Phylum Cnidaria
HW: Study notes & Define Vocab
Words.
Intro to Cnidarians
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How do you say it????
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Say it with me now: (ni DARE ee uh)
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What are cnidarians?
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Practicing saying this when you are with your
friends so you sound smart.
Examples: Jellyfish & Sea Anemones
Are there many species of cnidarians?
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Yes, there are 10,000 different species!
Body Structure
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Like sponges, they have:
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One body opening
Two cell layers
Unlike sponges:
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Their cell layers are organized into tissues that
have specific functions.
They have symmetry
Body Structure, cont’d
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The function of the two layers:
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Outer Layer: protects internal body
Inner Layer: mostly for digestion
Cnidarian Symmetry
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Cnidarians are radially symmetrical.
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Remember what that means????
Radially symmetrical organisms can detect
& capture prey from any direction.
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This is especially helpful for the sessile & slow
moving cnidarians.
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Some cnidarians are aquatic floaters.
Others are sessile.
 What’s that again????
Feeding & Digestion
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What’s a jellyfish look like?
They have long, stinging
tentacles.
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The tentacles have stinging cells
called cnidocytes. (NI duh sites).
They also have nematocysts.
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These are capsules that have
threadlike tubes that contain
poison & barbs.
The nifty nematocyst
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Nematocysts are like harpoons.
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They are propelled by osmotic pressure (water
pressure).
When a cnidarian is touched or otherwise
stimulated water rushes into the nematocyst (by
osmosis) and increases the water pressure,
forcing the barb out of the nematocyst.
The force is so strong that the barb can
penetrate a crab shell.
Feeding & Digestion cont’d
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After prey is hit with a nematocysts or
cnidocytes (tentacles) the cnidarian brings
it to its mouth (it’s only opening).
It then eventually makes it’s way to the
gastrovascular cavity.
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Cells in the gastrovascular cavity secrete
digestive enzymes to break down the captured
prey.
Any undigested material is then expelled through
the mouth. Yummy.
Response to stimuli
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Cnidarians have a primitive nervous system
known as a nerve net.
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These specialized cells can cause contractions
of muscle-like tissue.
The movement of tentacles during prey capture
is a result of these contractions.
Types of organ systems
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As we’ve mentioned Cnidarians have:
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They do not have:
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A primitive digestive system
A primitive nervous system
A circulatory system
A respiratory system
Excretory system
So while they are more complex than a sponge,
they are still not very complex creatures.
Comparison of Sponges & Cnidarians
Example
Body Plan
 Asymmetrical
Radial
Feeding/
Digestion
Filter
Feeder
Digestion takes place in
amoeboid cells
Captures
Movement
Sessile
Aquatic
Response
to Stimuli
No
nervous system
Cells react to stimuli
Simple
Reproduction
Sexual
Sexual
Asexual:
Fragmentation & budding
Symmetry
prey with
nematocysts & tentacles
Digestions takes place in
gastrovascular cavity
floating or sessile
nervous system
consisting of nerve net
Asexual:
budding
Vocabulary Review -- HW
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Define the following words using your notes:
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Radially Symmetry
Sessile
Cnidocyst
Nematocyst
Gastrovascular cavity
Nerve net