We dasnik an inexpensiw 1abnmtor)r exerci.ie thar lcres pla. mrians to demonstrate habitunrion and scnsitizaticm. Students drop waterfrom an eyedropper onto a plonarian's anteriw re@ and observe its contraction respaues. Initially. studenti practice the pmcedure until atmining high interobserver reliahilitv in scming the degree of contraction. Subsequently, rhey measure the decline in responciviry that occurs over repeated stimulus presentation~. Fcrllowing Imbin.mtion, the planarion is d r a m in and otir of the eyedropper sewerr11 rimes, resultine in sensitization and dishabituation of the contractile respnnie in accordance with drralprocess theory (Gmues FP Thompson, 1970). This ~~rucedure can also k used to demonstrate spontaneous recovq und shortw s u s long-term habituation processes.Difficulties associated with demonstrating fundamental learning processes to large groups of students without large costs have inspired many ingenious lahoratory exercises requiring simple equipment. O n e successful apprrrach uses planarians (e.g., Dugesia drmrtocephla) as experimental suhjects. As Katr (1978) noted, planarians occupy a unique position in evolutionary history as the earliest hilaterally symmetricanimals. Thesesimple, aquatic, invertebrate flatwornis evince synaptic nerve cunduction and are capahle of learning. Planarians are small ( I to 2 cm long), nonthreatening to students, inexpensive, and easy to care for (see Kenk, 1967). Descriptions of planarian anatomy (e.g..Pearse. Pearse, Buchshaum, & Buchsbaum, 1987;Stachowitsch, 1992) and behavioral hiology (e.g., Jenkins, 1967) are readily availahle. These animals exhibit habituation. classical conditioning, and instrumental condirinning (reviewed hyComing&Kelly, 1973). Descriptionsirfplanarianbased lahoratory exercises for educarional settin~s are available for classical and instruniental conditioning ( r . g . , Ahramson, 1990; Karz, 1978) but not for hahituarion and rensitiiation. The latter phenomena can he shown using other invertebrates (e.#., Ahramsc~n, 19YO), hut rhe specialized equipment required may make such exercises impracticable with large grnups nf srudents.This article extends the literature concerning the use of planarians in the undergraduate learning lahoratory to include the pnrcesses of hahituation and sensitization. As defined hy Groves and Thompson (19701, hahitvation refers to the decrease in resprnsiveness to a stin~ulus thar can occur as a result of repeated exposure to char stimulus. Conversely, sensitization refers to an increase in resplnsivenessas a restilt of stimulation. Groves and Thompson's dual-pn~cess theory proposed that hahituation is the simplest form of true leaming, resulting from experience-hased changes in a specific stimulus-response system. Sensitization, in contrast, was hypothesized to result from state changes in a central arousal system that does nc)t reflect a learning process.A planarian swimming in a Petri dish initially exhihits a reflexive contraction response when a water droplet is released onto or near its...
