Cnidaarien

Cnidaarien, also known as cnidarians, are a diverse phylum of aquatic animals that includes jellyfish, corals, sea anemones, and hydras. They are found primarily in marine environments, with some freshwater representatives. Cnidarians are characterized by radial symmetry, diploblastic tissue organization (ectoderm and endoderm with a gelatinous mesoglea in between), and the presence of specialized cells called cnidocytes that contain venomous organelles called nematocysts used for prey capture and defense.

Body plans vary, but most cnidarians exhibit two basic forms: the sessile polyp and the free-swimming medusa. Many species alternate between these stages (especially within Medusozoa), while Anthozoa (sea anemones and corals) are typically polyps with no free-swimming medusa stage. They possess a gastrovascular cavity with a single opening that functions as both mouth and gut. The nerve system is a diffuse nerve net rather than a centralized brain, and they respond to chemical and physical cues in their environment.

Cnidarians reproduce sexually or asexually. Asexual methods include budding and fragmentation, which often lead to colonial forms such as corals. Sexual reproduction typically produces free-swimming larvae that colonize new habitats. Ecology and importance vary by group: corals build extensive calcium carbonate reefs that provide structure and habitat for other organisms, while jellyfish and other cnidarians play roles as predators in marine food webs. Many corals harbor symbiotic algae (zooxanthellae) that contribute to their nutrition and reef productivity. The group has a long fossil record dating back to the late Precambrian, with diverse forms flourishing in the Paleozoic and beyond.