1. HW: Study notes & Define Vocab Words.
Phylum Cnidaria
HW: Study notes & Define Vocab Words.

2. Intro to Cnidarians How do you say it???? What are cnidarians?
Say it with me now: (ni DARE ee uh)
Practicing saying this when you are with your friends so you sound smart.
What are cnidarians?
Examples: Jellyfish & Sea Anemones
Are there many species of cnidarians?
Yes, there are 10,000 different species!

3. Body Structure Like sponges, they have: Unlike sponges:
One body opening
Two cell layers
Unlike sponges:
Their cell layers are organized into tissues that have specific functions.
They have symmetry

4. Body Structure, cont’d The function of the two layers:
Outer Layer: protects internal body
Inner Layer: mostly for digestion

5. Cnidarian Symmetry Cnidarians are radially symmetrical.
Remember what that means????
Radially symmetrical organisms can detect & capture prey from any direction.
This is especially helpful for the sessile & slow moving cnidarians.
Some cnidarians are aquatic floaters.
Others are sessile.
What’s that again????

6. Feeding & Digestion What’s a jellyfish look like?
They have long, stinging
tentacles.
The tentacles have stinging cells
called cnidocytes. (NI duh sites).
They also have nematocysts.
These are capsules that have
threadlike tubes that contain
poison & barbs.

7. The nifty nematocyst Nematocysts are like harpoons.
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.

8. Feeding & Digestion cont’d
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.
Cells in the gastrovascular cavity secrete digestive enzymes to break down the captured prey.
Any undigested material is then expelled through the mouth. Yummy.

9. Response to stimuli
Cnidarians have a primitive nervous system known as a nerve net.
These specialized cells can cause contractions of muscle-like tissue.
The movement of tentacles during prey capture is a result of these contractions.

10. Types of organ systems As we’ve mentioned Cnidarians have:
A primitive digestive system
A primitive nervous system
They do not have:
A circulatory system
A respiratory system
Excretory system
So while they are more complex than a sponge, they are still not very complex creatures.

11. Comparison of Sponges & Cnidarians
Example
Body Plan
Asymmetrical
Radial Symmetry
Feeding/
Digestion
Filter Feeder
Digestion takes place in amoeboid cells
Captures prey with nematocysts & tentacles
Digestions takes place in gastrovascular cavity
Movement
Sessile
Aquatic floating or sessile
Response to Stimuli
No nervous system
Cells react to stimuli
Simple nervous system consisting of nerve net
Reproduction
Sexual
Asexual: Fragmentation & budding
Asexual: budding

12. Vocabulary Review -- HW
Define the following words using your notes:
Radially Symmetry
Sessile
Cnidocyst
Nematocyst
Gastrovascular cavity
Nerve net