DK Nature: Cnidarians
Jellyfish, sea anemones, and CORALS, along with tiny freshwater animals called hydras, all belong to the same phylum (group)—the cnidarians. All cnidarians are simple aquatic invertebrates with stinging tentacles, which they use to capture prey. Some are venomous and have been known to kill people.
Jellyfish swim using a kind of jet propulsion. They contract their hollow, saucer-shaped bodies (called bells) to force water out, which propels them forward. Their long. tentacle-like arms, which trail out behind them, are used to sting and capture prey.
Sea anemones capture food with their tentacles, then pass it to the mouth in the middle of the tentacle crown. Sea anemones spend their adult lives attached to rocks on the seabed or in pools on the shore. Some sea anemones have a muscular collar, which they can pull over their tentacles if threatened.
The phylum (major group) Cnidaria contains about 10,000 species, divided into three classes:
| Hydrozoans (including hydras, the Portuguese man-of-war, and fire corals) |
| Anthozoans (including all other corals, and sea anemones) |
| Scyphozoans, (jellyfish) |
Tropical coral reefs are the ocean’s richest habitats, but the creatures that create them are surprisingly small. They look like tiny sea anemones but have chalky skeletons.
A polyp is an individual coral animal. It looks like a miniature sea anemone, and feeds on the tiny plants and animals, known as plankton, that float in seawater. Most coral polyps live in communities that slowly build up to form reefs. These reefs provide homes for all sorts of other sea-living creatures.
Coral reefs are made from the skeletons of coral polyps. Most coral polyps have a hard, chalky body case that protects the soft parts inside. When they die, the chalky skeletons remain. Over thousands of years, they build up on top of one another to form a reef that may stretch for hundreds of miles.
