What Can We Learn About Invertebrates at the Pond?

• Lesson A - Presto Change-o
• Lesson B - Glorious Gastropods
• Lesson C - Spider Survey

Lesson B - Glorious Gastropods

FYI

Gastropod means “stomach-foot.” Snails and slugs are gastropods.

They belong to the Phylum Mollusk. Two body parts that are characteristic of mollusks are the mantle and the large, muscular foot. The mantle of snails and conches secretes material for making their shells. The shell provides camouflage and protection and helps conserve water. The mantle of slugs is vestigial, appearing as a raised “cap” or “saddle” behind the optical tentacles.

A gastropod uses its foot to attach to objects or to move. Muscle fibers in the foot alternately relax and contract, creating pedal waves that cause forward movement. The pedal gland on the sole of the foot secretes a trail of mucus, which allows gastropods to glide across surfaces. Most of the body surface of slugs also produces mucus. It helps prevent dehydration and is used for self-defense.

The following are other characteristics of interest in snails and
slugs.

• The mouth is on the bottom surface of the foot and contains a tongue-like radula. The radula is covered with rows of teeth that work like a file to break up food and scrape algae off surfaces.
• Slugs and snails do not have antennae. They have two pairs of tentacles which they can move independently in or out, up or down, to gather information. The large tentacles on the top of the head end in light-sensing organs. These are called optic tentacles. The smaller pair of stalks below the optic tentacles are sensory tentacles. They are used like whiskers on a mammal to feel the way ahead.
• Slugs live only on land. Snails may live on land or in the water. Slugs and land snails have lungs. Most water-dwelling snails use gills to breathe.
• Slugs and snails are hermaphrodites. That means that each animal has both male and female sex organs.

For the lesson below, leaders either may buy snails or collect snails or slugs from “the wild.” There almost certainly will be slugs in the schoolyard habitat most months of the year. If you begin the classroom lessons studying slugs, it allows learning and experimentation to extend to the habitat more easily.

MATERIALS

Part 1, 2 and 3

• Live slugs or snails (buy from a biological supply company or gather from the “wild”)
• Large (at least 10-gallon) aquarium with a solid lid
• Oatmeal, wheatgerm, and/or grass or lettuce grown in the aquarium, to feed the slugs or snails
• Journals, data sheets, or record pages; one for each learner

Part 2

• Hand lenses, one for each team
• Petri dishes, one for each team
• A variety of materials to create texture test surfaces.
• Rough: broken clay pot shards, sandpaper. Soft: fabric, dry or wet moss

Part 3

• Mini-aquaria, one for each team
• A variety of plant seeds to test for food preferences
• Apples, lettuce, or other foods to test for food preferences
• A variety of materials to create texture test surfaces.
• Rough: broken clay pot shards, sandpaper. Soft: fabric, dry or wet moss
• Other reasonable materials requested by learners, such as colored and white paper, scissors, a spray bottle, a ruler

PREPARATION

Leaders should review the following:

• FYI section
• 4-H Science Inquiry Model
• Learner evaluation criteria in the ODE Scientific Inquiry Scoring Guide (PDF) to assist in coaching learners

Buy land snails and/or collect slugs several weeks before the activity is scheduled: they need some time to begin to behave normally in their new home.

Set up an aquarium with materials that simulate the gastropods’ natural environment. The surroundings should be moist, but not wet. Keep a solid lid on the aquarium to help provide the proper humidity.

If you bought snails, feed them the material that came with them. Additional foods include oatmeal and wheat germ, and grass or lettuce that you can grow in the aquarium by sprinkling seeds on the soil’s surface.

You can involve learners in collecting the gastropods and setting up the aquarium to practice using observational skills. You may wish to reserve feeding live plants for learners to test as part of their inquiries.

PROCEDURE

Part 1

Using the information in the “FYI” section, lead a discussion about snails and slugs (gastropods). Adapt the information to the age of the learners. The first thing learners generally comment on is the slime. Ask: Did anyone ever handle a slug and then try to wash the slime off their hands? What happened? It probably didn’t work very well. Slug slime absorbs water to prevent the slug from drying out! Rub off slime with a dry towel or by rubbing dry hands together.

If the aquarium has not been where learners could see it, have them spend a few days observing the gastropods periodically in the aquarium. You may need to wipe the slime trails off the glass walls daily. Ask learners to record their observations in their journals, data sheets, or record pages. Also, ask them to record any questions they have about the gastropods and their behavior. At the end of this observation period, ask learners to share their questions and observations.

Part 2

Place the gastropods in slightly moist Petri dishes, one for each team of learners. It is important to use a Petri dish or other clearsurfaced container so learners can observe the sole of the gastropod’s foot. Pass out the hand lenses and ask learners to use their journals, data sheets, or record pages to record their observations. They need to sit quietly for the gastropod to begin moving around normally. Coach learners to lift the Petri dish and look under the gastropod to locate the mouth and observe the motion of the foot. They should be able to see the pedal waves moving along the sole of the foot.

Ask learners to draw a picture of their gastropod and identify its parts on the picture.

Ask the teams to include in their observations how the gastropods respond to different simple stimuli. Do they react to movement? Light? Food? The type of surface they are on? How fast do they move? At the end of this observation period, ask learners to share new questions and observations. Put the gastropods back in the aquarium until the next activity period.

Part 3

Explain to learners that each team will be given a mini-aquarium and a gastropod. They are to follow the steps outlined in the 4-H Science Inquiry Model to create an experiment design, including a materials list and data sheet. Set a time frame for the experiments to be completed. Experiments could test food preferences, like apples or lettuce; or, learners may grow plants to test preferences. Some gardeners believe that slugs will not cross a barrier of marigolds in a garden. Is this true?

Additional differences that may be tested include various surfaces, damp or dry areas, or preferred types of hiding places. Avoid experiments that could harm the gastropod, such as applying salt. When all the teams have completed their experiment design, ask them to share them aloud with the group. Allow learners to ask the teams questions about their designs.

Pass out the materials requested by each team and facilitate, as needed, while they set up their experiments. Be sure each team records their set-up procedure. Allow time each day of the experiment period to view the mini-aquaria and record what is observed. At the end of the experiment period, ask the teams to analyze and interpret the data they collect and give a summary report to the group. Ask each team to explain how they could apply what they have learned to the habitat area. What additional inquiries would the learners like to make in the habitat area?

EXTEND THE LEARNING

A Palette of Fun (4-H 713L): Batik; Scissors Snipping

• Buy or collect pond snails and land snails. Compare their form, their movement, and their responses to various stimuli and food sources. Are there pond snails in the habitat area pond? Why or why not? If there are no snails there, should they be introduced? Why or why not? Which of the experiments that were made on the land gastropods could be tried also on the pond snails?
• Learners can do library research to study other members of the Phylum Mollusk. Characteristic body parts of a mollusk are a mantle; a thin, fleshy layer of tissue covering the mollusk’s soft body; and the large, muscular foot. Bivalves and cephalopods are mollusks. Bivalves are animals such as clams, scallops, and mussels. Examples of cephalopods are octopus, squid, and chambered nautilus.
• Many fossils that originate in marine environments have bivalve shells. Learners can find marine fossils at Beverly Beach on the Oregon coast north of Newport. Take a field trip to collect some fossils, and then identify them.

REFERENCE

Field Guide to the Slug. 1994. The Western Society of Malacologists. David George Gordon. Seattle, WA: Sasquatch Books.

“Snail Trails.” May 2002. Pamela Galus. Science Scope. Volume 25, Number 8.

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