Planarian Behavior and Sensory Systems A version to print out is HERE

Members of the phylum Platyhelminthes have a vermiform (wormlike shape). The platyhelminths have bilateral symmetry and meet the environment in an aggressive, head-on fashion. Cephalization, a specialization of the anterior end of the animal, can be seen in the clustering of sensory structures and clumps of nerves (ganglia or "brains") in the head region. Many of the elaborations you will see are related to increases in mobility, higher activity, and general increase in responsiveness of the animals.          


Figure 1. The nervous and sensory systems of a planarian.

Planarian Nervous System. The planarian nervous system is arranged in a ladder-like structure with a pair of ventral nerve cords connected in segments by commissural fibers (Figure 1). At each ladder segment along the nerve cord a swelling called a ganglion controls all the musculature and receives sensory information in the immediate area. Ganglia at one level communicate with those anterior and posterior to themselves along their nerve cord and also communicate with those on the other side via the commissural fibers. This coordinates the motor and sensory information at each level along the length of the animal. At the anterior end of the planarian the ganglia are elaborate into a brain. Portions of the brain are specialized for various functions. The mechanosensory neurons are connected to the auricles, the ear-like extensions at the sides of the head that serve as a sense of touch. The mechanosensory portions of the brain are also connected to a statocyst (no present in all species) to allow gravity detection. The chemosensory portions of the brain are also connected to the auricles that also serve as sensory organs to pick up chemicals in the water. This allows the animal to movie toward the smell of potential food or away from a noxious stimulus. When you watch the living planarian, not that it moves it's head back and forth. This is similar to "sniffing" the environment by using its auricles. Light-sensory portions of the brain are connected to the eyespots while interneurons make the decisions about what information should be passed from one part of the body to another.

The eyespots are simple cups lined with pigment cells and filled with a few photoreceptors (Fig 1). These simple eyes do not form an image but they are effective in telling the animal what direction light is coming from. Planarians prefer dim light. In the above diagram the eyespot that is shown is from the left side of the animal. If light were to come from the right, that light would be shadowed by the pigment cells. The other eye, however, would receive the light and the animal would turn away to the left.


Examination of a Living Planarian. Work in groups of two for this section. Wash and rinse a watch glass and then use an eye dropper to gently transfer a planarian to the bottom of the container (remove your animal from the container labeled "nonfed"). Use a dissecting microscope or hand lens to observe the behavior of your specimen (record your observations in the Results Section). How does your specimen appear to move? Use a dissecting needle and gently touch your specimen. Compare the responses of the animal when touched at the posterior end versus a light touch on the auricle. What do you conclude from the animal's response? If you flip your specimen over, does it right itself? Cover half of the watch glass with a card, and then illuminate your animal from above. Describe the orientation of the animal to the level of illumination (does it move straight toward the preferred illumination, or does it take a meandering route?).

Remove the light barrier and introduce a small piece of rinsed raw liver (or egg yolk) to the side of the arena opposite the planarian. A drop of cider vinegar can also be used as an attractant. Describe the route taken by the planarian toward the food source. What type of movements does the head make? What do you think is the significance of these head movements? Describe the planarian's initial feeding (or that of another group's organism if yours did not feed). Return your animal to the container marked "Fed Planarians."

Figure 2. Planarian light maze

Transfer an animal to a Y-maze (Fig 2).

  1. Allow the planarian to wander through the maze for several minutes. Determine if there is a preferred arm of the maze.

  2. Cover the maze as shown above with the green and red filters. Release the planarian and allow it to make a choice between the two colors. When it makes a choice, record it and then move the filters so that it has to make a choice again. Do this five times.

  3. Your planarian makes its choice by moving to the darker environment. If, for example, it moves into the green filter area, it can not see the color green. A grey filter has been supplied with the maze. Use this filter to show that the animals are not responding only to the color. Design this experiment yourself. Explain what you have done and the results of your experiments.