The purpose of this experiment is to observe the effects of electricity and light on the decision making of Dugesia dorotocephala, commonly called planarian. Planarians are invertebrate flatworms commonly found in fresh water streams. The experiment is to show whether planarians have a bias towards light or dark and can associate an electrical shock with dark. To determine if planarians have any light bias, the planarian will be tested and trained in a simple maze.

    Planarians are a part of the phylum Platyhelminthes, which is more commonly known as flatworms. The characteristics of this phylum are bilateral symmetry, moderate cephalization, no body cavity, and three layers of tissues. There are three classes in the phylum Platyhelminthes; turbellarian, trematoda, class cestoda. Planarians are under the class turbellarian. Planarians are classified as turbellarian because they are free living and live in ponds and lakes. Other organisms in the phylum Platyhelminthes include flukes and tapeworms, which are both parasitic Platyhelminthes (Raven and Johnson, 1990).

    Planarians regenerate from any small surviving piece of their body. Stem cells, also known as neoblasts, are undifferentiated cells located throughout a planarian’s body. When they are sliced the planarian produce more neoblasts than before they were cut. During regeneration the stem cells are signaled to divide and differentiate. This process replaces lost cell types. The stem cells begin to differentiate during the S phase of interphase of mitosis. During the S phase, DNA is replicated into two identical copies (Newmark & Sanchez 2000). Planarian can be made to reproduce into organisms with two posterior ends and no anterior end or visa-versa. They can also be made to reproduce into organisms with any number of heads. During regeneration the anterior end reproduces faster than the posterior. Grafting can be performed to grow a head on the posterior end of a planarian. After a two and one half-month period a new pharynx will form in the opposite direction of the original pharynx. A two headed planarian is easily replicated by slicing the planarian with a scalpel in a longitudinal cut starting at the anterior end and ending slightly before the pharynx (Buchbaum, 1991).

Figure 1

Demonstration of regeneration of double-headed planarians

    Planarians feed by extracellular digestion. This means that planarians ingest their food then digest it outside of the gastrovascular cavity. They use their sensory cells located on their anterior end. Because their pharynx is located on their ventral side, they must mount their food to ingest it. Planarians usually prey on small live animals, dead bodies of larger animals, and small pieces of raw meat (including hot dogs and liver). Struggling prey can be held in place by the slimy secretions on the pharynx of the planarians. The pharynx is then extended posteriorly and is inserted into the pray. The enzymes from the pharynx soften the prey’s tissues and allow the pharynx to tear the tissues into microscopic pieces that can then be swallowed. Planarians only need to eat about once per seven to eleven days (Buchbaum, 1991).

Figure 2

Demonstration of planarians feeding on raw liver

    Past high school students, at Woodrow High School, have performed experiments on planarian’s memory and behavior. In their experiment an electric shock was used to train planarian to always turn right in a simple T shaped maze. Four different groups were tested, the control being the no stimulus (NS) group. The other control group was the random stimulus (RS) group. The experimental group was the conditioned stimulus (CS) group where the planarians were exposed to a six-volt electric shock. The last group was a phase two (P2) group used in the second half of the experiment. Once the planarian were separated into their respective groups, a planarian from the NS group was placed in the maze for about ten minutes to lay down a layer of mucus on the maze to allow for easier movement of the planarian being tested. A planarian from the CS group was then placed in the maze by itself and given an electrical shock, if it tried to go left or straight, yet was not shocked if it went to the right. This process was repeated three to five times for every planarian in the CS group. Planarians from the RS group were given an electrical shock at random times during the experiment. The shock was given regardless of whether the planarian went left or right. The process was repeated three to five times for every planarian in the RS group. The planarian in the CS group were cut into small pieces and fed to the planarian in the P2 group. The P2 group was then tested in the same manner as the original CS group. The results of the NS group averaged 42.77% right per run. The results of the CS group averaged 55.3% right per run. The results of the RS group averaged 38% right per run. These results proved their hypothesis correct that the RS group would have the lowest right percent and the CS group would have the highest right percent. The P2 group averaged 70% right per run. This can be explained by the fact that the planarians gained more memory from the planarian they ate and therefore had better results in terms of percent of planaria going to the right per run. (http://www.woodrow.org/teachers/bi/1998/planaria/studentlab.html).

Picture of T-Maze that Woodrow High School used

    Very little knowledge has been acquired about stem cells and their role in planarian’s regeneration. Biologists know that neoblasts are signaled to divide and differentiate when the cells are wounded, yet they do not know how the cells signal the neoblasts because of the inability of the planarians to incorporate exogenous deoxyribonucleic acid (DNA) precursors. In an experiment done by Phillip Newmark and Alejandro Sanchez Alvarado, from Department of Embryology, at Carnegie Institute of Washington, they showed that planarians are capable of incorporating thymidine analogue bromodeoxyuridine (BrdU) to allow neoblasts to be labeled specifically during the S-phase of mitosis. The BrdU can be used to track movement and differentiation of neoblasts and has been studied before by other scientists. The evidence exhibited in the experiment shows that neoblasts are arrested in the G2 phase of mitosis by double labeling with BrdU and Mitosis specific marker. The BrdU labeling experiments show evidence that there is not a large, slow cycling population of stem cells in the intact planarian. The problems of metazoan regeneration and the control of stem cell proliferation has been studied more since the ability to label specifically regenerative stem cells and the use of double stranded ribonucleic acid (RNA) to inhibit gene expression in the planarian (Newmark & Sanchez 2000).

Figure 4

Diagram of neoblasts in planarian

    In an experiment done by Kentaro Kato, Hidefumi Orii, Kenji Watanabe, and Kiyokazu Agata, from the RIKEN Center for Developmental Biology, in Kobe, Japan, planarians were tested to see whether the stem cells differenticiate by themselves or have the ability to induce regeneration with pre-existing differentiated cells. For the experiment they irradiated a small grafted piece of planarian where all the stem cells were eliminated and attached it to an intact planarian with dorsoventral reversed orientation. In the scientist’s findings Ifb-expressing cells were generated from stem cells produced on the dorsal side of the planarian, when they normally are produced in the ventral tissue. The conclusion of their experiment is that stem cells may simply differentiate depending on its surroundings and differentiated cells present positional cues that induce morphogenesis (Kentaro Kato, Hidefumi Orii, Kenji Watanabe, and Kiyokazu Agata, 2001).

Results of Kato’s experiment