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Name ____________________________________
Date ___________
Activity: Observing a Cnidarian – the Hydra
Goals:
1. To observe and draw an illustration of a hydra.
2. To identify nematocysts (stinging cells) and the
mouth of a cnidarian.
Background Information: The Greek word “cnidos” means “stinging nettle,” and that’s
how the Phylum Cnidaria got its name and definition. Cnidarians are characterized by
stinging cells called nematocysts or cnidocysts, which when disturbed or hunting prey
eject a barbed thread and often poison as well.
Cnidarians are basically bag-shaped organisms, with a mouth but no anus, and usually
have tentacles containing stinging cells around the mouth. In cross section, a cnidarian
body consists of two cell layers: an ectoderm or outer layer, and an endoderm or inner
layer. Between the two there is a layer of jellylike material called mesogloea.
The hydra is a type of cnidarian. It is classified as a hydrozoan, one of three classes of
cnidarians. The other two classes are the anthozoans, meaning “flower animals” which
include anemones and the scyphozoans which are the “jellyfish.” (This information and
picture are taken from http://www.oceanicresearch.org/cnidarian.html.)
Materials: prepared slides of hydra
Procedure:
Part 1: Full View of a Hydra; Hydra Structure 2
1. Mount the prepared slide of hydra on the microscope stage. Focus your specimen
under low power and then observe your specimen at 100X.
2. Try to get a view of the hydra’s whole body into your field of view. Decide which
magnification provides the best overall view of the hydra and draw what you see.
3. Label the mouth, a tentacle, a stinging cell, the base or bottom of the hydra.
4. Caption your drawing.
Caption:
_____X
Part 2: Tentacle with Stinging Cell
1. Position your slide so that a tentacle with a stinging cell is in the center of your
field of view. Try to focus on the stinging cell under 400X if possible.
2. In a separate drawing, capture a detailed image of one or more stinging cells and
label it nematocyst (stinging cell).
3. Find and label one trigger hair in the stinging cell.
4. Caption your drawing.
Caption:
_____ X
Part 3: Questions:
a. Describe the general shape of the body of the hydra.
b. Is the mouth on the tentacle or base end of the hydra? Why do you think
the mouth is located there?
c. Explain how cnidarians capture prey and defend themselves. In your
explanation, refer to specific body structures.