically free-swimming and bell-shaped, and the polyp, which is usually sessile and cylindrical.
Through their fascinating life cycles, including both sexual and asexual reproduction, cnidarians play essential roles in marine ecosystems, serving as both predators and prey. Their stunning diversity, from the vibrant colors of coral reefs to the translucent bodies of jellyfish, showcases the remarkable adaptations these organisms have developed over millions of years.