Final answer:
Cnidaria is a phylum of marine organisms that exhibit radial or biradial symmetry. They have specialized cells called cnidocytes that contain nematocysts for immobilizing prey with toxins.
Step-by-step explanation:
Cnidaria is a phylum of marine organisms that includes over 11,000 species, including jellyfish, sea anemones, and coral. They exhibit radial or biradial symmetry and are diploblastic, meaning they develop from two embryonic layers. Cnidarians have specialized cells called cnidocytes that contain nematocysts, which are used to immobilize prey with toxins. These cells are concentrated around the mouth and tentacles of the animal. When touched, the cells fire the toxin-containing coiled threads that can penetrate and stun the predator or prey.