Planaria (Platyhelminthes) are free-living flatworms that live in freshwater. They are typically found under rocks and debris in streams, ponds, and springs. Planarians are interesting to study for a variety of reasons. First, planarians have bilateral symmetry with two nerves extending the length of the body, an enlarged "brain" (ganglion cell), and two eye spots. The eye spots are sensitive to light. Planarians move away from the light and are most active in the dark. Second, light and water current can be used demonstrate kinesis (change of speed) and taxis (change of direction). Planarians are also sensitive to shock. Therefore, planarians can be conditioned with several stimuli and responses. Finally, planarians are inexpensive and easy to care for making them an excellent animal for lab experiments.

Planaria diagram

Conditioning Planaria

To condition planaria to recoil to a light, present a light for 3 sec. Administer a shock from a 9 volt battery for the final second of the learning trial (i.e., 2 sec light alone and 1 sec with the light and shock). Once the trial is complete, remove the light and shock for 15 sec. Repeat the light + shock pairings for 25 trials. Test the response to the light. It may be necessary to take a break from conditioning for one to several hours before repeating another block of 25 conditioning trials.

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References for research articles using planarians in research