Frog Dissection Lesson for Middle School Biology
This is the 21st lesson in a series of 32 hands-on lessons covering middle school biology. This lesson focuses on frog dissections. I used this plan while teaching a 55 minute middle school biology class. Each lesson plan includes homework assignments and a variety of hands-on activities to make each lesson engaging & memorable. Use these fun lessons with your classroom, homeschool, after-school program, or co-op!
These lessons are written for a class that meets once a week. If your class meets 5 days a week, simply do this lesson one day a week and use the homework assignments (at the bottom of the page) for the work for the other days of the week.
1. Pass out tickets for students who did their homework and who did the extra credit. Go over homework questions. Allow students to share what they found interesting or important when observing live frogs for homework. (I give out tickets for students who volunteer to share their answers.)
2. Quickly review amphibians, taking brief notes.
- Have students name the 3 orders of amphibians: 1) Anura - frogs & toads, 2) Caudata - salamanders, and 3) Apoda - caecilians).
- What are some traits of an amphibian? (cold-blooded, 4-legged, vertebrate, usually feels slimy, usually lives on land as an adult)
- What is distinct about amphibians? (They live in both land & water, and they go through metamorphosis.)
- What is metamorphosis? (The changing of an immature hatchling, which doesn't resemble its parents, into an adult, which looks likes others of that specials.)
- Share something you found interesting while reading from the book about anurans (frogs & toads). Share something interesting about salamanders. Share something you found interesting about caecilians. *Give tickets to each student who shares.
3. To dissect the frogs, I spoke through this dissection guide and had students verbally answer the questions as they observed and dissected their specimens.
You will need:
- frogs (I got one for each group of 4-5 students) - I purchased these preserved from homesciencetools.com
- dissection kits such as a hard, plastic disposable plate, a paring knife or scalpel, a small probe, and sharp scissors
- disposable gloves
- Color: What color is the dorsal side? the ventral side? Why? (Camouflage) Some frogs can even change their colors slightly through pigment cells in the chromatophores.
- Toes: How many toes are on the hind legs? (5) Are they webbed? (Yes) How many toes are on the forelegs (4) Are they webbed? (No) Why do you think they were designed this way?
- Eyes: Open the eyes and find the nictitating membrane, which is the clear membrane attached to the bottom eye and is like a pair of googles for the frog to easily see underwater. What color is it? What color is the eyeball? You can remove the eyeball if desired. The eyes actually aid in digestion. Have you noticed that a frog always closes its eyes when swallowing? The eyeballs add a bit of extra pressure to help the food slide down the esophagus.
- Tympanic membrane: Find the circles behind each eye. Those are the tympanic membranes. In which other animal did we see those? (grasshoppers) What are they used for? (eardrums). If your tympanic membrane is the same size as the eyeball, you have a female. If it's larger than the eyeball, you have a male. Do you have a female or male?
- Skin: How does the skin feel? Is it smooth, scaley, slimy, etc.?
- Nares: The nares are basically what we call nostrils. These are the external nares. In a moment we'll see where they lead.
Inspecting the Frog's Mouth
- Open the frogs's mouth. Cut it open if needed.
- Tongue: What shape is it? Where is it attached to the mouth? How does it move? How far can you stretch it? It doesn't look long, but it is very stretchy. It kind of rolls up over the prey and shoves it into the esophagus.
- Esophagus: Find it. Where does the esophagus lead to?
- Glotis: Find the slit between the tongue and esophagus. That's the glotis. Who knows where it leads? Yes, it leads to the lungs. Open it with your probe. How does it compare to the esophagus?
- Teeth: How many sets of teeth do frogs have? (2) Who can name them? (Vomerine & Maxillary) The vomerine teeth are on the roof of the mouth. The maxillary are around the edge. Feel them. How do they feel different? How does the frog use its teeth? (It doesn't chew its food. It uses its teeth to hold animals in its mouth.
- Nares: Take your probing tool and put it through the frog's external nare to see where it comes out the internal nare.
- Making the incisions: Frogs are juicy, so make little incision are bottom of stomach & push out some of the juice. Cut the skin all the way up to the chin and then toward the arms. You can see the frog 8 pack. You can cut off the skin. Blood vessels are going down the middle. Now cut through the muscles right down that middle line. Try to have the cuts be shallow. Cut a T at the bottom of the stomach & then upward. Don't cut too deep. When you get through the ribcage, it'll get tough but you can still cut. Cut off the muscle. (If groups finish earlier than other groups, the can pull the skin off the frogs and play around with the leg muscles.)
- Ovaries & Oviducts - If you have a female with well-developed eggs, you'll need to remove the eggs first to see the other organs. If this isn't the case, we'll discuss these organs later in the dissection.
- Liver - It is largest internal organ. It has 3 lobes. Who remembers the function of a liver? (Secrets bile, which helps digest fat)
- Heart - Toward the top of the liver in a triangle shape. How many chambers does our heart have? Frogs only have 3 chambers: the left & right atrium and a ventricle at the bottom. Notice the large artery coming out from the top, the conus arteriosis.
Lungs are on either side under the heart and liver. They're small, gray, not inflated, & kind of spongy. Poke them with your finger.
- Fat bodies look like spaghetti or ramen noodles and are on each side near the stomach. Frogs don't store fat like we do, with fat under our skin. Frogs store fat in these. Remove them.
- Stomach - The food goes from the esophagus into where? (the stomach) Since a frog gulps down its food whole, the food doesn't start to get digested until it's in the stomach. Where does it go next?
- Small intestine - The straight part is the duodenum. The curved part is the ileum. It's held in there by plastic wrap looking tissue called mesentery. Mesentery is full of blood vessels that carry away the nutrients to the rest of the body. Where does food go next?
- Large intestine - The large intestine gets wide and finally leads out the exit hole of a frog, the cloaca, which literally means sewer. It only has one exit hole, which is used for solid waste, urine, sperm, and eggs.
- Spleen - Lift the small intestine and you'll fund a round, dark reddish spherical object. That's the spleen. Does anyone remember the function of the spleen? (It stores blood and is part of the circulatory system.)
- Gall bladder - Going back up, next to the liver you'll see a greenish-brown sac that looks like a booger. That's the gall bladder. What does the liver produce? (bile) The gall bladder stores the bile that the liver produces.
- Ovaries & Oviducts - If you have a female, her ovaries will be behind the stomach and intestines. Behind the ovaries will be small, tightly coiled tubes called oviducts.
- Kidneys - You'll be able to see flat, oval-shaped organs on the back wall of the frog, near its spine. Those are the kidneys. which filter waste from the blood.
Dissecting the Digestive System, Time to Explore, & Clean-up
- Stomach - Cut open the stomach and find out what this frog's last meal was. Since frogs eat their food whole, you might be able to recognize the animal. Also note the texture inside the stomach.
- Intestines - Stretch out the intestines. How long are they in comparison to how long the frog's body is?
- Esophagus - Stick your probe through the esophagus and see where it comes out the other end.
4. Time to Explore - Take a few minutes to explore any areas of your frog. If you haven't pulled off the skin to look at the leg muscles, be sure to do that! They are impressive.
5. Clean up.
6. Specifically mention the Extra Credit #3 for next week: bringing in a reptile pet (contained) or related reptile item to show and discuss with us in class next week. If you have a pet, bring it in and share about what it eats, how it behaves, etc. If you responsibly capture an outdoor lizard or turtle, identify it and research it a bit and share what you find. If you bring in shed snake skin or a snake rattle, an empty turtle or tortoise shell or egg, etc., be prepared to tell us a little bit about it. (Make sure to always wash your hands after handling reptiles and items related to reptiles and they can carry salmonella.)
- Friday: Read pp. 288-304. Answer 6 questions of your choice from p. 304.
- Monday: Research a specific species of lizard you don’t know much about. Sketch and color a picture of the lizard. Write its name (common and scientific) and five interesting facts about it. You can go to http://www.lizardtypes.com/lizards/types-of-lizards/ to find lots of lizard species.
- Tuesday: Research a specific species of turtle you don’t know much about. Sketch and color a picture of the turtle. Write its name (common and scientific) and five interesting facts about it. You can go to https://www.allturtles.com/turtle-species/ or https://www.allturtles.com/sea-turtles/ to find turtle species options.
- Wednesday: Research a specific species of snake you don’t know much about. Sketch and color a picture of the snake. Write its name (common and scientific) and five interesting facts about it. You can go to https://animalsake.com/types-of-snakes-with-pictures to find snake species options.
- *Extra Credit #1: Research a specific species of crocodilian you don’t know much about. Sketch and color a picture of the crocodilian. Write its name (common and scientific) and five interesting facts about it. You can go to http://www.crocodilian.com/cnhc/csl.html to find crocodilian species options.
- *Extra Credit #2: Complete the Reptiles Crossword Puzzle. https://www.biologycorner.com/worksheets/reptiles_crossword.html
- *Extra Credit #3: For biology class next week we will spend part of the time doing a Reptile Show & Tell. If you would like to bring in a reptile pet (contained) or related item (like a snake rattle, alligator or snake skin, empty turtle or tortoise shell or egg, etc.) and share about the pet(s) or item, I'd love for you to do so!
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