Teach lesson
Planarian Detectives: identify the mystery substance
Short lower-secondary activity: students open Planarians in detective mode, count line crossings for control E and two mysteries, identify the high- and low-movement mysteries, and write a cautious evidence-based conclusion.
Learning Outcomes
Define a fair rule for counting line crossings during a planarian observation.
Use substance E as the water control when comparing two mystery substances.
Calculate crossings per minute from a manual count and a fixed observation time.
Identify the high-movement and low-movement mysteries before learning the real names.
Write a conclusion limited by the evidence, without claiming direct human effects.
Student activity preview
Activity Content
Preview only. In a class session, students can fill in responses and submit their work to the teacher.
Case: two clues and one control
6 min
You will work as a data detective. The remote lab shows planarians, small flatworms, moving after they are placed in different solutions. Each liquid is pond water mixed with one substance; the control is water without an added mystery substance. In this activity you do not see the solution names at first: you see letters.
Planarians do not have a brain like ours, but they do have a simple nervous system that coordinates how they orient and move. In a much simpler form, they share an important idea with other animals: nervous signals help produce observable responses. That makes them useful for a small, measurable biology question: when the chemical environment changes, does movement change too?
The lab uses those aqueous solutions to change the planarian's chemical environment in a controlled way. Your job is not to guess from names or make claims about people; it is to observe a planarian, count line crossings, and use those data as clues.
What a planarian looks like
Reference image: a planarian is a small flatworm with a soft body, a triangular head, and two dark dots that look like eyes. In the lab, you will see it from farther away inside a dish of liquid.
The scene you will observe
In the lab, you will observe a dish like this. The planarian is the dark shape near the edge; the grid helps you count line crossings consistently.
The case has three samples:
- E is the control: water. It is the reference.
- A is a mystery substance.
- C is another mystery substance.
Your challenge is to decide, before learning the real names, which mystery shows more movement than the control and which mystery shows less movement than the control.
Respect for living organisms
This is a remote observation using recordings of real planarians. Use careful language: you may describe observed movement, but do not claim harm, intention, pain, or effects in people unless your evidence shows that.
When you open the lab from this activity, what should you see on the selection screen?
Before measuring, write a short hunch. Do you think A or C will move more than E? What evidence would make you change your mind?
Fair counting rule
7 min
Measuring behavior is not just watching. To make the comparison fair, use the same rule for E, C, and A.
Line-crossing rule
Always count the same event: the same body point fully crosses the same reference line. Keep the same observation time too.
For this activity, use a 300 s window, which is 5 min, for each sample. Record the time you actually observe and use it in your calculation.
Which rule lets you compare E, C, and A most fairly?
Lab: collect the clues
17 min
Open the lab from Teach and use the manual counter. The route is fixed: measure E first, then C, then A. For each sample, count crossings with your agreed rule and calculate crossings per minute.
Open Planarians in detective mode
Open the Planarians lab from this block.
On the selection screen, work with the letter cards.
Select Substance E and count crossings for 300 s.
Restart the observation and repeat with Substance C.
Restart the observation and repeat with Substance A.
Return to Teach and complete the table with your counts, rates, and notes.
Movement clues
Complete one row for each letter. E is the water control; C and A are mysteries. Enter the time in seconds, the final crossing count, the rate in crossings per minute, and whether the letter is above, below, or similar to E.
| Letter | Role in the case | Observation time s | Final crossings | Crossings per minute min^-1 | Comparison with E | Observation note or doubt |
|---|---|---|---|---|---|---|
Crossing rate
\text{crossings per minute}=\frac{\text{final crossings}}{\text{minutes observed}}
Choose one row from your table and show the crossings-per-minute calculation. Remember: 300 s is 5 min.
Claim before reveal
7 min
Before learning the real names, decide from the clues. Your conclusion does not need to be perfect; it needs to be supported by data.
Clue summary
Submit a brief summary of your clues. It may be text, a photo of a mini graph, or a table: it must include the rates for E, C, and A, and mark which one is highest and which one is lowest compared with the control.
Write your detective claim in 4-6 sentences:
- which letter shows more movement than E;
- which letter shows less movement than E;
- at least three numbers from your table;
- one reason your data still have uncertainty.
Which sentence avoids exaggerating the evidence?
Final check after the reveal
4 min
Move to this phase only after writing your detective claim. Wait for your teacher to reveal the letter code in class; the key is not written here so the detective mode stays fair.
After the teacher reveal, write a 3-5 sentence reflection. Include: the real name of your high-movement letter, the real name of your low-movement letter, whether your evidence supported that identification, and one limit you would keep when explaining the result.