Final answer:
Cnidarians are aquatic invertebrates with radial symmetry and tentacles that contain stinging cells called cnidocytes. They have two body forms, polyp and medusa, with jellyfish and corals being common examples. Their simple tissues can form colonies and play a crucial role in marine ecosystems.
Step-by-step explanation:
Cnidarians: Radial Symmetry and Stinging Cells
Cnidarians are a diverse group of aquatic invertebrates that include species such as jellyfish, corals, and sea anemones. These organisms are known for their distinctive body tissues, radial symmetry, and tentacles equipped with cnidocytes, which are specialized stinging cells. The stinging mechanism is quite sophisticated, involving nematocysts that fire a coiled thread, sometimes barbed, and deliver toxins to immobilize prey or deter predators. Cnidarians exhibit two main body forms: the sessile polyp and the roaming medusa. Polyps, like corals, generally attach to a surface and have their mouths and tentacles facing upward. Medusae, like jellyfish, are typically free-swimming with their tentacles and mouth located on the underside of their bell-shaped bodies.
Cnidarians are not only integral parts of marine ecosystems but also bear significant medical and biological interest due to their simple yet effective biological mechanisms and radial body plans. Despite their simple tissue structure, some cnidarians can form complex colonies, such as coral reefs, which serve as critical habitats for a multitude of marine life. Their radial symmetry allows for interaction with the environment in all directions, a feature that distinguishes them from bilaterally symmetrical animals.