Name:______________ Hr.:__
Purpose: To examine and consider:
1) the internal structure of a frog
2) how an adult frog is adapted to live in water and on land
Materials: (per group of two) * scalpel * forceps
* preserved frog * probe * scissors * pins * dissecting tray
Procedure:
1. Place your frog on your dissecting pan ventral side up.
2. Secure it by pinning its four legs to the pan.
Your first incision will be made down the middle of your frog’s belly from the pelvis to the throat. As you make this cut, be sure to cut through SKIN ONLY!!
3. Begin your dissection by carefully lifting the belly skin with the forceps at
point A, and make a shallow cut through the lifted skin with a scalpel.
4. Insert the rounded edge of the scissors into the incision, and cut
the skin along the dark line in fig. 1 up to the tip of the chin (point B).
5. Cut skin from point C to points D alongside your frogs forelegs, and from
point E to points F towards your frog’s knees.
6. Fold back the flaps of skin and pin them to the pan. If needed, use your scalpel to free skin from the body wall.
You are now ready to cut through the muscle layer!
7. Cut a small opening through the muscular body wall at point 1 with the scalpel. Hold the wall with the forceps as you cut.
8. Insert the round tip of the scissors into the incision, holding the scissors almost parallel to your frog’s body. Continue cutting to point 2 where the sternum (breastbone) begins. CAUTION: Your cuts must be very shallow
or internal organs will be damaged!! Keep cutting up to point 3.
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9. Cut the body wall from points 2 to 4 on both front legs. With the forceps, lift back this layer of body wall and cut it off. This cut should expose the
breast bone underneath.
10. With scissors cut through the bone at points 3. With forceps and scalpel, remove the whole bone.
11. Continue cutting through the body wall to the same points that you did the
skin, being careful not to damage the organs.
12. Pin back all body walls and examine your dissected frog.
Frogs store their fat in long thin finger-like bodies (called “fat bodies”) that are yellow or orange in color. Removal of these is often needed to see the internal organs.
If your frog is a female, the body cavity may be full of black eggs
and ovary tissue. If these are present, carefully remove the eggs and tissue and set them aside.
The Digestive System
The digestive system includes the mouth, esophagus, stomach, intestines, cloaca, anus, liver, gall bladder, and pancreas. The digestive system breaks food down into very small pieces so the sugars, vitamins, and minerals can be absorbed by the blood vessels in the small intestine. The body then uses these nutrients to nourish cells.
**** CHECK OFF EACH ORGAN AS YOU FIND IT!!
The ____ liver should be easy to find because it is the LARGEST
ORGAN in the abdominal cavity of your frog (it’s also the largest organ in humans!). It should be reddish-brown. Find it and count the number of lobes (sections) it has. ______
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The liver has many functions. It removes waste products and harmful materials, like alcohol, from the blood. It also forms bile to break down fats.
The ____ gall bladder is a small greenish sac attached to the liver. It stores and secretes bile into the small intestine to help break down fats.
- Beneath the liver, find the large, white ____ stomach. It should be on the right side as you look at your frog. The stomach connects to the ____ small
intestine. The straight part of the small intestine (near the stomach) is called the duodenum (due-uh-DEN-um); the remaining, coiled section of the small intestine is the ileum (ILL-ee-um). Separate some of the coils of the ileum and you will see that thin, transparent membranes connect them. Such membranes are called
____ mesenteries. The small intestine eventually widens to form the ____ large
intestine. In the frog, the large intestine is a straight tube that leads to the
anus. The lower portion of the large intestine is called the cloaca. Both feces and urine are excreted from the cloaca.
The last digestive organ to locate is a bit more difficult to find. In the mesentery along the inner curve of the stomach, locate the pinkish ____
pancreas. The pancreas produces enzymes to break down proteins, starches, and fats. It also produces insulin that controls the body’s use of sugar.
The Circulatory System
The circulatory system includes the heart and the blood vessels.
The blood carries sugar (glucose) and oxygen to body cells and carries carbon dioxide and other waste products away from the body cells.
The frog’s ____ heart is a small triangular shaped organ between the front legs and anterior to the liver. The heart is the pump that pushes the blood to the lungs and to the body and then back to the heart.
It may be easier to examine your frog’s heart if you remove it by cutting it away from the body. Before you remove this, or any other organ, be sure you
know where it came from (and don’t lose track of it or otherwise dispose of it, just set it aside when done looking it over)
(You may also want to cut through the thin membrane surrounding the heart to get a better look.)
Hopefully you found all three chambers…  Your frog’s heart (like all amphibian hearts) has two _____ atria (AY-tree-uh -- plural for “atrium”), a right and a left, and one ____ ventricle (VEN-trih-cull). Blood coming from your
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frog’s body (except blood from the lungs), collects in the right atrium. Blood coming in from the lungs collects in the left atrium. Both atria contract
(remember the heart is a great big muscle!!) and the blood is forced into the frog’s ventricle. Then the ventricle contracts and the blood is pumped through the arteries to all parts of the frog’s body.
Humans have a four chambered heart consisting of two atria and two ventricles. This is more efficient because the two ventricles prevent oxygenated and unoxygenated blood from mixing. Therefore, only oxygenated blood is sent to the body and only unoxygenated blood is sent to the lungs.
Another difficult organ to find is the last organ you need to find in the frog’s circulatory system. The ____ spleen (M) is a small, round, reddish organ that is located within the mesentery of the coiled part of the small intestine. The spleen removes old or damaged red blood cells from the blood plus it stores blood for times when more blood is needed, such as when a large cut into the body occurs.
The Respiratory System
The respiratory system is responsible for getting oxygen into the body and removing carbon dioxide from the body.
When frogs are in water, they get oxygen from air going through their skin directly into their bloodstream.
When on land, frogs get oxygen by breathing air into their lungs by swallowing movements; the floor of the mouth moves up and down as if the frog was swallowing. They also get oxygen into their body by letting the oxygen in their mouth get absorbed directly into the blood vessels in the mouth’s lining.
However, an active frog needs more oxygen than it can get from its skin and mouth alone. Therefore, every so often it closes its nostrils by means of tiny valves of skin, and in this way, air is forced down into the lungs when it swallows.
____ The two reddish-brownish-pinkish lungs are sac-like structures located on either side of the heart.
HAVE YOU BEEN KEEPING TRACK OF YOUR ORGANS USING YOUR ORGAN
TABLE?? IF NOT, NOW’S A GOOD TIME TO CATCH UP!!
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The Excretory System
The excretory system provides a way for various wastes to be removed from the body.
Carbon dioxide is removed from the bloodstream by the lungs and is exhaled out of the mouth.
One organ responsible for the removal of wastes in the blood (called urea ) is the ____ kidney. Just like we do, our frog has two kidneys, and these long, flat, and narrow reddish-brownish colored organs are located in the lower part of the frog’s abdomen. To locate them, carefully lift up the frog’s digestive organs. Each kidney lies tight against the frog’s back, on each side of the frog’s backbone. Once urea is dissolved in water, it is called urine.
Urine travels from each kidney through a tube called a ureter to the frog’s urinary bladder, a small sac at the posterior end of the frog’s abdominal cavity. The walls of the urinary bladder look like tissue paper. The urine then travels from the bladder into the cloaca and then out the anus to the outside of the body.
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Going Further: (In other words, if you have time you may try one or more of the following)
1. Using your scissors, carefully remove the liver. Cut through the upper end of the stomach and the lower end of the large intestine. Then remove the stomach and intestines. Cut open the stomach and a section of intestine to examine its lining and internal features. List the contents and your observations on your answer sheet.
2. How long do you think the small intestine is? Your guess: _____ cm.
Then stretch out the small intestine and measure it. Actual length = ____ cm
Remove the skin from one of your frog’s hind legs and examine its muscles.
Use a probe to separate the thigh muscle. Notice that the muscle
tapers at the end into a tough white cord, or tendon. A tendon attaches muscle to bone. Move the leg back and forth and observe how the muscles respond.
Chip away the top of your frog’s skull with your forceps. Uncover the brain
and find the cerebrum. How does your brain connect to the
spinal cord (Phylum Chordata, remember?!  ) How large is
your frog’s brain.
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