Download frog dissection

Biology
Dissection
THE FROG
Name___________________
Note: The organism you are cutting up today was once alive and vibrant, a living being. Dissection of a
complex organism is not a right, but a privilege. Please be aware that this privilege is one that many students
do not have.
Instructions: Read through the packet carefully while performing the lab. Make sure you
follow the directions and locate each underlined structure before moving on to the next step.
You are responsible for being able to locate and identify each of the underlined terms. You
should also be able to describe their function.
Part I – External Anatomy
Skin. The frog’s skin is often described as slimy. This is because the frog’s skin is also a respiratory surface
and is covered with mucus which can absorb oxygen and water, so the frog does not need to drink. The mucus
also prevents the frog from drying out when its out of the water and makes it slippery – helping it escape
predators.
Eye. The frog’s eye is covered by a special third membrane called the nictitating membrane which protects the
eye from water when the frog is submerged and keeps it moistened when out of water.
Nostrils. Called external nares, these lead directly to the mouth and give the frog its excellent sense of smell
Ear. Frogs also have an ear, which is really a membrane structure which detects changes in air pressure, similar
to the human ear drum. The membrane is called the tympanum.
Tongue. Carefully open the frog’s mouth and locate the tongue. Notice that the tongue is attached at the front
of the mouth not the rear of the mouth as in humans. Why might this be the case?
Legs. Notice the size of the frog’s legs. These powerful legs allow the frog to jump up to six feet, the
equivalent of human jumping a distance of 30 yards! Locate the cloaca, an orifice where the legs meet. Feces,
urine, eggs AND sperm all leave the through this orifice, so it’s difficult to determine the sex of the frog from
the outside! However, a large thumb pad on the “hand” of the forelimb usually indicates a male.
Before proceeding, record the following information, to be used in your lab report:
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Based on your examination of the frog’s external anatomy, identify three features that you could use to
recognize an amphibian. Answer only based on the dissection and your prior knowledge – do not do
research on the answer. One clue: other amphibians include salamanders and toads; lizards and snakes,
by contrast, are reptiles.
What can you tell about this animal’s lifestyle? Where does it live? What sort of things does it eat?
How is it adapted for its niche in the environment?
Part II – Internal anatomy Part A – Digestive System
1. Place the frog dorsal side down and insert pins into its feet to keep the frog in place. Then make incisions
with sharp scissors as seen in the picture below. Be careful not to cut to deep – you may tear and damage the
internal organs! The two posterior lateral (across the body) cuts need to be made long enough so that the pieces
of skin and muscle fold easily aside and out of the way. Pin them down.
2. The fist major organ that you’ll notice is the multi lobed liver. The liver will be darker in color and have
three lobes: the right lobe, the left anterior lobe, and the left posterior lobe. The liver produces glycogen
and a fat dissolving substance called bile. The bile is stored in the gall bladder, a small, thin sack located
beneath the liver. Locate it.
3. Locate the largest of the digestive organs, the stomach. It has a half moon shape and is usually found on the
left (frog’s left!) side of the body cavity. The stomach is continuous with the small intestine (named for its
diameter, not length!) The small intestine is coiled up and is connected with the large intestine, where the final
stages of digestion occur. Waste is excreted through the cloaca, which you located earlier.
4. The small pancreas (you may not be able to see it) attached to the “curve” of the stomach produces acid
neutralizing substances and these substances are delivered to the stomach from the gall bladder along with bile
via the bile duct.
5. Locate the “fingery” looking structures called fat bodies, which are usually underneath the stomach. These
are made out of fat cells (adipose cells) and are used to store energy.
6. Digestion really begins with at mouth. Open it and observe the tongue. Frogs do not have teeth, so food is
swallowed whole! Food enters the esophagus, a tube that is continuous with the stomach. Food is moved down
the esophagus by a process called peristalsis, which is coordinated muscular contraction of muscles in the
esophagus.
7. Food enters the stomach and then is stored briefly in the pylorus, which looks like a lump on the stomach.
The food then leaves the stomach and enters the small intestine through the duodenum. Nutrients are absorbed
from the food by the small intestine. The leftovers enter the large intestine, which is shorter and fatter, where
water is absorbed from the food. Waste then exits via the cloaca.
8. Locate the dark red spleen in the tissues of the mesentery, the membranous tissue that connects the segments
of the small intestine. The spleen serves as a holding area for the blood.
9. Carefully remove the liver, stomach, and intestines IF NECESSARY; Do not to damage other organs or cut
important blood vessels!
Before proceeding, record the following observations to be used in your lab report:
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What does the frog’s digestive system tell you about what it eats?
What role do you think the gall bladder and pancreas play in digestion? If you could not look up the
answer in a reference book, how could you find out?
If you have not located each of the organs above, do not continue to the next section!
Part II – Internal anatomy Part B – The Heart and Circulatory System (Use numbered picture below)
1. The circulatory system is comprised of the heart, blood vessels, and blood. First, locate the two small pink
lungs, which are on either side of the heart. Carefully remove one without damaging surrounding tissue and
blood vessels. Use a dropper to inflate the lung with air (you might not be able to do this).
2. Locate the heart. The heart is a complex organ made out of one of the three classes of muscle: cardiac
muscle (the others are: smooth muscle, which makes up the tubing, like blood vessels and intestines; skeletal
muscle, which controls locomotion). The heart has three chambers: the right and left atria, and the ventricle(7).
Locate the apex (point) of the heart, which is part of the ventricle. (FYI – the mammalian heart has FOUR
chambers – 2 atria and 2 ventricles)
3. The right atria (5) receive oxygen poor blood via a vein (6) (veins bring blood TO the heart; arteries take
blood away from the heart!). The left atrium (4) receives oxygenated blood from the lungs via a pulmonary
vein(3).
4. Locate the two branches of the aorta (1, 2) (an artery), which transports blood through the body cavity and
gives off many branches. Also locate the “huge” ventral abdominal vein (not numbered).
5. Note how the frog circulatory system has 2 loops, the Pulmocutaneous (pulmo – lungs, cutaneous – skin) and
the Systemic loops.
Heart – ventral side
If you have not located each of the organs above, do not continue to the next section!
Part II – Internal anatomy Part C – The Urogenital (Reproductive) System
Female
1. You will have noticed immediately if you have a female frog. Locate the ovaries (a thin membrane) which
contain hundreds of darkly colored eggs.
2. The eggs move through the oviducts (spaghetti-like tubes) before exiting through the cloaca.
Male
1. If your frog lacks these large egg bodies look carefully for two yellow small bean shaped organs. These are
the testes, where the sperm is produced.
2. The sperm pass through the vas efferentia to the kidneys and ultimately out through the cloaca.
3. (If your frog is a female, remove the eggs carefully and) locate the two dark kidneys, which are usually
surrounded by blood vessels. The kidneys filter the blood of excess salts and other chemicals and send that
waste via the ureters to the thin, sac-like bladder.
4. Locate the adrenal glands, which are small structures attached to or nearby the kidneys. The adrenal glands
produce and release hormones that can provide the frog with a quick burst of energy in an emergency.
5. Like all organs, the kidneys are connected to the circulatory system by blood vessels. Locate the renal artery
(which brings blood from the heart) and the renal vein, which takes blood back to the heart (via the lungs).
If you have not located each of the organs above, do not continue to the next section!
If you have a female frog, be sure to look at a male, and vice versa!!
Before proceeding, add the following observations to your notes, to be used in your lab report:
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Were you able to trace the pathway of the blood in the circulatory system? If not, what made it
difficult? In what way is the real frog different from a diagram? Are all frogs the same?
How do frogs reproduce? In what ways is their reproductive behavior evident from their anatomy?
What does it say about their habitat?
Internal Anatomy continued… Part D – The Muscular and Skeletal Systems
Introduction. An organism’s muscles serve two main functions: (1) Muscles are attached to bones by tendons
which pull on the bones during contraction and thus cause the bone to move (articulate), and (2) the muscles
also give protection to the organs. Ligaments are not muscle tissue, but give support and stability to joints.
Muscles work in antagonistic pairs. This means that when one muscle contracts, the opposite muscle of the pair
relaxes to allow the resulting articulation (relative movement of limbs). Without this process, movement would
be impossible.
1. Carefully remove the skin from one leg of your frog without damaging the muscles beneath. Use scissors
for the task. Make sure you remove the skin all the way down to the frog’s foot.
2. Look at the frog’s leg from the ventral (the underside of the frog). Locate the muscles listed it the pictures
below. It may be necessary to separate muscles from one another, as they are held together and covered by
membrane. Use pins as in the picture below to mark the muscles you have found. (Do not poke the muscles
with the pins!)
Ventral Muscles of Frog thigh
1 sartorius
2 adductor longus (thin, flat)
3 adductor magnus
4 gracilis major
5 gracilis minor (hard to find)
6 semitendinosus (beneath adductor longus,
also hard to find)
Ventral muscles of Frog shank
1 gastrocnemius
2 tibialis posticus
3 extensor cruris (hard to find)
4 tibialis anticus longis
5 tibiofibula bone
3. After locating the ventral muscles, flip your frog over and locate the dorsal muscles.
Frog thigh – Dorsal view
1 gluteus
2 piriformis (hard to find)
3 triceps femorus
4 semimembranosus
5 biceps femoris
Frog shank – Dorsal view
1 gastrocnemius
2 peroneus
4. The frog’s leg is made up of four major bones, listed here proximal (nearer the torso) to distal (further from
the torso): femur, tibiofibula, calcaneus, and astragalis. The calcaneus and astragalis run side by side in the 3rd
segment of the frog’s leg.
5. Cut neatly through the gastrocnemius and peroneus to expose the tibiofibula bone.
6. Cut neatly through all the ventral thigh muscles to expose the femur. You should also notice the femoral
artery (pink) and the femoral vein (blue). Which of these two supplies the leg with blood? Which one drains
the leg of blood?
Lab Report
In your lab report, type out the answers to the questions/observations you noted during the dissection. You
do not need to do additional research to answer these questions – they should be based solely on your
observations during dissection and your intuition and prior knowledge. Then answer the following questions,
for which you might need your book (chap.18 in Honors or 26 in regular Bio) or the class web page to answer.
Use these links: http://tinyurl.com/p96sr4p and the class biodiversity links at
http://www.gonzaga.org/Document.Doc?id=3585.
1. Describe several key distinction features of amphibians. Name some other species of amphibians in
your answer as examples.
2. The image on the left depicts the forearm bones of an amphibian ancestor.
How does it compare to today’s amphibians? How about to those of other
vertebrates?
3. Describe the characteristics of proposal amphibian ancestors. Search for “Lissamphibians” and/or check
out www.ucmp.berkeley.edu/vertebrates/tetrapods/amphibintro.html
4. How do frogs survive the winter? (see www.exploratorium.edu/frogs/woodfrog/index.html)
5. In what way is frog reproduction similar to that of fish? What makes them different, in terms of
reproduction, from other land vertebrates (especially reptiles)?
6. How are frogs similar to other land vertebrates?
7. Describe the pathway of amphibian circulation through the heart and through the body. Compare the
pathway to a fish and a mammal.
8. In the past 10 years there have been many reports of declines in amphibian populations and discoveries
of mutated frogs. Describe several possible reasons for these declines. Why are frogs especially
vulnerable to environmental problems? Check out http://armi.usgs.gov/index.php and
http://www.pwrc.usgs.gov/naamp/.
9. Design an experiment using frogs as a test subject. Explain in detail how you would go about
conducting the experiment, using the skills you learned earlier in the year.
10. Explore endangered frogs with Google Earth. Go to this website:
http://services.google.com/earth/kmz/edge_existance_n.kmz and open the file in Google Earth (you
might need to download the free program from http://www.google.com/earth/). Summarize what you
learn about at least three different endangered animals (they do not need to be amphibians) on different
continents.