Unit 4.2
Phylum Cnidaria
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Phylum Cnidaria
• Radial symmetry
• Mouth at oral end
surrounded by tentacles.
• One opening into and out
of gastrovascular cavity.
• Cnidocytes that discharge
nematocysts
• Two body forms
– Polyp
– Medusa
• Two germ layers
• Nerve net
• Still no circulatory or
respiratory systems
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Radial symmetry
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Mouth and Tentacles
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Fig. 13.2
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Generalized Cnidarian Life Cycle
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Cnidocyte Structure and Nematocyst Discharge
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Diploblastic - 2 germ layers
– Epidermis - outer covering (ectoderm)
– Mesoglea - middle non-living jelly-like layer
– Gastrodermis - lines gastrovascular cavity
(endoderm)
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Cnidarian Body Wall
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Nervous System
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Class Hydrozoa
• Freshwater & marine.
• Cnidocytes present only on
epidermis.
• Medusa (if present) with a
muscular velum that aids in
propulsion.
• Asexual reproduction by budding.
• Sexual reproduction via gametes
produced by epidermis & released
into water.
• Hydra, Ctenophores, Obelia,
Man-of-War
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Class Hydrozoa
Hydra
Encapsulated
embryo
Ovaries
Budding
Spermaries
Sexual cycle
Asexual cycle
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Hydra
Body Wall
Gastrovascular
cavity
Gastrodermis
Mesoglea
Epidermis
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Obelia
• Freshwater or
marine.
• Live in colonies
rather than free
living like hydra.
• Colonies contain
two types of polyps,
feeding and
reproductive.
• Dual stage life cycle
like other
Cnidarians.
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Obelia
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Obelia
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Obelia
Gonangium
Medusa bud
Medusae
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Class Hydrozoa
Gonionemus
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Class Hydrozoa
Gonionemus
Velum
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Class Hydrozoa
Physalia
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Class Hydrozoa
Physalia
• Commonly called the
Portuguese Man-ofWar.
• Not a jellyfish or even
a single organism.
• Made of a colony of
hydrozoans that
perform specialized
functions.
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Class Hydrozoa
Physalia
• Gastrozooids –
Feeding & digestion.
• Dactylozooids – Prey
capture.
• Gonozooids –
Reproduction.
• Pneumatophore –
Gas-filled float.
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Class Hydrozoa
Physalia
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Class Scyphozoa
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Class Scyphozoa
• True Jellyfish
• Tentacles up to 70
meters in length
• Cnidocytes present in
gastrodermis &
epidermis
• Thick mesoglea
contains amoeboid
cells
• Gametes produced by
gastrodermis
• All marine
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Class Scyphozoa
Aurelia – Moon Jellies
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Aurelia Life History
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Fig. 13.18
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Class Cubozoa
• Medusa dominant &
cuboid
• Tentacles arise at four
corners from blade-like
pendalium.
• All marine
• Strong swimmers which
prey primarily on fish
• Stings of some may be
fatal within minutes to
humans.
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Class Cubozoa
Gonad
Pedalium
Tentacle
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Class Cubozoa
The Box Jellyfish
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Class Cubozoa
The Box Jellyfish
• The most venomous
poison known to man.
• One brush against the
skin will inject enough
venom to put a full
grown adult in the
hospital for over a
week.
• Even a full morphine
drip can not relieve
the pain.
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Class Cubozoa
The Box Jellyfish
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Class Cubozoa
The Box Jellyfish
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Class Anthozoa
• Sea Anemones & Coral
• Medusa stage virtually
absent
• Solitary or colonial
• Some produce protective
skeletons
• Cnidocytes on tentacles and
lining the gastrovascular
cavity
• Mesoglea contains
amoeboid cells like all other
Cnidarians
• All Marine
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Class Anthozoa
Sea Anemone
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Class Anthozoa
Metridium
Tentacles
Mouth
Pharynx
Septum
Gastrovascular cavity
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Symbiosis
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Class Anthozoa
Corals
• Protective skeleton of
calcium carbonate
• Polyp retracts when
not feeding
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Zooxanthellae
• Photosynthetic
dinoflagellates
(brown)
• Live in corals
• Provide nutrients
for coral by
photosynthesis
while gaining a
home
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• Mutualism
Corals
Colony of interconnected polyps
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Class Anthozoa
Meandrina
Brain Coral
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Class Anthozoa
Gorgonia
Sea Fan
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Class Anthozoa
Tubipora
Pipe Organ Coral
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Class Anthozoa
Actinodiscus
Mushroom Coral
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Class Anthozoa
Acropora
Staghorn Coral
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Coral Reefs
•
•
•
Formed over thousands of
years from successive layers of
coral skeleton deposits
(calcium carbonate forms
underwater mountains of coral
animal skeletons)
The underwater equivalent of
the Amazon jungle- very high
species diversity and biomass
Reefs contain sponges, colonial
hydrozoans, anemones, many
varieties of coral, fish, many
types of worms we’ve not
discussed, not to mention
bryozoans, ctenophores,
protists, bacteria, etc etc..
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Coral Reef Ecosystem
Photo © McGraw-Hill Higher Education, Barry Barker, Photographer
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Cladogram of Cnidaria
Anthozoa
Scyphozoa
Hydrozoa
Cubozoa
Medusa cuboid
Loss of medusa
Polyp stage reduced
All marine
Radial symmetry &
cnidocytes
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The End
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