2Objectives Describe the characteristics of cnidarians. Explain how cnidarians reproduce.Compare and contrast the two body forms of cnidarians.
3Characteristics of Cnidarians Cnidarians are common in most ocean habitats, and a few live in freshwater habitats.Some cnidarians live attached to a surface and look like flowers or tiny trees.
4Others, like the jellyfish, swim freely in the ocean. Yet in spite of this diversity, all cnidarians have features in common.
5Cnidarians have two main body forms, the polyp (PAHLihp) and the medusa (muh DOO suh). Medusas swim freely, and polyps usually live attached to a surface.
6Some cnidarians exist only as polyps and some only as medusas. Some are medusas for most of their lives after being polyps for only a short time.In other cnidarians the two forms are equally important in the life cycle.
7Both cnidarian body,forms have radial symmetry. They have a top and bottom but no back or front.This body plan helps cnidarians sense in any direction.
8Cnidarians have several features that make them different from sponges.First, cnidarians have a mouth that leads to a digestive cavity.The mouth is the cavity's only opening.
9Cnidarians also have tissues. The cells covering the outside of a cnidarian make up one kind of tissue.The cells lining the inside are a different kind.Both contain simple muscle fibers and nerves.The two layers are separated by jellylike material.
10Cnidarians' most striking adaptation is their tentacles, which are lined with stinging cells. Stinging cells help cnidarians defend themselves and catch food.When prey is stunned or killed by the stinging cells, the tentacles carry the food to the mouth.
11Reproduction of Cnidarians Cnidarians can reproduce both sexually and asexually.When it occurs, asexual reproduction is carried out by polyps.A small piece of the animal breaks off and grows into a new polyp.This process is called budding.
12To reproduce sexually, cnidarians release eggs and sperm into the water. Each fertilized egg forms a larva.The larva grows into a new polyp or medusa.
13Many cnidarians have a life cycle that has both a medusa stage and a polyp stage. In these cnidarians, it is the medusa that reproduces sexually.In cnidarians without a medusa stage, the polyp reproduces sexually.
14Diversity of Cnidarians Cnidarians are divided into three main groups.Each has a different kind of life cycle.Within each group there are many shapes and sizes, and differences in habitat.
15The Hydra is one of the few freshwater cnidarians.
16Sea AnemonesOne group of cnidarians includes the sea anemones (uh NEHM uh NEES).Their scientific name means "flower animals."They are polyps all their lives.
17Some fish can live safely inside the tentacles of sea anemones. The anemones protect the fish.The fish, in turn, lure other fish that the anemone might eat.
18HydrozoansThis sea fan belongs to a second group called the hydrozoans (HY droh ZOH uhnz).A single fan contains many polyps.Most hydrozoans have both polyp and medusa stages.
19Reef-building CoralsTropical reefs are formed by tiny cnidarians called corals.Corals are in the same group as sea anemones.
20Reef corals spend their whole lives as tiny polyps living together in colonies. They make hard skeletons of calcium carbonate.
21Jellyfish Jellyfish make up the third cnidarian group. They spend most of their lives as medusas.
22Staghorn CoralThe staghorn coral has a shape very different from that of the reef coral.Yet it, too, is made of tiny polyps and a hard skeleton.
23Coral ReefsCoral reefs are found in warm ocean waters around the world.They provide homes for a greater variety of living things than any other place in the ocean.
24The Great Barrier Reef off the coast of Australia is over 2,000 km long. Many tropical islands were once coral reefs that have been left dry by changing sea levels.
25The surface of all living reefs is coated with coral polyps, each with its own hard skeleton. Below the surface are layers and layers of skeletons left by coral polyps that are now dead.The reef grows as new live corals replace ones that die on the surface.
26Algae live inside the polyps and make part of the polyps' food. The algae also give the coral its color.Coral must therefore live in shallow water so that their algae can get enough sunlight.
27In many places where there are coral reefs, scientists are worried that the coral polyps are dying off.They are not being replaced by new living corals.Scientists are not sure why this is happening, but some blame pollutants in the ocean water.
28Check and ExplainWhat features do cnidarians have that sponges do not?2. In which cnidarian body form does asexual reproductionoccur?3. Compare and Contrast - How are the structures of polyps and medusas alike and different?4. Infer - How do you think each of the following help a jellyfish survive: being transparent, having stinging cells, having radial symmetry, being able to swim?