Cnidaria

Cnidaria is a diverse phylum of aquatic animals that includes jellyfish, corals, sea anemones, and hydras. Most cnidarians are marine, but a few members occur in freshwater. They are characterized by cnidocytes, specialized cells that discharge nematocysts to capture prey or deter predators.

Adults exhibit radial symmetry and tissue-level organization but lack true organs. They are diploblastic, with two embryonic cell layers—the epidermis and gastrodermis—separated by a gelatinous mesoglea. They possess a single opening to a gastrovascular cavity that serves both digestion and circulation.

Cnidocytes and nematocysts are a defining feature, enabling effective prey capture and defense. Nematocysts can deliver toxins or entangle prey. The nervous system is a diffuse nerve net, coordinating contractions and responses to stimuli.

Cnidarians display two principal body plans: the polyp, typically sessile and columnar (as seen in corals and sea anemones), and the medusa, a free-swimming form (as in many jellyfish). Many hydrozoans alternate between both forms, while anthozoans (corals and sea anemones) exist only as polyps. Reproduction occurs sexually or asexually, with budding, fragmentation, or sprouting methods in various groups.

Ecology and evolution: cnidarians occupy a wide range of marine habitats and, in the case of reef-building corals, play key ecological roles in marine ecosystems. Some corals host symbiotic dinoflagellates (zooxanthellae). The phylum is divided into major lineages: Medusozoa (scyphozoa, hydrozoa, cubozoa, and staurozoa) and Anthozoa (Hexacorallia and Octocorallia). Fossils indicate cnidarians were among early-diverging animal lineages in the late Precambrian to early Cambrian; molecular data place them among the earliest eumetazoans.