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Copyright Pearson Prentice Hall

Biology

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26-3 Cnidarians

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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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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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Within each cnidocyte

is a nematocyst—a

poison-filled, stinging

structure that contains

a tightly coiled dart.

Filament

Barb Nematocyte

Trigger

Filament

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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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A polyp is a cylindrical

body with armlike

tentacles. In a polyp, the

mouth points upward.

Polyps are usually

sessile.

Polyp

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A medusa has a motile,

bell-shaped body with the

mouth on the bottom.

Medusa

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The gastroderm is the inner lining of the

gastrovascular cavity, where digestion takes place.

Gastrovascular cavity

Gastrovascular cavity

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Both polyps and medusas

have a nerve net, a

loosely organized

network of nerve cells. Nerve cells

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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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In the life cycle of Aurelia, a common jellyfish, the

female releases eggs into the water, and the male

releases sperm.

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Fertilization occurs in open water.

Each zygote grows into a free-swimming

larva.

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The larva eventually

attaches to a hard

surface and develops into

a polyp.

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The polyp eventually

buds and releases young

medusas that begin the

cycle again.

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Groups of Cnidarians


Cnidarians include:

• jellyfishes

• hydras and their relatives

• sea anemones and corals

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Jellyfishes

The class Scyphozoa contains the jellyfishes.

Jellyfishes live their lives primarily as medusas.

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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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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

Resources/ch26_sectn03_quiz.qtbResources/ch26_sectn03_quiz.qtbResources/ch26_sectn03_quiz.qtb

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26-3

The characteristic that defines the cnidarians is

a. bilateral symmetry.

b. stinging cells.

c. a gastrovascular cavity.

d. cephalization.

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26-3

Which of the following statements is generally

true of polyps and medusas?

a. Polyps are sessile, and medusas are motile.

b. Polyps are motile, and medusas are sessile

c. Both polyps and medusas are sessile.

d. 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

a. polyp.

b. larva.

c. medusa.

d. gemmule.

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Cnidarians, such as the sea anemone, move

using

a. water currents.

b. an exoskeleton.

c. a hydrostatic skeleton.

d. an endoskeleton.

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26-3

The stinging cells found in all Cnidarians are

called

a. choanocytes.

b. cnidocytes.

c. polyps.

d. medusas.

END OF SECTION

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