cnidarians

Cnidaria is a diverse phylum of mostly aquatic, predominantly marine animals characterized by radial symmetry, diploblastic tissue organization, and a gelatinous mesoglea. They range from small hydroids to large jellyfish and include the reef-building corals. Most cnidarians are aquatic; a few freshwater species exist, such as Hydra.

Cnidarians display two main body plans: the sessile polyp (mouth oriented upward) and the free-swimming medusa (umbrella-shaped). The body is filled with a gastrovascular cavity that serves digestion and distribution of nutrients, with a single opening at the mouth. Most species alternate between polyp and medusa in their life cycle; however, anthozoans (sea anemones, corals) exist only as polyps, and others vary in their life cycles.

Cnidocytes, specialized stinging cells, line tentacles and contain nematocysts that eject when triggered, delivering venom and aiding prey capture or defense. The nervous system is a diffuse nerve net, lacking a central brain. Digestion begins in the gastrovascular cavity, with extracellular digestion followed by intracellular digestion.

Cnidaria comprises four major classes: Anthozoa (sea anemones, corals), Scyphozoa (true jellyfish), Cubozoa (box jellyfish), and Hydrozoa (hydroids, freshwater Hydra, and many colonial forms). About 11,000 described species exist. Most are marine; corals host symbiotic algae (dinoflagellates) called zooxanthellae, which are essential for reef ecosystems. Some hydrozoans form colonial colonies.

Ecological and human relevance: corals build reefs that support diverse ecosystems; jellyfish influence marine food webs; cnidarians are sources of bioactive compounds. They can sting humans, and environmental changes such as climate warming and acidification threaten coral reefs.