Download Characteristics of amphibians

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Frog Dissection Pre-Lab
Why are we dissecting a frog?
Frogs and humans are vertebrates and they have very similar organ systems. Although all of the
internal organs are not exactly the same, it is helpful to learn about anatomy through dissection. We will
be looking at each body system and exploring the individual organs of the frog. We will be making
comparisons between the frog and yourself.
Some individuals argue that dissecting a frog is cruel. Dissection would be a cruel practice if the
frogs were mistreated. The frogs that we use in lab for our dissection were bred in Mexico for the sole
purpose of scientific study. The companies use a very safe preservative to make the frogs as safe as possible.
The frogs we will be using in class have been prepared for dissection- all blood has been drained from their
bodies. We still need to wear gloves and wash our hands while working with the frogs.
Computers can be used to simulate a dissection. This method is very popular with some students. I
do not believe that the computer experience is anywhere near as valuable as the actual dissection. But, I do
think that it is a good tool. We will be going through a ‘Virtual Dissection’ to help prepare for our actual
dissection.
The frogs we use, gave their lives for science. They are ‘Organ Donors’ who would like us to learn
more about them (and ourselves). In order to best use this opportunity to learn and show respect for the
frog, we must follow all instructions and safety procedures. But, For our safety, we will be wearing gloves
and goggles during the dissection. Aprons will be available for students. EVERYBODY must wash their
hands before they leave the room. Hair should be tied back. No gum chewing or eating at all.
1. Why are we doing a dissection?
Characteristics of amphibians
Because their eggs are not in a shell, they must develop in a wet environment. Most amphibians do not
have scales. Their skin is thin, smooth, and moist. They do not drink water! Instead, they absorb it through
their skin. This is a major reason why most amphibians prefer to live in damp environments (or near
water). Amphibians can breath through their lungs or through their skin.
Ecological Indicators
Amphibians are often called ecological indicators. When large numbers of amphibians begin to die or show
deformities, this may indicate a problem with the environment. Sometimes deformities are caused by other
living organisms. They are good ecological indicator because their skin is responsible for gas and water
exchanges- and thus they are extremely sensitive to changes in air and water quality.
2. Frogs are amphibians, and do not drink water. How do frogs get their water?
3. Why are amphibians considered to be a good ecological indicator?
Frog Life Cycle
http://allaboutfrogs.org/weird/general/cycle.html
Below you will find questions organized by section.
Use the website above to answer.
4. Spawn (egg-mass). Describe how a male frog fertilizes eggs.
5. Egg. Frogs lay thousands of eggs at a time because most will not survive. Roughly 5% of all egg
laid, even hatch. How long does a tadpole egg take to hatch (on average)?
6. Tadpole. Tadpoles are born with gills, and spend ALL of their time in the water. How long before
the gills start getting grown over (and covered) by skin?
7. Tadpole with Legs. When do legs start to sprout?
8. Young Frog. This life stage looks just like frog, but still have a tail. How many weeks old is a frog in
this stage?
9. Frog. How long does it take a frog to fully develop from tadpole to frog?
10. Frog. What other factors can influence the grow rate of frogs?
Virtual Frog Dissection
http://www.mhhe.com/biosci/genbio/virtual_labs/BL_16/BL_16.html
Below you will find questions organized by section. Use the website above to answer.
Introduction
11. Why Dissect. Why are frogs a good model to use when studying the digestive system (as well as
other systems)?
12. Natural History. Frogs and humans are both vertebrates. What does this mean they both have?
13. Dissection Tools. What are the probe and scissors used for?
Click on Menu on the bottom at the bottom of the page when you are ready to move on to the next section
External Anatomy
14. Orientation. Is it possible to tell if a frog is male or female by external appearance?
15. Skin. What does the mucus do for the skin?
External Anatomy (continued)
16. Head. Where are the tympanic membranes (eardrums)? Do frogs have a pinna?
17. Cloaca. What materials would pass through a cloaca.
18. Legs. How many hind leg toes does a frog have?
Click on Menu on the bottom at the bottom of the page when you are ready to move on to the next section
Internal Anatomy
19. The Initial Cut. Are you suppose to push the pins in at an angle or straight down? What is the
benefit of pushing the pins in this way?
20. The Initial Cut. Describe the first cut you will be making. Include where you will be cutting and
how deep.
21. The Initial Cut. Why are there so many blood vessels in between the skin and muscle layers?
22. Digestive System. Which organ is the pancreas located closest to?
Internal Anatomy (continued)
Digestive System.
23. The movie asks you to remove the intestines. What other organ(s) is/are removed with the
intestines when you click on the tweezers?
24. Respiratory System. What does the skin do in frogs that it does not do in humans?
25. Respiratory System. Where are the lungs located in a frog (relative to the heart)?
26. Circulatory (Cardiovascular) System. How many chambers does a frogs heart have?
27. Circulatory (Cardiovascular) System What is a frogs heart missing when compared to a humans
(what chamber)?
28. Circulatory (Cardiovascular) System Why is the three chambered heart not as efficient as a four
chambered heart?
29. Reproductive System. In your own words, describe where are the testes located?
30. Reproductive System In your own words, describe where are the ovaries located?
31. Excretory System. What happens to blood that enters the kidneys?
32. Excretory System What organ connects the kidneys to the (urinary) bladder?
Internal Anatomy (continued)
33. Nervous System. This video is long, and parts without sound. Please watch patiently. What makes
up a frogs nervous system?
34. Muscular System. The upper leg muscles in a frog are responsible for moving which part of the
frog’s body?
35. Skeletal System. How many bones are found in the axial region?
36. Skeletal System. How many bones are found in the appendicular?
Follow-up.
After you are done with the website portion of this pre-lab, complete the following questions:
37. What is the purpose of this lab?
38. What will we be doing during this lab to stay safe?
39. Sketch the pattern of where you should cut to
first incision on your frog (cut thru the integument).
make the
Draw
directly on the picture to the right.
Additional Resources:
http://www.froguts.com (under DEMO)
http://frog.edschool.virginia.edu/
http://www1.teachertube.com/viewVideo.php?video_id=155411&title=Frog_Dissection_Instructions