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A Constructivist Version of the Snail & Elodea Lab

By James Linhares

Type of Entry:

  • Lesson/Class activity

Target Audience:

  • hands-on
  • Inquiry lab
  • Group cooperative learning
  • Assessment of skills in testing a hypothesis

Target audience:

  • Life Science
  • Biology
  • Environmental Studies


This activity has six phases. First, a pre-determined number of students are asked a series of questions in an interview format as a pre-test. The second phase presents a series of hypotheses and some simple materials. In groups of 2 or 3, students focus on one hypothesis and design and run an experiment to test that hypothesis. During the next class period, the groups, after collecting data and making conclusions, present their experimental designs and results to the rest of the class. Discussion follows concerning the use of controls and of controlling variables. This is phase three.

Phase four consists of mostly teacher talk (class notes) about experimental design and about the interdependence of plants, animals, and decomposers. Phase five is a class brainstorm on setting up a mini-environment that will sustain both a snail and a piece of elodea over an extended period of time. Students proceed to set these up. Phase six extends for much of hte school year as we refer back to the mini-environments and add information and details on what happens among the plants, animals, and decomposers within the simulated ecosystem.

This "hands-on" activity is certainly not new or original. But I have developed it as an introductory assessment of what students know about biology and designing experiments. It also gives them a concrete experience around which they can organize the concepts that we develop throughout the year.


The consturctivist model of teaching suggests learning what the students know at the outset of instruction and then building upon that knowledge. This activity is designed to be used very early in the school year and provides a great deal of information on how well the students understand the importance of having a control when setting up an experiment. It also gives the teacher powerful information on the depth of knowledge that the students have regarding the interdependance of plants, animals, and decomposers in an ecosystem.

In many ways this 2-3 day activity can introduce the entire course of biology. A tremendous amount of what we cover from September to June provides the student with more detailed information about what happens inside the snail and elodea tubes.



  • Snails. Obtain from aquarium supply stores or collected from ponds.
  • Sprigs o fElodea (Anarchis) Obtained from aquarium stores
  • Culture tubes with screw-top caps
  • Other containers of various sizes
  • Pond water and/or tap water
  • Bromylthymol Blue reagent in dropper bottles


Pre-lab questions:

In the contructivist model it is important to determine students' pre-set conceptions. This is best done in a one-on-one interview when possible but can also be done with the whole class. This series of activities helps me learn a great deal about both the students' knoledge and skills at designing an experiment and about their knowledge of ecological principles. We start it the first week of school. In the interview, these are some of the questions asked:

  • Why do you breathe?
  • What do you breathe?
  • Do plants breathe?
  • What do plants breathe?
  • Do animals and/or plants breathe or respire as much in the dark as in light?
  • What would happen if we put this snail in this tube and sealed it?
  • What would happen if we put this piece of aquarium plant in the tube and sealed it? Both the snail and the plant?

These are starting questions that always lead to other interesting questios.


Testing a Hypothesis:

This is the activity of the year where we use the steps 1) Pose a problem, 2) Solve the problem, and 3) Convince your peers.

I give them little direction except that bromylthymol blue is blue in the absence of CO2 and is yellow to clear when CO2 is present. Each group of 2 or 3 students is given one question.

  • Do snails give off CO2?
  • Do snails give off CO2 in the dark?
  • Does elodea give off CO2? Does elodea give CO2 in the dark?

Their job is to design and carry out an experiment to answer their question.


Reporting and discussing results:

During the next class period they observe any changes in their tubes and make conclusions about the results. They then present their experimental designs and conslusions to the rest of hte class. This is a very good first assessment of their work habits, their collaboration and communication skills as well as their knowledge of setting up a control in an experiment and in controlling variables. We make plans to repeat the experiments if needed.


Teacher talk:

After two or three days of their wrestling, and struggling, with designing experiments and with th econcepts associated with the carbon and oxygen cycles I sit them down and give them some direct information. This should make more sense to them as they now have some concrete experiences ot help synthesize this new informaiton with concepts formed previously. (I emphasize that plants also use )2 and why that is so.)


Designing a mini-environment:

With some more specific information about how plants, animals, and decomposers support each other in nature, we return to the mini-environments and brainstorm what would be needed ot form a self-sustaining system. I pose the question: Can a snail and a piece of elodea support each other in a closed container? Our goal is to have a healthy environment for 1 month. Students set these up using whatever container and materials they wish. We observe them daily and discuss their progress.



Filling in the details:

We refer to the mini-environments throughout the entire school year. Assessment questions for the units on cells, photosynthesis & respiration, and ecology involve explainingkl in detail what goes on inside the containers. I often hold up one of hte containers as I am lecturing or making a point. The snail & elodea tubes can be used to create an unifing theme which can be carried throughout many of the tyopics that we study during the school year. To me, education is making connections. This activity is my way of connecting all that we do from September to June.

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