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Chapter 7 - Cnidarians Animals with stinging tentacles, including:
jellyfish, corals, sea anemones, and hydra
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.
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.
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
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
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
Includes 4 groups: jellyfish (1), sea
anemones (2), coral (3) and hydra (4).
3 classes:
1. Scyphozoa
2. Anthozoa
3. Hydrozoa
Phylum Cnidaria
Cnidarians Jellyfish
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.
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.
Respiration in Jellyfish
The membranes of jellyfish are thin: oxygen
diffuses directly from water into the animal’s cells.
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.
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.
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).
Comb Jellyfish
Phylum: Ctenophora
They have 8 rows of long cilia.
They have tentacles for
movement/catching food.
Bioluminescent.
Cnidarians Anemones
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).
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.
Sea Anemones: Feeding
http://www.montereybayaquarium.org/videos/Video.as
px?enc=0ZZ+8rD1FkZFMg4UwjaBnQ==
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.
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
Cnidarians The Corals
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.
Symbiosis: Zooxanthellae!
Symbiotic zooxanthellae - a photosynthetic algae that
gives the corals sugars and oxygen .
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.
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.
TED Talk Lesson
“Conserving our spectacular, vulnerable coral
reefs” - Joshua Drew
Go to this link: http://ed.ted.com/on/rHi5crC4
Cnidarians Hydroids
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)
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.
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.
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
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