26-3 Cnidarians
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26-3 Cnidarians
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.
They include organisms such as hydras, jellyfishes,
sea anemones, and corals.
Cnidarians get their name from the cnidocytes, or
stinging cells, located along their tentacles.
Cnidarians use cnidocytes for defense and to capture
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prey.
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26-3 Cnidarians
What is a Cnidarian?
Within each cnidocyte is a
nematocyst—a poisonfilled, stinging structure
that contains a tightly
coiled dart.
When a small organism
brushes up against the
tentacles, thousands of
nematocysts explode,
releasing enough poison
to paralyze or kill the prey.
Trigger
Filament
Nematocyte
Barb
Filament
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26-3 Cnidarians
What is a Cnidarian?
3 Types of nematocysts:
•Penetrant: harpoon-like structure used to
penetrate tissues of prey or predators.
•Glutinant: sticky surfaces used to stick to prey
•Volvent: lasso-like string that is fired at prey as a
projection
•All are injectant: used like a hypodermic needle to
inject toxins to stun prey.
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26-3 Cnidarians
Form and Function in Cnidarians
Form and Function in Cnidarians
Body Plan
Cnidarians are radially symmetrical. They have a
central mouth surrounded by numerous tentacles that
extend outward from the body.
Cnidarians typically have a life cycle that includes two
different-looking stages: a polyp and a medusa.
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26-3 Cnidarians
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.
Polyp
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26-3 Cnidarians
Form and Function in Cnidarians
A medusa has a motile,
bell-shaped body with the
mouth on the bottom.
Medusa
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26-3 Cnidarians
Form and Function in Cnidarians
Cnidarian polyps and medusas each have a body
wall that surrounds an internal space called a
gastrovascular cavity.
Gastrovascular
cavity
Gastrovascular
cavity
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26-3 Cnidarians
Form and Function in Cnidarians
The gastroderm is the inner lining of the
gastrovascular cavity, where digestion takes place.
Gastrovascular cavity
Gastrovascular cavity
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26-3 Cnidarians
Form and Function in Cnidarians
The epidermis is the outer layer of cells.
Epidermis
Epidermis
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26-3 Cnidarians
Form and Function in Cnidarians
The mesoglea is a layer that lies between the
epidermis and gastroderm. It varies from a thin,
noncellular membrane to a thick, jellylike material that
contains cells.
Mesoglea
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26-3 Cnidarians
Form and Function in Cnidarians
Feeding
A cnidarian pulls its food through its mouth and
into its gastrovascular cavity, a digestive
chamber with one opening.
Food enters and wastes leave the body through
that same opening.
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26-3 Cnidarians
Form and Function in Cnidarians
The digestion that occurs in the gastrovascular cavity
is extracellular, meaning that it takes place outside of
cells.
Partially digested food is absorbed by the
gastroderm.
Digestion is completed intracellularly, within cells in
the gastroderm.
Any materials that cannot be digested are passed out
of the body through the mouth.
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26-3 Cnidarians
Form and Function in Cnidarians
Respiration, Circulation and Excretion
Following digestion, nutrients are usually transported
throughout the body by diffusion.
Cnidarians respire and eliminate the wastes of cellular
metabolism by diffusion through their body walls.
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26-3 Cnidarians
Form and Function in Cnidarians
Response
Cnidarians gather information from their environment
using specialized sensory cells.
Both polyps and medusas have a nerve net, a
loosely organized network of nerve cells that
together allow cnidarians to detect stimuli such as
the touch of a foreign object.
The nerve net is usually
distributed uniformly throughout
the body, but sometimes is
concentrated around the mouth
or in rings around the body.
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26-3 Cnidarians
Form and Function in Cnidarians
Cnidarians also have statocysts, which are groups
of sensory cells that help determine the direction of
gravity.
Ocelli are eyespots made of cells that detect light.
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26-3 Cnidarians
Form and Function in Cnidarians
Movement
Sea anemones have a hydrostatic skeleton.
A hydrostatic skeleton consists of a layer of
circular muscles and a layer of longitudinal
muscles that, with the water in the gastrovascular
cavity, enable the cnidarian to move.
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26-3 Cnidarians
Form and Function in Cnidarians
Reproduction
Most cnidarians reproduce both sexually and
asexually.
Polyps can reproduce asexually by budding.
One type of budding begins with a swelling on the side
of an existing polyp, which grows into a new polyp.
In another type of budding, polyps produce tiny
medusas that separate and become new individuals.
In most cnidarians, sexual reproduction takes place
with external fertilization. External fertilization takes
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place outside the female's body.
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Form and Function in Cnidarians
The sexes of cnidarians are often separate individuals.
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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26-3 Cnidarians
Form and Function in Cnidarians
Fertilization occurs in open water.
Each zygote grows into a freeswimming larva.
The larva eventually
attaches to a hard surface
and develops into a polyp.
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26-3 Cnidarians
Form and Function in Cnidarians
The polyp eventually
buds and releases young
medusas that begin the
cycle again.
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26-3 Cnidarians
Groups of Cnidarians
Groups of Cnidarians
All cnidarians live under water, and nearly all live in
the ocean.
Cnidarians include:
• jellyfishes
• hydras and their relatives
• sea anemones and corals
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26-3 Cnidarians
Groups of Cnidarians
Jellyfishes
The class Scyphozoa contains the jellyfishes.
Scyphozoans means “cup animals.”
Jellyfishes live their lives primarily as medusas.
The polyp form of jellyfishes is restricted to a small
larval stage, and no elaborate colonies ever form.
The largest jellyfish ever found was almost 4
meters in diameter and had tentacles more than 30
meters long.
Jellyfishes reproduce sexually.
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26-3 Cnidarians
Groups of Cnidarians
Hydras and Their Relatives
The class Hydrozoa contains hydras and related
animals.
The polyps of most hydrozoans grow in branching
colonies that can extend more than a meter.
Within the colony, polyps are specialized to perform
different functions.
In the Portuguese man-of-war, one polyp forms a
balloon-like float that keeps the entire colony afloat.
Other polyps in the colony produce tentacles
equipped with nematocysts to sting prey (and
humans).
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26-3 Cnidarians
Groups of Cnidarians
The most common freshwater hydrozoans are hydras.
Hydras differ from other cnidarians in this class
because they lack a medusa stage.
Instead, they live only as solitary polyps.
Hydras reproduce asexually, by budding, or sexually,
by producing eggs and sperm in the body wall.
Many hydras get their nutrition from capturing,
stinging, and digesting small prey.
Some hydras get their nutrition from symbiotic
photosynthetic protists that live in their tissues.
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26-3 Cnidarians
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.
Anthozoans all have a central body surrounded by
tentacles, a form that gave them their name,
anthozoa, meaning “flower animal.”
Many species are colonial, or composed of many
individual polyps.
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26-3 Cnidarians
Groups of Cnidarians
Anthozoans reproduce sexually by producing eggs
and sperm that are released into the water. The
zygote grows into a ciliated larva that becomes a new
polyp.
Some species also reproduce asexually by budding
or splitting into two halves.
Sea anemones are solitary polyps that live at all
depths of the ocean.
They use nematocysts to catch a variety of marine
organisms as food. Many shallow-water species also
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26-3 Cnidarians
Groups of Cnidarians
Most corals are colonial, and their polyps grow
together in large numbers.
Hard coral colonies are usually founded when a
single motile larva settles onto a hard surface and
develops into s single polyp.
New polyps are produced by budding, and as the
colonies grow, they secrete an underlying skeleton of
calcium carbonate, or limestone.
These colonies grow slowly and may live for
hundreds or thousands of years.
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26-3 Cnidarians
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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Ecology of Corals
The “stony” or “hard” corals that build coral reefs
require high levels of light—these corals rely on algae
that capture solar energy, recycle nutrients, and help
corals lay down their calcium carbonate skeletons.
Symbionts provide as much as 60 percent of the
energy corals need. This allows coral reefs to live in
water that carries few nutrients.
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26-3 Cnidarians
Ecology of Corals
Many coral reefs suffer from human activity:
•
Recreational divers can damage reefs.
•
Silt and sediments from logging, farming, mining,
and construction smother corals.
•
Chemical fertilizers, insecticides, and industrial
pollutants poison corals.
•
Overfishing upsets ecological balance.
•
Stresses that makes coral reefs susceptible to
other threats.
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26-3 Cnidarians
Ecology of Corals
Coral bleaching has become more common in recent
years.
High temperatures can kill the algae that live in the
tissues of the corals, leaving behind only transparent
cells atop ghostly white skeletons.
In the past, this was a rare and short-term event from
which many corals recovered.
Over the past 20 years, bleaching has become more
common and severe, causing many corals to die.
Researchers fear that the rising ocean temperatures,
produced by global warming, may be contributing to
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this problem.
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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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26-3
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
Groups of sensory cells that help cnidarians
determine the direction of gravity are known as
a. nerve nets.
b. statocysts.
c. ocelli.
d. cnidocytes.
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