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Eye to Eye With Garden Snails

By Kathy Liu



ABSTRACT

This Snail Unit encourages students to explore the external characteristics and behavior of snails. It effectively gets students past the "ugh, slime" reaction to recognizing individual differences in snails and challenges students to learn enough about the snail to be able to predict their behavior under a variety of conditions. Detailed observations are requested as are preparation and testing of hypotheses. This unit works very well with all levels of students and with heterogeneously grouped students. This Snail Unit consists of six lessons:
  1. Introduction to a Snail
  2. How do snails move? How fast is a snail's pace?
  3. What and how do snails eat?
  4. Are snails attracted to, or repelled by particular substances?
  5. Can snails be enticed to travel faster or in a certain direction?
  6. How are snails like other animals? How are they different?

TYPE OF ACTIVITY

  • hands-on
  • inquiry lab
  • authentic assessment
  • group/cooperative learning
  • TARGET AUDIENCE

  • Life Science
  • Biology
  • Integrated science ( level 1)
  • Environmental studies
  • Special needs (ESL, LEP, NEP)
  • Special education
  • INFORMATION FOR THE TEACHER

    If you plan to use garden snails in your classroom, collect the snails well before you want to use them. Sudden changes in the weather can send an apparently plentiful supply of snails into hiding so that none seem to be available.

    Once collected, snails can be kept in a variety of ways.

    METHOD 1

    Active snails can be kept for classroom viewing in a terrarium made from an aquarium, a large, wide mouth jar, or a fish bowl. The top should be securely covered with screen or part of a nylon stocking. Lids which have holes punched in them do not allow adequate ventilation and the sharp edges on the interior are a safety risk . Snails "chew" their way out of containers covered with paper. Although the contents of the terrarium can be an elaborate recreation of a garden environment, or something as sparse as only food and water, most will probably be between these two extremes.

    Water can be supplied in a shallow plastic lid, in a deeper dish which contains a water soaked sponge, or by generously sprinkling lettuce or other fresh leafy food with water.

    Food can also be put into a shallow plastic lid. Snails are vegetarians and eat many kinds of plant material. Cornmeal, oatmeal, and fresh green leaves are all appropriate foods. Chalk, egg shells, or snails should be powdered and added to the food to provide the calcium necessary for healthy shells.

    Cleaning to remove moldy food, the build up of mucus, and droppings, should be done on a regular basis.

    Overcrowding should be avoided. Too many snails in too small a space, or inadequate ventilation can cause the humidity to rise to unacceptable levels and the snails to die.

    The life cycle of the snail can be observed in the classroom. Adult snails can be recognized by a small lip which is added to the open end of their shell when growth is complete. Because snails are hermaphroditic (each snail has both male and female sex organs), any two snails can mate. Each fertilizes the eggs of the other. The eggs are laid in a hole which the snail digs in the soil. Translucent white ggs about the size of small peas ore translucent white are laid in damp soil.

    METHOD 2

    If snails are kept in a clean, dry, container covered, they will not require continual attention. (I have kept class sets of snails in a couple of shoe boxes for weeks at a time.) Plastic or metal lids which have holes poked in them will be too humid. If the humidity is too high, the snails will be active or at even higher levels, the snails will die. Over crowding can also raise the humidity to unacceptable levels. If there is room for the snails to align themselves in a single row around the container opening, the container is probably large enough.

    Snails can be maintained in this manner, without food or water, for weeks or even months. In nature, snails seal themselves off from adverse conditions and emerge to feed when temperature and humidity are favorable.

    Snails kept without food and water for an extended time need to be "awakened" before you need them for class. To awaken a snail which is attached to a surface, gently remove the snail from the surface and dip it into water. Then set the snail on a damp surface. The snail may emerge from its shell within a few minutes, or it may take six to eight hours, depending, in part, on how long the snail has been without food and water. Snails may emerge without special treatment if damp lettuce or other food and water is left with them over night. Those which are slow to emerge may need to be dipped in water a second or third time.

    Snails which withdraw into their shells during class can usually be coaxed out by a brief dip in a shallow container of water. The lids from frozen juice cans are convenient to use because they hold enough water to wet the snail, but hold so little water that spills are not a problem.

    These activities have been used successfully with college level biology majors and with elementary school classes. Below each lesson description are comments specific to the individual lesson.

    Materials (Applies to all lessons described) In addition to the materials listed in each lesson, students will need snails, shallow containers of water, wet and dry paper towels, paper on which to record observations.

    LESSON 1: INTRODUCTION TO A SNAIL

    OBJECTIVE

    To become familiar with details of snail external structure.

    MATERIALS

    Magnifying lenses, plain paper, rulers, pencils or colored pencils

    STUDENT ACTIVITIES

    1. Examine shell shape, color texture, size, using sense of sight and of touch.
    2. Examine snail body, find tentacles, eyes, foot.
    3. Make drawing of snail. Label parts.
    4. Let snail crawl on transparent surface and look at the underside of the snail. Find the pneumostome
      (breathing hole).
    5. Draw snail as seen from underside.

    BACKGROUND INFORMATION AND TEACHING TIPS

    One way to present the snails is to put them on a head of lettuce and invite students to choose the snail they want to work with. Snails can be marked with some identifying mark so that students can find the same snail for future lessons. Nail polish, permanent marking pens, or typewriter correction fluid all work for this purpose.

    Students should be encouraged to touch the snails, as well as look at them. Reluctant students can be given their snails in plastic strawberry baskets. These containers allow squeamish students to hold the snail without touching it, Even the most tentative touch will help the student become involved. Have students have examine the shell and the body of the snail with their unaided eyes first, then challenge them to find additional detail with the help of magnifying glasses and/or dissecting scopes. Later lessons seem to go more smoothly if the students have plenty of time to look and touch during this lesson.

    Asking students to make a detailed drawing forces them to look more closely than they might otherwise. I require that the students make their drawings such that the shell is at least 6 inches across. Discussion of the purposes and styles of drawing is especially useful if this is the first drawing of the course, especially if you have recognized cartoonists or artists in your class.

    LESSON 2: HOW DO SNAILS MOVE? HOW FAST IS A SNAIL'S PACE?

    OBJECTIVE

    To observe and describe snail motion and speed.

    MATERIALS

    Stop watch or clock with second hand

    STUDENT ACTIVITIES

    1. Watch snails crawl across table and across glass. Observe and describe the motion of the foot. Allow a snail crawl to crawl on your hand and describe how it feels.
    2. Determine the snail's preferred direction of motion. Compare to at least 3 other snails.
    3. Measure the speed of the snail as it crawls along a ruler.
    4. Discuss repeat ability of results and repeat measurements.
    5. Calculate average speed. Calculate feet per minute and miles per hour.
      Some convenient conversions:
      cm/min x 1.97 = feet/hour; ft/min x 0.0114 = mi/hr;
      cm/min x 0.6 = meters/hr.

    BACKGROUND INFORMATION AND TEACHING TIPS

    Students should feel the motion of the snail on their hand and compare what they feel with what they see. Depending upon time available and your emphasis, this lesson can be combined with the previous one and kept very brief, or it can become much more involved as students measure snail speed and calculate the average rate of a snail's pace. Experimental results should be repeatable, and repeated under identical conditions should give approximately the same results each time. Lead students to discovery that experimental conditions, individual variation, and unknown factors can all influence experimental results. If time allows, compare the rates of large and small snails. Have students record their results in chart form and later graph the data. Another extension would be to investigate how snails respond to obstacles placed in their paths.

    LESSON 3: WHAT AND HOW DO SNAILS EAT?

    OBJECTIVE

    To compare and contrast human and snail what and how snails and people eat.

    MATERIALS

    Lettuce, cornmeal, cornstarch. Wet cornmeal and put in on glass in a thin layer. Mix cornstarch with water and put in on other pieces of glass. Let dry over night (can be used immediately if paste is thick enough to stick to glass). Students can do this for themselves but techniques should be demonstrated.

    STUDENT ACTIVITIES

    1. Put snails on a crisp leaf of lettuce. Listen for the scratching sound when it begins to eat.
    2. Watch through glass as snails eat cornmeal, then
    3. Watch as snails eat cornstarch. Count the strokes of the radula per minute. Examine the marks let in the cornstarch by the scraping radula, or some .
    4. Design an experiment to determine how much food a snail eats in a day, other period of time.
    5. Compare snail diet and human diet. Snails need calcium for their shells, just as humans need calcium for bones. Discuss how snails and humans meet nutrient requirements.

    BACKGROUND INFORMATION AND TEACHING TIPS

    Ask grocery store produce person for an untrimmed head of iceberg lettuce and put a group of snails on the lettuce. In a quiet classroom, a snails eating a head of lettuce can seem quite noisy.

    Each student or small group of students can watch the eating process if the food is mixed with water, painted onto a microscope slide or other piece of transparent glass or plastic. A thicker paste, applied with the fingers or a spatula can also be used. As snails eat, students can see the snail's radula scrape the food off the glass. Tooth marks should be visible in the cornstarch left on the glass. (Snails have thousands of teeth on their radula(tongue). Have students count the strokes of the radula per minute.

    Snails eat both cornmeal and cornstarch. If time limitations force you to choose between the two, use cornstarch. The coarser texture of the cornmeal gives less detail. However, having students compare the marks left in the two substances encourages more detailed observation than simply having them look at one or the other. This lesson can be done as a demonstration, and the class can view the tooth marks in the cornstarch via an overhead projector, but much detail is missed this way.

    LESSON 4: ARE SNAILS ATTRACTED TO, OR REPELLED BY PARTICULAR SUBSTANCES?

    OBJECTIVES

    To test snail reactions to a variety of textures, tastes and smells. Students will learn basic ideas of experimental procedure including single variable and repeatability of results.

    MATERIALS

    Test substances which can include: sand, sawdust, flour, chalk dust, sandpaper, sticky tape, white glue, milk, lettuce, slice of apple or orange, or any other substance you or your students. DO NOT INCLUDE salt, sugar, laundry detergent, boiling water, or other substances which might be harmful to students or snails.

    STUDENT ACTIVITIES

    1. Introduce the concept of single variable and have students design experimental set-up to test single substances.
    2. Have students decided on how they will evaluate snail response to decide if the response is positive, negative, or neither and design a chart on which to evaluate their data.
    3. Review concept of repeatability of results as it applies to this lesson. Have students repeat tests and compare results of successive trials.
    4. Have students compare their results with those obtained by other students. (This is clearer to students if the information is shared via a chart on an overhead transparency or the chalk board.

    BACKGROUND INFORMATION AND TEACHING TIPS

    This lesson provides opportunities for creative and analytical thinking as students decide what substances to test, how to test them, and how to interpret the snail's response. Not all results will be decisive. The teacher should set limits on the types of substances to be tested. Substances known to be lethal (e.g. salt, laundry detergent and sugar) to snails or harmful to students should not be allowed.

    Encourage comparisons such as different textures (glass and sandpaper), different powders (flour and chalk dust), or different temperatures (ice cold and warm, not hot).

    Discuss the concept of variables and the need for a single variable in order to have meaningful experimental results. Also, discuss the need for repeatable results. Have students produce a list of some of the factors which might lead to variable results.

    Design of the test situation should be left to the students. Each group of students agreeing on a particular test design might compare their results with others testing the same substance in a different way. One possible test situation is to put a snail in the center of a ring of the test substance. Another method is to put the substance directly in front of the crawling snail.

    Encourage your students to test their own ideas. Many have become conditioned to feel that there is only one right way to do something.

    LESSON 5: CAN SNAILS BE ENTICED TO TRAVEL FASTER OR IN A CERTAIN DIRECTION? HOW CAN WE GET A SNAIL TO WIN THE "GREAT SNAIL RACE"?

    OBJECTIVE

    To evaluate and apply information gained in previous lesson to influence the speed and direction of a snail.

    MATERIALS

    Test substances from previous less which students feel will make their snails go faster or straighter.

    STUDENT ACTIVITIES

    1. Use rulers for race tracks and have students work with their snails in practice races.
    2. Have students design a race course and decide on ground rules for the snail races
    .

    BACKGROUND INFORMATION AND TEACHING TIPS

    This lesson provides an opportunity for students to evaluate information gained in previous lessons and put into "practical use." Snails move according to their own reflexes and instincts. The better the student understands those reflex and instincts, the greater the probability the student will get his or her snail to crawl directly to the finish line during a snail race. Time trials or practice races give students a chance to their ideas.

    A snail race can be an activity at the end of this day, or it can be a featured event and take a whole day. Have students design their race course and ground rules for the races which can be as simple as match races between pairs of snails, or a variety of "track and field" events. One snail race was covered by the local newspaper and a photograph of the winning snail crawling across the nose of its "trainer" made the front page of the paper. A feature article and more photos followed on the front page of a later section. You might have a student contact the sports editor of your local paper with the details of your event.

    LESSON 6: HOW ARE SNAILS LIKE OTHER ANIMALS? HOW ARE THEY DIFFERENT?

    MATERIALS

    Invertebrates for comparison: Slugs, water snails, familiar or readily animals uch as sow bugs, pill bugs, earwigs, crickets or meal worms. Spiders should not be handled by students. Earthworms, caterpillars, and shrimp or crayfish are other ideas. Containers for invertebrates. Magnifying lenses.

    STUDENT ACTIVITIES

    1. Examine shape, color, size, and texture as in Lesson 1.
    2. Compare invertebrates to each other and to their natural habitats.
    3. Using drawings or words record observations.

    BACKGROUND INFORMATION AND TEACHING TIPS

    Water snails can be obtained from most aquarium stores or biological supply houses and maintained in a classroom aquarium. Slugs can be found in damp places where they are protected from drying. Look under rocks and fallen logs or boards which have been on the ground for an extended period of time. Slugs need to be kept on damp paper towels or other moist surface while they are being observed.

    Comparing other invertebrates to a snail can lead to comparing physical structure to how animals get their food can lead to a discussion of adaptations to environmental niches.


    ACKNOWLEDGMENTS

    All of the material in this unit was developed with the help of collaborating teachers and students. I would especially like to thank Cynthia Peterson and Elizabeth Johnson for their part.


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