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Introduction to Hydra

Hydra under microscope

Carolina Labsheets™

This lab introduces students to Phylum Cnidaria. The activity also serves as an introduction to animals.

Needed Materials

Hydra (132800, for 30 students)

Daphnia pulex (142314) or freshly hatched brine shrimp

Concavity Slides (632250)

microscope slides


dropping pipets

dissecting needles

razor-sharp scalpels (625980), single-edged razor blades (626931), or utility knifes

Bibulous Paper (634050) or paper towel

0.1 M acetic acid, or 1 volume of household vinegar diluted in 5 volumes of water

Hydra, l.s. (306052)


compound microscopes

Hydrozoa Anatomy Diagram

Optional Materials

You may substitute Deep-Well Slides (603730E) for the concavity slides. Because of their greater capacity, however, you would need to transfer several Daphnia to a Deep-Well Slide in order to have a good chance to observe feeding (or, use a pipet to suction enough water out to restrict the movement of a single Daphnia). If stereomicroscopes are not available, hand lenses will suffice. If your compound scopes are equipped with 4x scanning objective lenses, the 40x option may also substitute for stereomicroscopes. You may make copies of the Hydrozoa Anatomy sheet for reference when students are observing the stained longitudinal section and identifying structures.


Ensure that students understand and adhere to safe laboratory practices when performing any activity in the classroom or lab. Demonstrate the protocol for correctly using the instruments and materials necessary to complete the activities, and emphasize the importance of proper usage. Use personal protective equipment such as safety glasses or goggles, gloves, and aprons when appropriate. Model proper laboratory safety practices for your students and require them to adhere to all laboratory safety rules. Caution students about the use of the sharp scalpels or razor blades and supervise their use. Cultures remaining after the completion of the activities can be added to a classroom aquarium or disposed of by flushing down a sink with tap water. The chlorine and chloramine in most tap water will kill Hydra. If your tap water is not chlorinated, pipet 1 mL of household bleach (sodium hypochlorite solution) or isopropanol (rubbing alcohol) into the culture and wait 15 minutes before flushing down the sink.


If you are using brine shrimp instead of Daphnia for the feeding activity, set up the brine shrimp eggs for hatching at least 24 hours before the lab. See our Brine Shrimp Care Guide for instructions on hatching. The brine shrimp larvae must be rinsed in running tap water to remove the salt and then be transferred to springwater before feeding to hydra. Make the transfer immediately before the feeding activity; brine shrimp larvae survive only about 30 minutes in freshwater.

Do not feed Hydra for at least 4 days before the lab, in order to increase the likelihood that students will observe them feeding. Allowing for 2 days in transit, delay an additional 2 or more days after receipt of the culture before conducting the feeding activity. Although Daphnia are traditionally used for feeding Hydra, any small crustacean can be used. Use Daphnia pulex, not Daphnia magna, which are too large for the Hydra to ingest.

It may not be possible to complete all of the Observing a Living Hydra activity in one lab period. A convenient stopping point is after Step 5, the feeding activity. Students can pick up at Step 6 for the next lab. Even if the Observing a Living Hydra activity is completed in one lab period, it is best to use a fresh Hydra for Step 6. A Hydra that has just captured prey will have discharged many of its nematocysts.

Students can work individually or in pairs.

Set up the following workstations:

For pickup of Hydra For pickup of Daphnia For observing discharge of cnidocytes
Hydra culture Daphnia culture acetic acid
dropping pipet dropping pipet dropping pipet
concavity slide   scalpels or razor blades (use caution)
    bibulous paper


With green Hydra (132810), students will observe symbiotic green algae within the Hydra’s tissues. Place a Hydra in a drop of water on a slide. Add a coverslip to compress it and observe under low and high power. To release algae cells for better viewing, cut off a portion of a tentacle and place it in a drop of water on a slide. Add a coverslip and apply pressure with the eraser of a pencil to crush the tentacle. This will release the algal cells into the water.

Students can design their own experiments to test the response of Hydra to other stimuli. Possibilities include responses to pH, gravity, salt concentration, and light.

Answer Key to Questions asked on the Student LabSheet

  1. Using a dropper, transfer a Hydra to the well of a concavity slide. Add enough water to fill the concavity. Place the slide onto the stage of a stereomicroscope and observe the Hydra for a few minutes. Do you observe anything to indicate that the Hydra is alive?

    Yes, movement.
  2. The Hydra’s body consists of a column with arms (tentacles) at one end. How many tentacles are there?

    Answers will vary; usually there are five or six.

    Compare this number with the number other groups observe on their specimens.

    Do most of the Hydra have the same number of tentacles?

    Usually, the answer is yes.
  3. With the end of a dissecting needle, gently touch one of the tentacles. Describe what happens.

    Students may observe that the intensity of the reaction is related to the intensity of the stimulus; either the touched tentacle will contract, or the entire hydra will contract.
  4. Use the blunt end of the dissecting needle’s handle to tap on the dissecting scope’s stage. Describe what happens.

    The entire Hydra contracts.
  5. Use a dropper to add a Daphnia (or brine shrimp) to the concavity. Observe what happens and describe below.

    Answers will vary. Some will see the Hydra contract when the Daphnia bumps into it. Some will see a tentacle adhere to the Daphnia before the Daphnia breaks away. Most will see the capture of a Daphnia. The tentacles wrap around the Daphnia and draw it to the mouth. It will take several minutes for the Hydra to expand and ingest a Daphnia. If a Daphnia is too large, the Hydra may reject it.
  6. Place a Hydra on a microscope slide. With a sharp scalpel or razor blade, cut off part of a tentacle. Remove the Hydra from the slide. Add a drop of water over the tentacle if needed. Then add a coverslip and observe under low power (100x) and high power (400x). Notice the small, round cells that give a bumpy appearance to the tentacle. These are the stinging cells or cnidoblasts, which are characteristic of organisms in this phylum. Use the iris diaphragm of your microscope to reduce light and increase contrast to make the stinging cells more visible before proceeding.
  7. Now add a drop of acetic acid to one edge of the coverslip. While watching under the microscope, touch a piece of bibulous (absorbent) paper to the opposite edge of the coverslip. This will draw the acid under the glass. Describe what happens.

    The tentacle first contracts, and this may be followed by the firing of the nematocysts.
  8. How might this relate to your observations recorded under 5?

    Contact with a Daphnia triggers firing of a tentacle’s nematocysts, which sting and attach to the prey.
  9. Most animals exhibit either radial or bilateral symmetry. Animals with bilateral symmetry can be divided along the body axis into more or less identical left and right sides. Radial symmetry is similar to that of a wheel or jellyfish. The body can be divided along many different planes, so there are no distinct left and right sides. Does Hydra have radial or bilateral symmetry?

    Hydra has radial symmetry.
  10. List two or more animal characteristics that you have observed in Hydra.

    Movement, response to stimuli, ingestion of food.

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