Planarians

Planaria are non-parasitic flatworms of the biological family Planariidae, belonging to the order Seriata. Planaria are common to many parts of the world, living in both saltwater and freshwater ponds and rivers. Some are terrestrial and are found on plants in humid areas. These animals move by beating cilia on the ventral dermis, allowing them to glide along on a film of mucus. Some move by undulations of the whole body by the contractions of muscles built into the body wall.

They exhibit an extraordinary ability to regenerate lost body parts. For example, a planarian split lengthwise or crosswise will regenerate into two separate individuals. The size ranges from 3 to 12 mm, and the body has two eye-spots (also known as ocelli) that can detect the intensity of light. The eye-spots act as photoreceptors and are used to move away from light sources. Planaria have three germ layers (ectoderm, mesoderm, and endoderm), and are acoelomate (i.e. they have a solid body with no body cavity). They have a single-opening digestive tract, consisting of one anterior branch and two posterior branches in freshwater planarians. Because of this three-branched organization, freshwater flatworms are often referred to as triclad planarians.

The most frequently used in the high school and first-year college laboratories is the brownish Dugesia tigrina. Other common varieties are the blackish Planaria maculata and Dugesia dorotocephala. Recently, however, the species Schmidtea mediterranea has emerged as the species of choice for modern molecular biological and genomic research due to its diploid chromosomes and existence in both asexual and sexual strains. Recent genetic screens utilizing double-stranded RNA technology have uncovered 240 genes that affect regeneration in S. mediterranea. Interestingly, many of these genes are found in the human genome.

It should be noted that the term "planaria" is most often used as a common name. It is also the name of a genus within the family Planariidae. Sometimes, it also refers to the genus Dugesia.

Anatomy and physiology
The planarian has very simple organ systems. The digestive system consists of a mouth, pharynx, and a structure called a gastrovascular cavity. The mouth is located in the center of the underside of the body. Digestive enzymes secrete from the mouth to begin external digestion. The pharynx connects the mouth to the gastrovascular cavity. This structure branches throughout the body allowing nutrients from food to reach all extremities. They eat living or dead small animals that they suck with their muscular mouth. From there, the food passes through the pharynx into the intestines and digesting of the food takes place in the cells lining the intestine, which then diffuses to the rest of the body. Planare receive oxygen and release carbon dioxide by diffusion. The excretory system is made of many tubes with many flame cells and excretory pores on them. Flame cells remove unwanted liquids from the body by passing them through ducts that lead to excretory pores where the waste is released on the dorsal surface of the planarian. At the head of the planarian there is a ganglion under the eyespots. From the ganglion there are two nerve cords which connect at the tail. There are many transverse nerves connected to the nerve cords which make it look like a ladder. With a ladder-like nerve system, it is able to respond in a coordinated manner.

Reproduction
Planaria are hermaphrodites, possessing both testes and ovaries. Thus, they can reproduce asexually with their own gametes or sexually with another planarian. In asexual reproduction, the planarian detaches its tail end and each half regrows the lost parts by regeneration. However several problems can occur with this, so this does not happen very often. Instead, in sexual reproduction, each planarian transports its excretion to the other planarian, giving and receiving sperm. Eggs develop inside the body and are shed in capsules. Weeks later, the eggs hatch and grow into adults. Sexual reproduction is desirable because it enhances the survival of the species by increasing the level of genetic diversity.

Biochemical memory experiments
In 1955, Thompson and James V. McConnell conditioned planarian flatworms by pairing a bright light with an electric shock. After repeating this several times they took away the electric shock, and only exposed them to the bright light. The flatworms would react to the bright light as if they had been shocked. Thompson and McConnell found that if they cut the worm in two, and allowed both worms to regenerate each half would develop the light-shock reaction. In 1962, McConnell repeated the experiment, but instead of cutting the trained flatworms in two he ground them into small pieces and fed them to other flatworms. Incredibly these flatworms learned to associate the bright light with a shock much faster than flatworms who had not been fed trained worms.

This experiment intended to show that memory could perhaps be transferred chemically. The experiment was repeated with mice, fish, and rats, but it always failed to produce the same results. The perceived explanation was that rather than memory being transferred to the other animals, it was the hormones in the ingested ground animals that changed its behaviour. McConnell believed that this was evidence of a chemical basis for memory, which he identified as memory RNA. McConnell's results are now attributed to observer bias. No double-blind experiment has ever reproduced his results.

Books

 * Abramson, C. I., Kirkpatrick, D. E., Bollinger, N., Odde, R., & Lambert, S. (1999). Planarians in the classroom: Habituation and instrumental conditioning. Washington, DC: American Psychological Association.
 * Sheiman, I. M., & Tiras, K. P. (1996). Memory and morphogenesis in planaria and beetle. Westport, CT: Praeger Publishers/Greenwood Publishing Group.

Papers

 * Abramson, C. I. (1990). Classical conditioning. Washington, DC: American Psychological Association.
 * Alvarado, Alejandro Sánchez, "Quick Guide – Planarians" Current Biology Vol. 14 (R737-8), No. 18 (Retrieved online)
 * Arees, E. A. (1986). Absence of light response in eyeless planaria: Physiology & Behavior Vol 36(3) 1986, 445-449.
 * Ash, J. F., McClure, W. O., & Hirsch, J. (1973). Chemical studies of a factor which elicits feeding behaviour in Dugesia dorotocephala: Animal Behaviour Vol 21(4) Nov 1973, 796-800.
 * Best, J. B., Abelein, M., Kreutzer, E., & Pigon, A. (1975). Cephalic mechanism for social control of fissioning in planarians: III. Central nervous system centers of facilitation and inhibition: Journal of Comparative and Physiological Psychology Vol 89(8) Oct 1975, 923-932.
 * Best, J. B., Elshtain, E., & Wilson, D. D. (1967). "Single Monophasic Square-wave Electric Pulse Excitation of the Planarian Dugesia Dorotocephala": Erratum: Journal of Comparative and Physiological Psychology Vol 63(3) Jun 1967, 515.
 * Bianki, V. L., Sheiman, I. M., & Zubina, E. V. (1990). Preference of movement direction in T-maze in planaria: Zhurnal Vysshei Nervnoi Deyatel'nosti Vol 40(1) 1990, 102-107.
 * Buttarelli, F. R., Pontieri, F. E., Margotta, V., & Palladini, G. (2002). Cannabinoid-induced stimulation of motor activity in planaria through an opioid receptor-mediated mechanism: Progress in Neuro-Psychopharmacology & Biological Psychiatry Vol 26(1) Jan 2002, 65-68.
 * Carney, R. E., & Mitchell, G. L. (1978). Reactions of planarians after cannibalization of planarians exposed to four stimulus combinations: Journal of Biological Psychology Vol 20(2) Dec 1978, 44-49.
 * Cebria, F. (2008). Organization of the nervous system in the model planarian Schmidtea mediterranea: An immunocytochemical study: Neuroscience Research Vol 61(4) Aug 2008, 375-384.
 * Coward, S. J., & Piedilato, J. W. (1972). The behavioral significance of the rhabdite cells in planarians: Journal of Biological Psychology Vol 14(1) Jul 1972, 5-7.
 * Dawson, J. H. (1972). Behavioral effects of a magnetic field on goldfish and planaria: Dissertation Abstracts International Vol.
 * Fiore, C., Tull, J. L., Zehner, S., & Ducey, P. K. (2004). Tracking and predation on earthworms by the invasive terrestrial planarian Bipalium adventitium (Tricladida, Platyhelminthes): Behavioural Processes Vol 67(3) Nov 2004, 327-334.
 * Itoh, M. T., Shinozawa, T., & Sumi, Y. (1999). Circadian rhythms of melatonin-synthesizing enzyme activities and melatonin levels in planarians: Brain Research Vol 830(1) May 1999, 165-173.
 * Kato, C., Mihashi, K., & Ishida, S. (2004). Motility recovery during the process of regeneration in freshwater planarians: Behavioural Brain Research Vol 150(1-2) Apr 2004, 9-14.
 * Katz, A. N. (1978). Inexpensive animal learning exercises for huge introductory laboratory classes: Teaching of Psychology Vol 5(2) Apr 1978, 91-93.
 * Kessler, C. C. (1973). The effect of magnesium pemoline on learning in the planarian: Journal of Biological Psychology Vol 15(2) Dec 1973, 31-33.
 * Kimmel, H. D., & Carlyon, W. D. (1990). Persistent effects of a serotonin depletor (p-chlorophenylalanine) in regenerated planaria (Dugesia dorotocephala): Behavioral Neuroscience Vol 104(1) Feb 1990, 127-134.
 * Kimmel, H. D., & Garrigan, H. A. (1973). Resistance to extinction in planaria: Journal of Experimental Psychology Vol 101(2) Dec 1973, 343-347.
 * Kimmel, H. D., Olson, S. J., Stephenson, J. M., & Ray, R. L. (1976). Percent of longitudinal body contraction and latency of response onset in planaria as a function of intensity of electric shock: Physiological Psychology Vol 4(3) Sep 1976, 389-390.
 * Kimmel, H. D., Ray, R. L., & King, J. (1975). A television technique for quantifying conditioned and unconditioned responses of planarians: Behavior Research Methods & Instrumentation Vol 7(4) Jul 1975, 353-356.
 * Krebs, E. K. (1975). Factors in conditioning and orientation of planarians: Learning in a polar field: Journal of Biological Psychology Vol 17(2) Dec 1975, 21-23.
 * Kusayama, T., & Watanabe, S. (2000). Reinforcing effects of methamphetamine in planarians: Neuroreport: For Rapid Communication of Neuroscience Research Vol 11(11) Aug 2000, 2511-2513.
 * Levison, M. J., & Gavurin, E. I. (1979). Truly random control group in Pavlovian conditioning of planaria (Dugesia dorotocephala): Psychological Reports Vol 45(3) Dec 1979, 987-992.
 * Loomis, L. L., & Napoli, A. M. (1975). Transfer of training through two cannibalisms of planarians: Journal of Biological Psychology Vol 17(1) Jul 1975, 37-40.
 * Mason, P. R. (1973). Size and other factors determining planarian behaviour: Journal of Biological Psychology Vol 15(2) Dec 1973, 8-13.
 * Mason, P. R. (1975). Chemo-klino-kinesis in planarian food location: Animal Behaviour Vol 23(2) May 1975, 460-469.
 * Micklin, M. P., & May, J. G. (1975). An apparent frustration effect in planarians: Journal of Biological Psychology Vol 17(2) Dec 1975, 4-9.
 * Nazaryan, O. A. (1973). Retention of traces of previous experience in planaria after regeneration: Zhurnal Vysshei Nervnoi Deyatel'nosti Vol 23(1) Jan-Feb 1973, 120-125.
 * Nixon, S. E. (1974). Some behavioral observations on a cave dwelling planarian: Journal of Biological Psychology Vol 16(2) Dec 1974, 32-33.
 * Nogi, T., Yuan, Y. E., Sorocco, D., Perez-Tomas, R., & Levin, M. (2005). Eye regeneration assay reveals an invariant functional left-right asymmetry in the early bilaterian, Dugesia japonica: Laterality: Asymmetries of Body, Brain and Cognition Vol 10(3) May 2005, 193-205.
 * Pagan, O. R., Rowlands, A. L., Azam, M., Urban, K. R., Bidja, A. H., Roy, D. M., et al. (2008). Reversal of cocaine-induced planarian behavior by parthenolide and related sesquiterpene lactones: Pharmacology, Biochemistry and Behavior Vol 89(2) Apr 2008, 160-170.
 * Pescetto, G., & Dettore, D. (1981). Negative phototaxis and conditioning in the planarian Dugesia dorotocephala: Rivista di Neurobiologia Vol 10(2) Apr-Jun 1981, 287-295.
 * Peters, A., Streng, A., & Michiels, N. K. (1996). Mating behaviour in a hermaphroditic flatworm with reciprocal insemination: Do they assess their mates during copulation? : Ethology Vol 102(3) Mar 1996, 236-251.
 * Raffa, R. B., Dasrath, C. S., & Brown, D. R. (2003). Disruption of a drug-induced choice behavior by UV light: Behavioural Pharmacology Vol 14(7) Nov 2003, 569-571.
 * Raffa, R. B., & Desai, P. (2005). Description and quantification of cocaine withdrawal signs in Planaria: Brain Research Vol 1032(1-2) Jan 2005, 200-202.
 * Raffa, R. B., Stagliano, G. W., Ross, G., Powell, J. A., Phillips, A. G., Ding, Z., et al. (2008). The kappa -opioid receptor antagonist nor-BNI inhibits cocaine and amphetamine, but not cannabinoid (WIN 52212-2), abstinence-induced withdrawal in planarians: An instance of 'pharmacologic congruence': Brain Research Vol 1193 Feb 2008, 51-56.
 * Raffa, R. B., Stagliano, G. W., & Tallarida, R. J. (2006). Subadditive withdrawal from cocaine/kappa -opioid agonist combinations in Planaria: Brain Research Vol 1114(1) Oct 2006, 31-35.
 * Rawls, S. M., Baron, S., Ding, Z., Roth, C., Zaveri, N., & Raffa, R. B. (2008). Nociceptin attenuates methamphetamine abstinence-induced withdrawal-like behavior in planarians: Neuropeptides Vol 42(3) Jun 2008, 229-237.
 * Rawls, S. M., Rodriguez, T., Baron, D. A., & Raffa, R. B. (2006). A nitric oxide synthase inhibitor (L-NAME) attenuates abstinence-induced withdrawal from both cocaine and a cannabinoid agonist (WIN 55212-2) in Planaria: Brain Research Vol 1099(1) Jul 2006, 82-87.
 * Rawls, S. M., Gomez, T., & Raffa, R. B. (2007). An NMDA antagonist (LY 235959) attenuates abstine H., & Gleason, K. (1975). Determinants of planarian aggregation behavior: Animal Learning & Behavior Vol 3(4) Nov 1975, 343-346.
 * Stasko, A. B., & Sullivan, C. M. (1971). Responses of planarians to light: An examination of klino-kinesis: Animal Behaviour Monographs Vol 4(2) 1971, 47-124.
 * Umeda, S., Stagliano, G. W., & Raffa, R. B. (2004). Cocaine and kappa -opioid withdrawal in Planaria blocked by D-, but not L-, glucose: Brain Research Vol 1018(2) Aug 2004, 181-185.
 * Van Deventer, J. (1974). The behavior of two species of planaria in confined spaces: Psychological Record Vol 24(3) Sum 1974, 379-384.
 * Van Deventer, J. (1974). The feeding latencies of two species of planaria: Psychological Record Vol 24(3) Sum 1974, 373-377.
 * Vreys, C., Schockaert, E. R., & Michiels, N. K. (1997). Unusual pre-copulatory behaviour in the hermaphroditic planarian flatworm, Dugesia gonocephala (Tricladida, paludicola): Ethology Vol 103(3) Mar 1997, 208-221.
 * Wisenden, B. D., & Millard, M. C. (2001). Aquatic flatworms use chemical cues from injured conspecifics to assess predation risk and to associate risk with novel cues: Animal Behaviour Vol 62(4) Oct 2001, 761-766.