Phylum Cnidaria
Cnidos = “Stinging Needle”
www.onacd.ca
4 Classes of Phylum Cnidaria
HYDROZOA – Obelia, Hydra (above),
Portuguese Man O War
CUBOZOA – box jellies (sea wasps)
SCHYPHOZOA - Jellyfish
ANTHOZOA – anemones (above),
corals, sea fans
Identifying Characteristics of the
members of Phylum Cnidaria
• Non-coelomates & therefore do not possess any true
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body systems or organs. 3 germ layers include ectoderm
(protection), endoderm (inner lining) and mesoglea
Exhibit radial symmetry
Possess tentacles used in transportation and for
capturing food and stinging cells called nematocysts.
Possess a Gastrovascular Cavity (GVC)
Primitive Nervous System
Hermaphroditic : can reproduce sexually (do not self
fertilize) or asexually by regeneration or budding
Have a motile (medusa) and a sessile (polyp) stage in
their lives
Found in marine habitats
Gastrovascular Cavity (GVC)
• The inner cavity responsible for digestion,
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circulation, respiration and excretion.
Disadvantages of having a GVC include:
– There is only one opening….. The mouth is the anus…..
– Body systems of digestion, circulation, respiration and
excretion are not separated or specialized
Mouth and Anus
Tentacle
Tentacle
Gastrovascular Cavity
Calcified Shell (Coral)
Note: This diagram shows the
GVC in the polyp body type.
2 Body Types Present in Phylum Cnidaria
1. Polyp
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Sessile (no movement
as they are anchored to
the ground)
Tentacles face up
asexual
Ex. Hydra, sea
anemones, coral
2. Medusa
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Motile
Tentacles face down
sexual
Ex. Jellyfish, sea wasps
The stinging cells: Nematocysts
• Nematocysts are most
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commonly located at the
end of tentacles
Are composed of special
cells called cnidocytes
that have special
organelles (cnidocysts)
that produce a toxin
When a trigger is
stimulated it releases a
barbed needle that
penetrates the flesh and
injects toxins.
Nematocysts are used to
ward off predators or
attack prey
A discharged
nematocyst
Life cycle of a typical Jellyfish
#1-10 exhibit the formation of the
polyp life form.
#11 shows the polyp undergoing
asexual reproduction in the form of
budding. The top of the polyp
breaks off and goes on to form the
medusa stage of the jellyfish’s life.
(if this was an anemone the polyp
would settle on the ocean floor and
become a new polyp…. See picture of
brooding anemone on next slide)
#12-14 shows the formation of the
adult medusa
The medusa will then go on to
produce and release egg and sperm
into the water. The eggs will be
cross fertilized by the sperm of
another medusa and eventually
develop into a new polyp
This alternating between two life forms is termed
ALTERNATION OF GENERATIONS
Brooding Anemones
From a single anemone other polyps are forming which will
eventually break off and settle on the ocean floor to form
new anemones. This is why many of the same type of
anemone are often observed in the same area as the new
polyps are not capable of traveling far distances.
Ecological Importance of Cnidarians
• Filter and clean the water
• Form symbiotic
The clownfish are immune to
the stinging cells of the
clownfish anemone. Therefore
the anemone provides
protection and shelter for the
clownfish and in turn the
clownfish clean the anemone.
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relationships with other
ocean life
– Examples.
• Clownfish and
anemone
(remember Finding
Nemo?)
• Coral and many
types of algae
Coral will die as the water
temperature increases.
Death of coral often
precedes death of entire
ecosystems
Super Cool Killer Cnidarians
Portuguese Man O’ War
Physalia physalis
The Portuguese Man O’ War
• Looks like a jellyfish but is
actually a colony of
specialized polyps and
medusas
• The sting from their
tentacles causes
excruciating pain and
sometimes death
• Named for its air bladder
which looks like the sails of
a Portuguese fighting ship
Super cool fact: Loggerhead
turtles are actually immune
to their toxins and feed on
the Portuguese Man O’ War
A common sign to
observe near
Australian Beaches
Box Jellyfish
• Possess the most deadly
venom (toxins) in the
animal kingdom which
cause anaphylaxis shock
and death
• In Nov. – April they are
abundant in Australian
waters but it is not known
where they go for the
winter
• Through ultrasonic tagging
it has been found that they
sleep on the ocean floor
between 3pm and dawn to
conserve energy and avoid
predators
• Possess 22 very simple
light sensing eyes as well
as a more developed eye
0.1 mm across
Box Jellyfish Chironex flecker
This jellyfish has had an ultrasonic
tag attached (very carefully!) to it
in order to help learn more about
the migration patterns of these
cnidarians
Jelly FISH OUT OF WATER