15.3 Cnidarians. Objectives. Describe the characteristics of cnidarians. Explain how cnidarians reproduce. Compare and contrast the two body forms of cnidarians. Characteristics of Cnidarians. Cnidarians are common in most ocean habitats, and a few live in freshwater habitats.
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Cnidarians have two main body forms, the polyp (PAHLihp) and the medusa (muh DOO suh).
Cnidarians' most striking adaptation is their tentacles, which are lined with stinging cells.
To reproduce sexually, cnidarians release eggs and sperm into the water.
Many cnidarians have a life cycle that has both a medusa stage and a polyp stage.
Reef corals spend their whole lives as tiny polyps living together in colonies.
The Great Barrier Reef off the coast of Australia is over 2,000 km long.
The surface of all living reefs is coated with coral polyps, each with its own hard skeleton.
In many places where there are coral reefs, scientists are worried that the coral polyps are dying off.