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Biology
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26-3 Cnidarians
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What is a Cnidarian?
What is a Cnidarian?
Cnidarians are soft-bodied, carnivorous animals that have stinging tentacles arranged in circles around their mouths.
They are the simplest animals to have body symmetry and specialized tissues.
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What is a Cnidarian?
Cnidarians get their name from the cnidocytes, or stinging cells, located along their tentacles.
Cnidarians use cnidocytes for defense and to capture prey.
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What is a Cnidarian?
Within each cnidocyte is a nematocyst—a poison-filled, stinging structure that contains a tightly coiled dart.
Trigger
Filament
Nematocyte
Barb
Filament
Cnidarians are carnivorous animals that have stinging tentacles arranged around their mouths. Stinging cells called cnidocytes are used to capture and paralyze prey. Within each cnidocyte is a stinging structure called a nematocyst. Here, a sea anemone captures a fish that has brushed the trigger of the nematocyst. When an animal touches the trigger of a nematocyst, the filament inside uncoils and shoots a barb into the animal.
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What is a Cnidarian?
Cnidarians are carnivorous animals that have stinging tentacles arranged around their mouths. Stinging cells called cnidocytes are used to capture and paralyze prey. Within each cnidocyte is a stinging structure called a nematocyst. Here, a sea anemone captures a fish that has brushed the trigger of the nematocyst. When an animal touches the trigger of a nematocyst, the filament inside uncoils and shoots a barb into the animal.
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7. Form and Function in Cnidarians
Cnidarians are radially symmetrical.
Cnidarians typically have a life cycle that includes two different-looking stages: a polyp and a medusa.
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8. Form and Function in Cnidarians
A polyp is a cylindrical body with armlike tentacles. In a polyp, the mouth points upward.
Polyps are usually sessile.
Many cnidarians have both a polyp stage and a medusa stage.
Polyp
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9. Form and Function in Cnidarians
A medusa has a motile, bell-shaped body with the mouth on the bottom.
Many cnidarians have both a polyp stage and a medusa stage.
Medusa
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10. Form and Function in Cnidarians
The gastroderm is the inner lining of the gastrovascular cavity, where digestion takes place.
Gastrovascular cavity
Gastrovascular cavity
Many cnidarians have both a polyp stage and a medusa stage.
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11. Form and Function in Cnidarians
Both polyps and medusas have a nerve net, a loosely organized network of nerve cells.
Nerve cells
Cnidarians have nerve nets that consist of many individual nerve cells.
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12. Form and Function in Cnidarians
Reproduction
Most cnidarians reproduce both sexually and asexually.
Polyps can reproduce asexually by budding.
In most cnidarians, sexual reproduction takes place with external fertilization. External fertilization takes place outside the female's body.
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13. Form and Function in Cnidarians
In the life cycle of Aurelia, a common jellyfish, the female releases eggs into the water, and the male releases sperm.
Jellyfishes reproduce sexually by producing eggs and sperm. Depending on the species, fertilization is either internal or external. In Aurelia, fertilization is external, occurring after eggs and sperm are released into the water.
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14. Form and Function in Cnidarians
Fertilization occurs in open water.
Each zygote grows into a free-swimming larva.
Jellyfishes reproduce sexually by producing eggs and sperm.
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15. Form and Function in Cnidarians
The larva eventually attaches to a hard surface and develops into a polyp.
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16. Form and Function in Cnidarians
The polyp eventually buds and releases young medusas that begin the cycle again.
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Groups of Cnidarians
Groups of Cnidarians
Cnidarians include:
jellyfishes
hydras and their relatives
sea anemones and corals
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Groups of Cnidarians
Jellyfishes
The class Scyphozoa contains the jellyfishes.
Jellyfishes live their lives primarily as medusas.
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Groups of Cnidarians
Hydras and Their Relatives
The class Hydrozoa contains hydras and related animals.
Hydras differ from other cnidarians in this class because they lack a medusa stage. Instead, they live only as solitary polyps.
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Groups of Cnidarians
Sea Anemones and Corals
The class Anthozoa contains sea anemones and corals, animals that have only the polyp stage in their life cycle.
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Ecology of Corals
Ecology of Corals
Worldwide distribution of corals is determined by a few variables:
temperature
water depth
light intensity
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26-3
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26-3
The characteristic that defines the cnidarians is
bilateral symmetry.
stinging cells.
a gastrovascular cavity.
cephalization.
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26-3
Which of the following statements is generally true of polyps and medusas?
Polyps are sessile, and medusas are motile.
Polyps are motile, and medusas are sessile
Both polyps and medusas are sessile.
Both polyps and medusas are motile.
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26-3
During the life cycle of Aurelia, the zygote grows into a free-swimming
polyp.
larva.
medusa.
gemmule.
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26-3
Cnidarians, such as the sea anemone, move using
water currents.
an exoskeleton.
a hydrostatic skeleton.
an endoskeleton.
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26-3
The stinging cells found in all Cnidarians are called
choanocytes.
cnidocytes.
polyps.
medusas.
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