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Carolina LabSheets™: Introduction to Planaria

In this lab, students examine the anatomy and behavior of the planarian, a simple animal with bilateral symmetry. The activity does not cover regeneration.
For that, see our 132900 Carolina BioKits®: Planaria Regeneration or our Carolina™ Protozoa and Invertebrates Manual.

Needed Materials

Optional Materials

You may wish to print the Planarian Anatomy sheets for student use. Be aware that the sections shown in the review sheet are composites made from viewing hundreds of sections. It is unlikely that any one cross section will show all, or even most of the features on the sheet. It is also possible that a cross section may reveal features not shown on the sheet. You may, for example, find a section that goes through the eyes, showing the cup shape of the eyes, the layer of pigment granules, and nerve cells inside the eye cups. Often, the middle cross section will be through the pharynx.


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. Cultures remaining after the completion of the activities may be added to a classroom aquarium, or they may be flushed down a sink with tap water. The chlorine and chloramine in most tap water will kill planaria. 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.


Planarians tend to foul their water and require frequent water changes. See our Carolina™ CareSheet: Planaria for instructions on their maintenance.

Students may work individually or in pairs.

Prepare the 5% salt solution by dissolving 5 g of salt (NaCl) in water to make 100 mL of solution.

Set up one or more workstations with the following materials:

  • petri dishes
  • container of springwater
  • dropping pipets
  • planaria culture

You may wish to supervise the pickup of the planarians. Make sure that students use only the tips of the pipets. If a planarian is drawn too far into a pipet, it may attach and become impossible to remove without harming it.

Set up separate workstations with the following:

  • 5% salt solution in dropper bottle or with separate pipet
  • bibulous paper
  • microscope slides
  • coverslips

Optional: This lab can be done in conjunction with our Introduction to Hydra LabSheet to give a comparison of two simple animals (with planaria being more complex). Here is a list of a few ways that hydra and planaria are alike and different.

Characteristic Hydra Planaria
Tissue Layers Two Three
Organization Tissues Tissues and Organs
Symmetry Radial Bilateral
Sense Organs None Eyes
Digestive Tract Sac-like, one opening Highly branched, one opening

The activity Observing stained cross sections of planarians can be considered optional.

Planarians show a number of interesting behavioral responses. Students can design experiments to test how planarians respond to pH (and try to determine their favored pH), gravity, light, electrical and magnetic fields, and other stimuli. These activities are highly recommended. See the following website for instructions on making inexpensive observation chambers for conducting behavioral studies of certain small invertebrates such as planarians: http://www.eeob.iastate.edu/faculty/DrewesC/htdocs/toolbox-V.htm

Answer Key to Questions Asked on the Student Labsheet

Expect considerable variation in the answers to questions involving behavioral responses. Recall the adage, “Under a set of carefully constructed experimental controls, an organism will behave as it pleases.”

Is the planarian active or passive?

Active. It is in almost constant motion, gliding about on the bottom of the petri dish. Note: Although this is typical, planarians sometimes go through periods of inactivity.

Do planarians have radial or bilateral symmetry?

Planarians have bilateral symmetry

Do you observe anything to indicate that planarians have anterior and posterior ends?

Yes, the planarian moves with the same end forward at all times, and this end has a distinct head.

Do you observe anything to suggest that cephalization is characteristic of planarians? Explain.

Yes, there is a distinct head with eyes. This suggests, but does not prove, that nerve tissue is concentrated in the head region.

Planarians are often described as being flattened dorsoventrally. Describe what this means

The planarian’s body is very thin relative to its width; i.e., the dorsal and ventral surfaces of the animal are close together. Because of this feature, the group that includes planarians is often called flatworms.

Fill a dropping pipet with springwater. Place the tip about 1 cm behind the planarian and gently squeeze the bulb. (If you squeeze too hard, the planarian will be blown loose from the bottom of the dish.) Describe the reaction, if any, of the planarian to the puff of water.

The posterior contracts, and then the anterior extends.

Repeat this action to the front of the planarian. Is the reaction the same? Describe.

No, the anterior of the animal waves from side-to-side. The planarian may then begin to move forward again, or it may begin to move in a new direction.

Trials Rolls Right Rolls Left
Trial 1  
Trial 2  
Trial 3  
Trial 4  
Trial 5  

Does your planarian show a bias toward one side when it rolls?

Answers will vary. Data shown above are for a worm with a bias to the right. It is often stated that a planarian will always roll to the same side, either its left or its right; thus showing “handedness.” This is not strictly true. Some will roll to one side more often than the other; others show no preference. Discussion can lead to comparisons with human handedness and to some interesting reports and research projects.

Dip a strip of bibulous paper into a 5% sodium chloride (NaCl, table salt) solution. Place the end of the saturated paper into the water about 1.5 cm in front your planarian while it is moving. Record any change in movement of the planarian. If it seems to react to the sodium chloride, indicate whether the reaction is best described as a positive or a negative taxis

The planarian shows a negative taxis.

View Planarian Anatomy Sheets

View Student Lab Sheet

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