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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

pneumatocysts• Two body forms

– Polyp– Medusa

• Two germ layers• Nerve net

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Radial symmetryRadial symmetry

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Mouth and Tentacles

Cnidarians are the oldest existing animals that have specialized tissues.

• Cnidarians have two body forms. – polyps

In the polyp formof a coral, the tentaclesand mouth face upward.

– medusas

In the medusa form of a jellyfish, the tentacles andmouth face downward.

• The four major cnidarian classes are defined by their dominant body form.

– Anthozoans such as sea anemones have a dominant polyp stage.

• The four major cnidarian classes are defined by their dominant body form.

– Hydrozoans such as hydra alternate between forms.

• The four major cnidarian classes are defined by their dominant body form.

– Scyphozoans are jellyfish with a dominant medusa form.

• Cubozoans such as sea wasps have a dominant medusa form.

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• The four major cnidarian classes are defined by their dominant body form.

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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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• Cnidarians are made up of two tissue layers separated by mesoglea.

barbscoiled nematocyst

dischargednematocyst

• The outer tissue layer has three cell types. – contracting cells – nerve cells – cnidocytes (which contain

nematocysts)

gastrovascular cavity

mouth

mesoglea

tentacle

oral arms

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Nervous System• Nerve cells, arranged in a nerve net.

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Class Hydrozoa• Freshwater & marine.• Cnidocytes present only on

epidermis.• Medusa (if present) with a

muscular velum.• Mesoglea without ameboid cells.• Asexual reproduction by budding.• Sexual reproduction via gametes

produced by epidermis & released into water.

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Class Hydrozoa

• Hydra

• Obelia

• Gonionemus

• Physalia

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Class HydrozoaHydra

SpermariesSpermariesSpermariesSpermaries

OvariesOvariesOvariesOvaries

EncapsulatedEncapsulatedembryoembryo

EncapsulatedEncapsulatedembryoembryo

Sexual cycleSexual cycleSexual cycleSexual cycle Asexual cycleAsexual cycleAsexual cycleAsexual cycle

BuddingBuddingBuddingBudding

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GastrovascularGastrovascularcavitycavityGastrovascularGastrovascularcavitycavity

EpidermisEpidermisEpidermisEpidermis

MesogleaMesogleaMesogleaMesoglea

GastrodermisGastrodermisGastrodermisGastrodermis

HydraBody Wall

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Obelia

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Obelia

MedusaeGonangium Medusa bud

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Class HydrozoaGonionemus

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Class HydrozoaGonionemus

Velum

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Class HydrozoaPhysalia

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Class Scyphozoa• Tentacles up to 70

meters in length• Cnidocytes present in

gastrodermis & epidermis

• Thick mesoglea contains ameboid cells

• Gametes produced by gastrodermis

• All marine

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Class ScyphozoaAurelia

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Aurelia Life History

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Fig. 13.18

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Planula LarvaPlanula Larva

Ciliated larva that can swim to a new location

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Aurelia

EggsEggsEggsEggs

MesogleaMesogleaMesogleaMesoglea

GastrovascularGastrovascularcavitycavityGastrovascularGastrovascularcavitycavity

MouthMouthMouthMouth

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Aurelia

Mouth

Oral arm

Gastric pouch

Tentacles

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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

GonadGonadGonadGonad

TentacleTentacleTentacleTentaclePedaliumPedaliumPedaliumPedalium

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Class CubozoaChironex Sea Wasp

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Class Anthozoa• Medusa stage absent• Solitary or colonial• Some produce protective

skeletons• Gastrovascular cavity

subdivided by at least 8 mesenteries

• Cnidocytes on mesenteries• Mesoglea contains ameboid

cells• All Marine

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Class AnthozoaSea Anemone

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Class AnthrozoaMetridium

Class AnthrozoaMetridium

MouthMouthMouthMouth

TentaclesTentaclesTentaclesTentacles

PharynxPharynxPharynxPharynx

SeptumSeptumSeptumSeptum

Gastrovascular cavityGastrovascular cavityGastrovascular cavityGastrovascular cavity

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Symbiosis

Sea Anenome Feeding

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Class AnthozoaCorals

• 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

• Mutualism

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CoralsColony of interconnected polyps

CoralsColony of interconnected polyps

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Class AnothozoaMeandrinaBrain Coral

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Class AnothozoaGorgonia

Sea Fan

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Class AnothozoaTubipora

Pipe Organ Coral

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Class AnothozoaActinodiscusMushroom Coral

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Class AnothozoaAcropora

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

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Photo © McGraw-Hill Higher Education, Barry Barker, Photographer

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AnthozoaScyphozoa Cubozoa

Hydrozoa

Radial symmetry, cnidocytes, planula larva

Septa divide gastrovascular cavity

Medusa cuboidal

Polyp stage reducedLoss of medusa

Cladogram of Cnidaria

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The End