This is one of my favorite labs. Students get to view a planarian (also known as Dugesia) and observe its behavior. (See Planarian Lab ) You might be wondering what a planarian is…..it is a flatworm belonging to the Phylum Platyhelminthes, which means it is related to parasitic worms such as tapeworms and flukes. My students have already learned this, so one of their first questions is “What would happen if you ate this?”. I do not know why freshman are so fascinated by eating random things, but it seems almost every lab we do, specimen we study, or picture I show them is followed by that question. I assure them that the little flatworm swimming around in their petri dish is harmless, even if you ate it. I also follow that up with threats of dire punishment if anyone does eat their planarian. ( This is related to their eating fascination, young boys love to *dare* their friends to eat something gross. )
Once we have those preliminaries out of the way,students proceed to observe their flatworm and answer the questions on the lab handout. Halfway through, I dim the lights so that they can use a flashlight to test whether the worm prefers the light or the dark, most students have no trouble figuring out that the planarian moves away from the light as quickly as it can undulate its little body.
The highlight of the experiment is the final step where students cut their planarian in half so that they can observe it regenerate the two halves of its body (a process that usually takes a week or so). The conundrum I have as a teacher with this part of the lab is handing freshman potentially sharp and dangerous equipment – the scalpel. In the early days of my career I was so fearful of a student stabbing another student with the scalpel that I avoided this by giving them thin rulers to cut the body – flatworms are so fragile they really don’t need a sharp knife to cut in half.
Now, I see some merit in using scalpels as a teaching tool on its own – separate from the planarian lab. Freshman do need to learn how to use equipment and how to respect the laboratory. This is as good of a place to start as any. Before students receive their scalpels, I pause the lesson to give them instructions about how to hold the scalpel, how to hand the scalpel to someone (handle first) and also give them dire warnings about goofing around with the scalpels. Finally, they get their scalpels to use to cut the worm – what they don’t realize is that I have a separate box of scalpels that really aren’t sharp at all but they work just fine for this lab. Sure, with enough motivation, you could probably hurt someone with a dull scalpel, but the same could be said for a butter knife. The point here is that with careful instructions, and an observant eye, most freshman can handle using scalpels.
I’d also like to note that scalpels seem to have the same effect on students as latex gloves. Whenever students get to use those items, they really feel like they are in a real laboratory doing real science. I can only guess it is because the scalpel (and gloves) are iconic on tv shows about science, like CSI.