PHYLUM CNIDARIA
(Sea Anemones, Jellyfish, Coral, Hydras)
~10 000 species
FACT: Cnidarians have a single external opening that serves as both
the mouth and the anus!
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CNIDARIA

Cnidae refers to the creatures’ “stinging cells”

Are further up the “evolution ladder”
 meaning they are more complex than Porifera (sponges)

Three Classes
 Class Hydrozoa
 Class Scyphozoa
 Class Anthozoa
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CNIDARIA

MULTICELLULAR
 cells are specialized and organized into tissues and
sometimes even into simple organs
 all Cnidarian have specialized defense cells that sting =
cnidocytes
 cnidocytes (the cell) contain a unique organelle =
called nematocyst = “stinger”

GERM LAYERS
 Only endoderm and ectoderm
 Endoderm – forms inner gastrodermis
 Ectoderm – forms outer epidermis
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CNIDARIA

MOVEMENT



medusa form (bell-shaped) is motile (moving)
polyp form (vase-shaped) is sessile (non-moving)
some cnidarians exist in both forms throughout their life
cycle (alteration of generations)
medusa
(motile)
polyp
(sessile)
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CNIDARIA

Two body forms

Polyp




Tubular shape
Mouth faces upwards
Sessile
Medusa


Mouth faces downwards
Motile (drifters)
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CNIDARIA

REPRODUCTION

Asexual – budding

Sexual – monoecious, produce both sperm and eggs

Sexual – male (produce sperm)
- female (produce eggs)
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CNIDARIA

no body cavity because no true mesoderm

radial symmetry

non-segmented

have tissues
 gastrodermis (gas exchange and digestion)
 epidermis (protection)
 middle layer = mesoglea (jelly-like)

have a “nerve net”
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CNIDARIA

DETAILED STRUCTURE (Example of Hydra)
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CNIDARIA
•
BODY SYSTEMS

REPRODUCTIVE SYSTEM – a rudimentary system in some
organisms
 Asexual Reproduction
 budding (no system)
 Sexual Reproduction
 Monoecious = individuals produce both sperm (testis)
and eggs (ovary)
 Dioecious = male produce sperm (testis) and females
produce eggs (ovary)
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CNIDARIA
•
BODY SYSTEMS


REPRODUCTIVE SYSTEM and LIFE CYCLES
Alternation of generations
Aurelia (jelly fish)
http://trc.ucdavis.edu/biosci10v/bis10v/week9/moonjellemba.mov
Obelia (hydra)
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CNIDARIA
•
BODY SYSTEMS

DIGESTIVE SYSTEM – not a true system
 food enters gastrovascular cavity via mouth
 extracellular digestion (enzymes secreted into cavity)
 cells lining the cavity absorb digested nutrient and
smaller food particles
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CNIDARIA
•
BODY SYSTEMS

CIRCULATORY SYSTEM – not a true system
 nutrients and gasses diffuse directly across the thin cell
layers

RESPIRATORY SYSTEM – not a true system
 gas exchange directly between cells and water by
diffusion (O2 diffuses into and CO2 out of cells)
 Both across epidermis and gastrovascular tissues
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CNIDARIA
•
BODY SYSTEMS

EXCRETORY SYSTEM – not a true system
 waste diffuses from cells into gastrovascular cavity
 waste is then released through the mouth

MUSCULAR SYSTEM – NONE
 no muscle cells but some cells can contract and relax
(e.g. in the tentacles)

SKELETAL SYSTEM – NONE
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CNIDARIA
•
BODY SYSTEMS

Nervous System – a basic system
 a net of nerve cells
 react to presence of food and danger
 for feeding and protection
Hydra Feeding
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CNIDARIA
•
GENERAL INFORMATION

HABITAT
 mostly marine waters (marine = salt water)
 some freshwater species

SIZE
 as small as 0.5 cm tall
 bell of up to 2.4 m diameter and tentacle ranging upwards
of 50 meters
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CNIDARIA - Class Hydrozoa

colonial organisms
 hydras, Portuguese Man-of-War
 medusa and polyp forms
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CNIDARIA - Class Scyphozoa

“Cup Animals”
 jelly fish
 predominantly in the medusa form
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CNIDARIA - Class Anthozoa

“Flowering Animals”
 sea anemones
 coral
 predominantly in the polyp form
sea anemone
and clown fish
coral
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
http://www.youtube.com/watch?v=pYUZxS1b
ZR4 = Steve Irwin and Box Jelly
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