15.3 Cnidarians. Objectives Describe the characteristics of cnidarians. Explain how cnidarians...
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Transcript of 15.3 Cnidarians. Objectives Describe the characteristics of cnidarians. Explain how cnidarians...
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15.3 Cnidarians
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Objectives
• Describe the characteristics of cnidarians.• Explain how cnidarians reproduce.• Compare and contrast the two body forms of cnidarians.
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Characteristics 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.
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• Others, like the jellyfish, swim freely in the ocean. • Yet in spite of this diversity, all cnidarians have features in
common.
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• Cnidarians 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.
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• Some 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.
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• Both 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.
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• Cnidarians have several features that make them differentfrom sponges.
• First, cnidarians have a mouth that leads to a digestive cavity. • The mouth is the cavity's only opening.
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• Cnidarians 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.
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• Cnidarians' 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.
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Reproduction 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.
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• To 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.
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• Many 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.
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Diversity 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.
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• The Hydra is one of the few freshwater cnidarians.
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Sea Anemones• One group of cnidarians includes the sea anemones (uh
NEHM uh NEES). • Their scientific name means "flower animals." • They are polyps all their lives.
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• Some 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.
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Hydrozoans• This 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.
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Reef-building Corals• Tropical reefs are formed by tiny cnidarians called corals. • Corals are in the same group as sea anemones.
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• Reef corals spend their whole lives as tiny polyps living together in colonies.
• They make hard skeletons of calcium carbonate.
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Jellyfish• Jellyfish make up the third cnidarian group.• They spend most of their lives as medusas.
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Staghorn Coral
• The 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.
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Coral Reefs
• Coral 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.
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• The 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.
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• The 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.
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• Algae 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.
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• In 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.
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Check and Explain
• What features do cnidarians have that sponges do not?• 2. In which cnidarian body form does asexual reproduction
occur?• 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?