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Chapter 7 - Cnidarians Animals with stinging tentacles, including:
jellyfish, corals, sea anemones, and hydra
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Cnidarians
Cnidarians are soft-bodied animals.
Have stinging tentacles arranged in circles around
their mouth.
May live as single individuals (like anemones) or in
colonies (like coral).
Have radial symmetry
Many cnidarians have a life
cycle that includes two stages: 1. Polyp- a sessile flower-like
stage. 2. Medusa- a mobile bell-
shaped stage.
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Cnidarian Anatomy
Two layers of cells.
A saclike digestive tract
with one opening (where
food enters AND waste
exits!)
Tentacles which are long
flexible appendages
arranged in a ring around
a central mouth.
No brain, but they can
respond to stimulus.
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Nematocysts – Stinging Cells
Almost all cnidarians
capture their food using
stinging cells called
nematocysts, which
are on their tentacles
Each nematocyst is a
poison-filled sac
containing a coiled
“spring-loaded” dart
When another animal
touches a nematocyst, it
“fires,” paralyzing its prey
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Cnidarians do not have a
circulatory system,
excretory system,
nervous system or brain
They have a simple
nervous system, which is
concentrated around the
mouth.
They have eyespots,
which can tell light from
dark.
Cnidarian Anatomy
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Most cnidarians can reproduce sexually and asexually.
Asexually- by budding (new organism and detaches from
the parent)
Sexually- by broadcast spawning (sending sperm out
into the water)
Cnidarian Reproduction
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Includes 4 groups: jellyfish (1), sea
anemones (2), coral (3) and hydra (4).
3 classes:
1. Scyphozoa
2. Anthozoa
3. Hydrozoa
Phylum Cnidaria
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Cnidarians Jellyfish
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The Jellyfish Member of the class Scyphozoa.
Umbrella-shaped structure called the medusa, with tentacles hanging down from it.
Epidermis: outer layer
Mesoglea: jelly-like mass separating the membranes.
Gastrodermis: stomach skin.
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Jellyfish Movement
Jellyfish are considered
part of the plankton
population; however, they
can move.
Their muscles can
contract the medusa,
causing them to pulsate
through the water.
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Respiration in Jellyfish
The membranes of jellyfish are thin: oxygen
diffuses directly from water into the animal’s cells.
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How jellyfish obtain food
Cnidoblasts: stinging cells
in tentacles.
These cells can paralyze a
fish with a coiled thread
with a barb at the end
called a nematocyst.
A paralyzed fish can be
taken into the jellyfish by
its contracting tentacles.
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How jellyfish obtain food
•Jellyfish can also
catch food using the
medusa.
• The medusa can be
coated in a sticky
mucus that traps
plankton.
• Ciliated cells move
the plankton to the
mouth of the jellyfish.
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Jellyfish Reproduction
Adult jellyfish release eggs and sperm into
water (sexual reproduction).
Fertilization = planula larva
Polyp grows on a rock or other surface
(sessile stage).
Polyps clone themselves (asexual
reproduction).
Ephyra are formed.
Adult medusa develops (mobile stage).
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Comb Jellyfish
Phylum: Ctenophora
They have 8 rows of long cilia.
They have tentacles for
movement/catching food.
Bioluminescent.
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Cnidarians Anemones
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Sea Anemones Member of the class Anthozoa.
Anemones have stinging tentacles and a nerve net.
Radial symmetry
They live attached to a substrate with a muscular foot
(sessile).
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Sea Anemones: Feeding
Very similar to jellyfish.
They use stinging tentacles to obtain food.
Food is brought to the mouth and digested in
the digestive sac by enzymes.
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Sea Anemones: Feeding
http://www.montereybayaquarium.org/videos/Video.as
px?enc=0ZZ+8rD1FkZFMg4UwjaBnQ==
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Sea Anemones: Symbiosis
Shrimp and clownfish live among the tentacles:
they have a specialized skin coating that protects them from stings.
clown fish gain protection from anemone.
clown fish protect the anemone’s tentacles from being bitten off by other fish.
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Life cycle of anemones
Anemones do NOT have a medusa stage like the
jellyfish.
Polyp stage only (SESSILE).
They can reproduce asexually by splitting in half.
Pieces of a polyps base can break off to make a new
organism.
They can reproduce sexually by broadcast spawning
(releasing sperm into water) – fertilized eggs will settle
and develop into polyps.
http://www.youtube.com/watch?v=Vrq7LEVeDdQ
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Cnidarians The Corals
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The Corals
Member of the class Anthozoa.
Polyp stage only.
Coral consists of a small polyp that rests within
an expanding bed of limestone.
Grow in the tropics.
Coral are also the home to symbiotic algae
called zooxanthellae.
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Symbiosis: Zooxanthellae!
Symbiotic zooxanthellae - a photosynthetic algae that
gives the corals sugars and oxygen .
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How does coral grow? Reefs are a massive limestone structure made of calcium carbonate.
To build this structure corals need a source of calcium and carbon.
Polyp cells absorb the calcium from sea water.
Zooxanthellae take up CO2 from coral respiration.
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Coral Colonies
Coral polyps live in large
groups and are attached to
one another by a thin
membrane.
Membranes connect the
coral digestive systems so
that there is sharing of food
between polyps.
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TED Talk Lesson
“Conserving our spectacular, vulnerable coral
reefs” - Joshua Drew
Go to this link: http://ed.ted.com/on/rHi5crC4
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Cnidarians Hydroids
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Hydroids
In the class Hydrozoa.
Colonial animals: made up of
individual polyps that function
together.
Two types of polyps:
1. Feeding polyps (have
nematocysts)
2. Reproductive polyps
(have a brief medusa
phase)
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Portuguese man of war:
This organism is classified
with hydrozoans and not
jellyfish because it is a
colony made up of different
types of polyps.
Like the rest of the
cnidarians it has many
nematocysts on its
tentacles.
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Physalia – Different types of
polyps
A gas filled bag keeps the
colony afloat – one polyp
type.
Colony of stinging tentacles
– another type of polyp.
Some polyps serve a
reproductive function.
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Hydroid Reproduction
Dominant phase is a polyp.
They can reproduce sexually with a brief
medusa (free swimming sexual phase).
Can also reproduce asexually through budding.
http://www.youtube.com/watch?v=wfbhwq95Duc