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The Hydra: An Unusual Cnidarian
 Hydras are animals that belong to the phylum Cnidaria. The word Cnidaria
means “_stinging nettles_”. Cnidarians use _nematocysts__ to sting &
capture their prey then tentacles bring prey to their mouth.
 Cnidarians have _radial___ symmetry, and use a gut for digestion.
 Cnidarians come in 2 body forms; a polyp which is shaped like a __vase_,
or a medusa, which is shaped like _mushroom__ with tentacles. Unlike
most Cnidarians, hydras live their entire lives as polyps.
POLYP
MEDUSA
 The nervous system of hydras (and all Cnidarians) consists of a simple
network of nerves called a __nerve net___ that controls the movement of
the body and tentacles. Messages coming into hydras are gradually
spread throughout their bodies by the nerve net.
Materials:
Pipette
Hydra
Watch glass
Hand lens/ microscope
Toothpick
Daphnia
Procedure:
1. Use a pipette to place one hydra in a watch glass along with some of the
water in which it lives.
2. Observe the hydra with a hand lens or under the microscope under very-low
power (4X). How many tentacles did your hydra have? ________
3. Make a drawing of the hydra on page
2 of this lab, labeling the mouth,
body, tentacles, and basal disk.
4. While looking at the hydra, gently
touch the hydra in three different
places with a toothpick. Observe
what happens.
5. Place some Daphnia (water fleas) in
the dish. Observe what happens.
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Labeled picture of hydra
Analysis:
1. How many tentacles did your hydra have? Do all hydra have the same number
of tentacles? (check with other lab groups)
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2. How is the body shaped? Is that considered polyp or medusa form?
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3. Where is the mouth located?
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4. Where is the basal disk?
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5. When you touched the hydra, did anything happen to suggest that the touch
spreads to other areas? What is responsible for this reaction?
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6. What evidence is there to show that the hydra used stinging cells to get food?
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7. What are 2 ways that the tentacles help the hydra in feeding?
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8. Why is budding considered a form of asexual reproduction?
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