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Frog Dissection
Classify the following taxa of a frog:
Kingdom:
Phylum:
Class:
Order:
External Anatomy
1. Observe and describe the dorsal and ventral sides of the frog. Dorsal side color____________________.
Ventral side color________________________________.
2. Examine the hind legs. How many toes are present on each foot ________? Are the toes webbed _________?
3. Examine the forelegs. How many toes are present __________? Are the toes webbed? ______________?
4. Use a ruler to measure the hind legs and forelegs (cm). Hind legs ___________. Forelegs____________.
How does the length of the hind legs compare in proportion to the forelegs _____________________________?
5. Locate the frog’s eyes. Describe the location of the frog’s eyes _______________________________________.
How does the location of the frog’s eyes help improve its vision ______________________________________?
A frog doesn’t chew its prey but can use its eyes to help gulp down food. Examine and press down the eyes.
How do the eyes help in gulping food ____________________________________________________________?
6. The nictitating membrane is a clear membrane that is attached to the bottom of the eye and covers and
protects the eye when the frog is under the water. Use tweezers to carefully remove the nictitating membrane.
You may also want to remove the eyeball.
What color is the nictitating membrane __________? What color is the eyeball ____________?
7. Just behind each eye is a circular structure called the tympanic membrane. This structure is used in hearing to
detect sound waves. What comparable structure do mammals have to detect sound waves _______________?
Anatomy of the mouth
Use scissors to make deep, angled cuts in the muscles of the frog’s jaws. Pry the frog’s mouth open wide enough to
view the structures inside.
1. Locate the tongue. Does it attach to the front or the back of the mouth __________? A frog catches its prey by
using a method known as “flip and grab”. How is the location of the tongue utilized in this this method
__________________________________________________________________________________________?
2. The frog has two sets of teeth. There are two vomerine teeth, which are used to hold prey directly over the
gullet and are found on the roof of the mouth. There is also a set of maxillary teeth which line the edge of the
upper jaw and are used to make sure prey cannot escape when captured. Run your finger over both of them
and describe the difference between the two _____________________________________________________.
3. Find the gullet, which is the wide opening in the back of the frog’s mouth. The gullet opens to the esophagus
and the esophagus is the tube going to the stomach. Push a probe into the gullet and esophagus. If you push
far enough you will reach the stomach.
4. Below the gullet and behind the tongue is a small slit like opening known as the glottis. This opening leads to
the respiratory system of the frog. It is also where air is expelled into the mouth and into air sacs for
vocalization in male frogs. How does the opening of the glottis compare to the opening of the gullet
_________________________________________________________________________________________?
5. Close to the angles of the jaw are two openings, one on each side. These are the Eustachian tubes and they are
used to equalize pressure in the inner ear while the frog is swimming. Insert a probe into the Eustachian tube
and describe where it leads ____________________________________________________________________.
6. On the roof of the mouth and slightly above and on either side of the vomerine teeth are the two small openings
of the nostrils or nares. Insert your probe into a nostril and write down where it leads
_______________________________. The opening where your probe exited is known as the external nostril.
The external and internal nares are used together to help the frog acquire oxygen.
7. Label the following structures on the diagram below. Maxillary teeth, vomerine teeth, tympanic membrane,
external nare, internal nare, tongue, glottis, gullet (esophagus), Eustachian tubes
Dissection instructions
1. Place the frog on its dorsal side with its ventral side up. Place dissecting pins through the tip of the snout and
each of the legs.
2. Lift the skin with the forceps and insert the point of the scissors to the right of the midline near the pelvic region.
Make the first incision from the pelvis to the throat as show by line a. At the forelimbs and hind limbs, make
transverse cuts on the lines b, c, d and e. Use dissection pins to hold the flaps of skin back.
3. Once the skin has been cut you can see the thick, muscular tissue of the frog. Use the same cuts described
above to expose the internal organs of your frog.
Internal anatomy
If your specimen is a mature female, the body cavity may be filled with eggs and an enlarged ovary. You may need to
remove these to view the internal organs.
1. Fat bodies: These are spaghetti shaped structures that have a bright orange or yellow color. If you have a large
frog, these may need to be removed to see the internal structures. Frogs use fat bodies as food reserve in
winter and are used to nourish the eggs in a female.
2. Peritoneum: A spider web like membrane that covers and protects many of the organs. You will have to
carefully pick it off to see all of the organs.
3. Liver: The large reddish-brown liver is the largest structure of the body cavity. How many lobes does the liver
have ___________________________? The digestive function of the liver is to make bile, which is a digestive
enzyme used to break down fat.
4. Gall bladder: Lift the lobes of the liver and you will see a small, greenish colored sac. The gall bladder stores the
bile made in the liver. The bile duct connects the gall bladder to the small intestine.
5. Stomach: Curving from underneath the liver is the white, tubular stomach. The stomach is the first major site of
chemical digestion. It is where ingested food becomes digestive food known as chyme. Follow the stomach to
where it turns into the small intestine. The pyloric sphincter is a muscular valve that regulates the exit of chyme
from the stomach to the small intestine.
6. Small intestine: The first straight portion of the small intestine is called the duodenum and the highly coiled part
is called the ileum. The ileum is held together by a membrane called the mesentery. Note the blood vessels
running through the mesentery which are used to carry absorbed nutrients away from the intestine. Absorption
of digested nutrients occurs in the small intestine.
7. Large intestine: As you follow the small intestine down, it will widen into the large intestine. The colon is
considered part of the large intestine and is responsible for absorption of water and compaction of solid waste
material.
8. Cloaca: At the end of colon is the cloaca. The cloaca is the only opening a frog has for the exit of digestive
(poop) and excretory (urine) waste, along with reproductive gametes (eggs and sperm). The word cloaca
literally means sewer and all amphibians, reptiles, birds and egg laying mammals possess one. Placental
mammals like yourself do not have a cloaca as you have multiple openings for digestive, reproductive and
excretory products to leave your body.
9. Spleen: Return to the folds of the mesentery and locate the dark red, spherical object known as the spleen. The
spleen filters out dead red blood cells, stores white blood cells and destroys harmful bacteria.
10. Esophagus: Return to the stomach and follow it upward to where it gets smaller. This is the esophagus which
leads from the frog’s mouth to the stomach.
11. Pancreas: You will probably not be able to locate the pancreas but it is a thin, glandular, yellowish structure
located in the mesentery between the stomach and the intestine. It secretes digestive enzymes into the
stomach by way of the pancreatic duct.
12. Heart: You will need to make sure your midline cut goes all the way to the tip of the lower jaw. Make sure you
have the flaps of muscle pulled all the way back from your transverse cuts b and c.
The heart is located at the top of the liver. A frog’s heart has three chambers. The pointed muscular pumping
chamber is called the ventricle and is located at the bottom of the heart. The top of the heart has two thinned
walled blood collects chambers knows as atria (atrium). How many chambers does your heart have ________?
13. Lungs: Push your probe into the glottis and you are now in the trachea or “windpipe” of the frog. The trachea
branches into two tubular bronchi, which connect to each lung. The lungs are located underneath and behind
the heart and liver. They are brown and spongy. If you are careful, you can inflate the lungs by inserting a
dropper into the glottis and squeezing gently. Inside the lungs, oxygen and carbon dioxide are exchanged in
small air sacs known as alveoli.
14. Removal of the stomach: Cut the stomach out of the frog and open it up. You may find what remains of the
frog’s last meal. Describe the texture of the stomach ______________________________________________.
15. Measuring the small intestine: Remove the small intestine from the body cavity and carefully separate the
mesentery. Stretch out the small intestine and measure it in cm _____________. Now measure the length of
your frog in cm_________________________. How does the length of the small intestine compare to the
overall length of the frog
__________________________________________________________________________?
16. Villi: The last part of the small intestine is lined with many small, microscopic villi. These are small folds used to
increase the surface area for absorption of nutrients by the small intestine. They look like small fingers.
17. Label the following digestive structures below: Liver, gall bladder, esophagus, pancreas, spleen, mesentery,
large intestine, cloaca, duodenum, stomach, ileum
The frog’s reproductive and excretory system is combined into one system called the urogenital system. You will want
to look at the structures of a male and female frog.
1. Kidneys: These are flattened been shaped organs located at the lower back of the frog, near the spine. They are
often dark in color and are used to filter waste from the blood. Each kidney has an adrenal gland attached to it
which secretes numerous hormones for life’s function.
2. Bladder: The bladder is an empty sac used to store urine. Follow the ureter, which is a small tube leading from
each kidney to the bladder. The bladder attaches to the cloaca so excretory waste can easily be expelled from
the body.
3. Testes: In male frogs, these organs are small, white oval bodies located at the top of each kidney. Sperm are
created here in mature male frogs.
4. Oviducts: Females do not have testes. You may see a curly-q type structure around the outside of each kidney.
These are the oviducts and this is where eggs are produced in female frogs. These oviducts carry the eggs to the
cloaca where they will be released and fertilization can occur.
5. Cloaca: Previously mentioned as the structure where digestive, excretory and reproductive gametes are
released.
6. Label the following female urogenital structures below: Cloaca, bladder, oviduct, kidney, ovary (eggs), fat bodies
7. Label the following male urogenital structure below: Cloaca, bladder, testes, kidneys, ureter, fat bodies
Nervous system and brain of the frog
1. Turn the frog dorsal side up. Cut away the skin and flesh on the head from the nose to the base of the skull.
2. Scrape the top of the skull with a scalpel until the bone is thin and flexible. Be sure to scrape away from you.
With your scalpel held almost horizontally, carefully chip away the roof of the skull to expose the brain. Use
scissors to cut away the heavier bone along the sides of the brain.
3. Description of brain: Staring at the most anterior part of the brain, the olfactory nerves connect to the nostrils
and then to the olfactory lobes where odors are processed. Just posterior to the olfactory lobes are two
elongate bodies with rounded bases, known as the cerebrum. This is the frog’s thinking center and helps the
frog respond to its environment. Posterior to the cerebrum are the optic lobes, which function in vision. The
ridge just behind the optic lobes is the cerebellum and it is used to coordinate the frog’s muscles, movement
and balance. Posterior to the cerebellum is the medulla oblongata, which connect the brain to the spinal cord
and is in charge of involuntary processes such as heart rate, breathing and digestion of food.
4. Label the following parts of the frog’s brain on the lines below: medulla oblongata, optic lobes, olfactory lobes,
cerebrum, cerebellum, spinal cord
A.______________
B. _______________
C. ______________
D. ______________
E. ________________
F. _______________