Materials

■ silicone culture gum

■ microscope slide

■ hydra culture

■ 2 medicine droppers

■ compound microscope

■ methylene blue solution

■ stereomicroscope

■ filter paper cut into pennant shapes

■ concentrated beef broth

■ culture of Daphnia pulexor Daphnia magna

Background

1. How do animals respond to stimuli in their environment?

3. What characteristics do hydras share with other cnidarians?

4. How does a sessile animal, such as a hydra, obtain food?

5. What is a nematocyst?

Close-up Examination of a Hydra

1. ^ty CAUTION Slides break easily. Use care ▼ when handling them. Using a long piece of silicone culture gum, make a circular "well" on a microscope slide, as shown in the illustration below.

Silicone culture gum

CAUTION You will be working with a live animal. Be sure to treat it gently and to follow directions carefully. With a medicine dropper, gently transfer a hydra from the culture dish to the well on the slide, making sure the hydra is in water. The hydra should be transferred quickly; otherwise, it may attach itself to the medicine dropper. Allow the hydra to settle. As you go through the following steps, add water to the slide periodically to replace water that has evaporated, and keep the hydra wet.

Examine the hydra under the low-power setting of a compound microscope. Add a drop of methylene blue solution to the well containing the hydra to make the tentacles more visible. Identify and draw the hydra's body stalk, mouth, and tentacles in your lab report. In your lab report, make a data table like the one shown on the next page. As you complete the following steps, record your observations in your data table. As you continue to observe the hydra at low power, add a drop of vinegar to the well. Record what happens to the bumps on the tentacles. These bumps are cnidocytes.

6. Transfer the hydra to the culture dish labeled "Used hydras." Rinse the well on your microscope slide with water to remove all traces of methylene blue and vinegar.

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