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SNC2D: Frog Dissection
Formal Report Due:
Date: June 24, 2017
Your Name:
Partners:
Purpose:
In this lab, you will dissect a frog in order to observe the external and internal structures
of the frog anatomy.
No Hypothesis is necessary for this lab
Materials
• preserved frog
• dissecting tray and paper towels
• scalpel, scissors, pins, magnifying glass
Background
As members of the class amphibian, frogs may live some of their adult lives on land, but
they must return to water to reproduce. Eggs are laid and fertilized in water.
Observations:
External Anatomy:
1. Sexing your frog: Place a frog on a dissection tray. To determine the frog’s sex,
look at the hand digits, or fingers, on its forelegs. A male frog usually has thick
pads on its "thumbs," which is one external difference between the sexes, as
shown in the diagram below. Male frogs are also usually smaller than female
frogs.
Is your frog male or female? Explain: (Record your external observations that led you to
this determination)
2. Place the frog on its belly (ventral side) in the dissecting pan
3. Examine the hind legs and front legs of the frog. The hind legs are strong and
muscular and are used for jumping and swimming. The forelegs provide balance
and cushion the frog when it lands after jumping. Notice the difference between
the toes of the hind legs and those of the front legs. How many toes are on the
front legs_______________. How many are on the hind
legs__________________________.
4. Observe the forelimb (front limb) of the frog. Find an elbow, forearm, wrist,
hand and fingers. Draw a biological drawing of the frog’s forelimb. Label the
parts bolded above on this diagram. Label the forelimb and hind leg in
Figure 1.
5. Locate the large, bulging eyes. The frog has 3 eyelids. The 2 outer ones are the
color of the fog's body. They do not move. Locate the third eyelid. It is a
transparent membrane the protects the eye while permitting the frog to see under
water. It is call a NICTITATING MEMBRANE. Label the eye and the
nictitating membrane on Figure 1.
6. Behind each eye find the circular eardrum called a TYMPANUM. They locate
the two openings into the nasal cavity. The nasal openings, are also call
EXTERNAL NARES, found toward the tip of the snout will closes when the
frog is under water. Label the mouth, tympanum, and the external nares on
Figure 1.
7. Feel the frog's skin. It is smooth, moist and thin. The frog can breathe directly
through its skin as well as with its lungs. Turn the frog onto its ventral side and
notice the color difference. What colour(s) and pattern(s) are on the frog's back?
Explain why the skin is like this? What colour is the skin on the frog's underside?
Explain why? What type of tissue is the frog’s skin?
8. After the dissection, briefly list the function of each of the parts in Figure 1
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Figure 1 – External Anatomy of Frog
INTERNAL MOUTH STRUCTURES:
9.
Place the frog on its dorsal side in the dissecting pan, pin down the legs, and cut
the corners of the mouth. CAUTION: Be careful when using scissors.
10. Locate the TONGUE. Is it attached to the front or the back of the
mouth?_______________ In a live frog, the tongue is sticky and is used to catch
insects. Pull on the tongue. Notice that it is still flexible.
11. Feel the inside of the upper jaw ( maxilla) and the lower jaw (mandible). The
teeth you feel are the MAXILLARY TEETH. Locate the 2 VOMERINE
TEETH on the upper jaw. They are located toward the front of the upper jaw and
between the internal nares (internal nostril openings).
Briefly explain what the maxillary teeth and vomerine teeth are used for?
12. Push carefully on the eyes observe how they fill a space in the mouth. The eyes
help hold the prey as a frog is swallowing it.
13. Locate a vertical opening toward the back of the mouth. This is the GLOTTIS.
It is the opening to the trachea (windpipe) that leads to the lungs.
14. Find the GULLET (throat) it leads to the opening of the esophagus. On both
sides of the gullet, near the cut jaws are opening to the EUSTACHIAN TUBES.
Use your probe.
Where does the Eustachian tube lead?
What is its purpose?
15. Locate and label each part in Figure 2 the vomerine teeth, the maxillary teeth,
the internal nares, the tongue, the openings to the Eustachian tubes, the
esophagus, the pharynx, and the slit-like glottis. See Figure 3 for aid.
Figure 2
Figure 3
16. After the dissection, briefly list the function of each of the parts in Figure 2
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DISSECTING THE FROG:
17. Place the frog on its dorsal side
and secure it in place with
dissecting pins through each of
the legs.
18. With your scissors make a
cut (through the skin only) along
the midline of the belly from the
pelvis to the throat.
19. Now make transverse cuts
through the skin below each of the
fore limbs and above each of the
hind legs. If needed you may pin
the skin back. Notice the blood
vessels under the skin.
20. Notice the abdominal muscles.
Now cut through the muscle layer
and repeat the incisions you mad
in step 18 and 19. BE CAREFUL
NOT TO CUT TOO DEEP AND
DAMAGE THE UNDERLYING
ORGANS.
21. You will have to cut through the sternum (breastbone). Open and re-pin the frog.
22. If your frog is female, the body cavity maybe be full of black eggs. You may
have to remove these to one side in order to continue your dissection.
INTERNAL ANATOMY:
23. The digestive system consists of the organs of the digestive tract and the
digestive glands. Swallowed food moves from the mouth down the esophagus
and into the stomach and then into the small intestine. Bile is a digestive juice
made by the liver and stored in the gall bladder. Bile flows into a tube called the
bile duct. Digestive enzymes from the pancreas flows into this duct. Both bile
and pancreatic enzymes flow into the small intestine. Most digestion and
absorption of food into the bloodstream takes place in the small intestine.
Indigestible materials pass through the large intestine and then into the cloaca,
the common exit chamber of the digestive, excretory, and reproductive systems.
24. Use Figure 4 below to help in locating and identifying the organs of the digestive
system: esophagus, stomach, small intestine, large intestine, cloaca, liver, gall
bladder, and pancreas.
Figure 4
25. Locate the largest organ in the abdominal cavity it is the reddish brown LIVER.
How many lobes does the liver have? ___________
26. Locate the greenish sac attached to the liver. This is the GALL BLADDER.
What is stored in the gall bladder? What does bile digest?
27. Beneath and to the right of the liver is a J shaped STOMACH. With your
scissors open the J of the stomach to observe what the frog may have eaten. Was
there anything in the stomach? What do you think the frog ate?
28. The stomach attaches to the small intestine. The straight part of the small
intestine is called the DUODENUM and the coiled section is the ILEUM. The
coils of the ileum are connected by thin transparent membranes with blood
vessels. This tissue is called the MESENTERY. Mesentery helps keep your
intestine from knotting up. After cutting the small intestine away from the
large intestine, measure how long your small intestine is in cm and inches.
______________cm. _______________________ inches.
Name the two sections of the intestine:
1.
2.
29. The small intestine widens to form the LARGE INTESTINE. The large
intestine is a straight tube leading to the anus. The lower portion of the large
intestine is called the cloaca. Waste, urine and sex cells are expelled here.
6. In the mesentery along the inner curve of the stomach locate the pinkish
PANCREAS. In the mesentery find a reddish spherical structure call the spleen.
The spleen filters out worn out red blood cells and platelets from the blood.
30. After the dissection, briefly state the function of each of the parts in step 23.
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31. The respiratory system consists of the nostrils, trachea and bronchi which opens
into two lungs. Locate the LUNGS, 2 reddish brown sac like structures towards
the dorsal side. Lung Inflation: Insert the tip of a pipette into the glottis in the
mouth. When you squeeze the pipette you should see the lungs inflate if you have
not damaged the lung. Were you able to inflate the lungs? _________. Briefly
describe their characteristics and function.
32. Again, referring to the Figure 4, locate the parts of the circulatory system that are
in the chest cavity. Find the left atrium, right atrium, and ventricle of the heart.
Find an artery attached to the heart and another artery near the backbone. Find a
vein near one of the shoulders.
33. The circulatory system consists of the heart, blood vessels, and blood. The heart
has two receiving chambers, or ATRIA (singular: atrium), and one sending
chamber, or VENTRICLE. Blood is carried to the heart in vessels called veins.
Veins from different parts of the body enter the right and left atria. Blood from
both atria goes into the ventricle and then is pumped into the arteries, which are
blood vessels that carry blood away from the heart. The heart is located between
the lungs. Compare the thickness of the atria and the ventricle.
Why is the ventricle so much thicker than the atria?
34. After the dissection, Briefly indicate the function of the bolded words above
(in step 32):
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35. Use a probe and scissors to lift and remove (cut out) the entire intestinal tract and
liver.
36. The urinary system consists of the FROG’S KIDNEYS, URETERS, URINARY
BLADDER, AND CLOACA The kidneys are organs that filter wastes from the
blood and excrete urine. Connected to each kidney is a ureter, a tube through
which urine passes into the urinary bladder. The urinary bladder is a sac that
stores urine until it passes out of the body through the cloaca. Label the
KIDNEYS, URETERS and URINARY BLADDER on Figure 5.
Figure 5
37. The reproductive system in the female consists of OVARIES which produce
egg and the OVIDUCTS which carry eggs to the cloaca. In the male it consists
of TESTIS which produce sperm, sperm ducts which transport sperm to the
cloaca. LABEL TESTES, OVARY, OVIDUCTS AND EGGS ON FIGURE
5.
38. Closely examine the kidneys notice there is a light colored band of tissue running
through the middle of each kidney. This tissue is the adrenal gland.
39. Voluntary muscles, which are those over which the frog has control, occur in
pairs of flexors and extensors. When a flexor of a leg or other body part contracts,
that part is bent. When the extensor of that body part contracts, the part
straightens.
40. The central nervous system of the frog consists of the brain, which is enclosed in
the skull, and the spinal cord, which is enclosed in the backbone. Nerves branch
out from the spinal cord. The frog’s skeletal and muscular systems consist of its
framework of bones and joints, to which nearly all the voluntary muscles of the
body are attached.
41. Fat bodies are orange/yellow in color and are stored food. LOCATE AND
LABEL THE FAT BODIES ON FIGURE 5.
Disposal
42. Properly dispose of your materials according to the directions from your teacher.
43. Clean up your work area and wash your hands before leaving the lab.
Discussion Questions
1. Compare and Contrast human and amphibian body structures: (Hint: It may be
easier to make a Venn Diagram)
2. With use of correct terminology for parts of the heart, explain in detail how
the frog's heart functions. Also, explain how this is both similar and different
compared to a human heart. -- Research must be properly sourced/cited.
3. Part of what makes a good scientist is the ability to make connections across
fields of study. Read the excerpt from "The Silence of the Frogs". While
considering the upcoming unit on climate change and the grade 9 science unit
on ecosystems, suggest how dissections could possibly help scientists in other
fields of study besides biology. You may also want to consult the following
site for more information.
http://www.carcnet.ca/english/amphibians/amphissues.php
Excerpt from "The Silence of the Frogs"
WHY ARE FROGS DISAPPEARING?
The worldwide disappearance of frogs is a bit of a
puzzle. In some areas, scientists don’t really know
what is causing the problem. In other areas, they
have identified some probable causes.
1. LOSS OF HABITAT
In Canada, frogs in more heavily populated areas,
such as southern Ontario, seem to be in great
danger. The loss of habitat, places where a species
can live, is most often mentioned as the main
cause. Frogs need
wetlands, ponds, or lakes with clean water so they
can breed and lay their eggs. As adults they need a
place such as a forest or a field, where they can
catch insects. They also need a safe path between
the two. The growth of cities and other human
activities, such as farming and industry, takes away
all of these things. Humans drain wetlands, cut
down trees, build on fields, and build roads
between ponds and woods. A highway separating a
woodlot from a pond or lake can claim the lives of
many frogs as they move between their feeding and breeding areas. Cutting down some of the trees that surround a lake
creates problems for amphibians by exposing them to predators as they make their way between the water and the
denser trees farther from the lake. From 1984 to 1986 scientists studied an area where a swamp and a forest were
separated by a road. When trees
bordering the road were cut in 1986, researchers noticed a huge decline in the number of frogs and other amphibians.
2. AIR AND WATER QUALITY
A second cause for the decline in frog numbers is pollution. This is because frog skin is thin and it is not protected by
feathers, fur, or scales. Frogs have lungs, but they also breathe through their skin, which must be thin to allow oxygen
through. Pollutants can also pass through their thin, moist skin. Acid rain, caused mostly by pollutants released by
vehicles and industry, is just one example.
Acidity also affects frogs’ ability to reproduce. Researchers have noted that if the water is even slightly acidic, it reduces
the mobility of frog sperm cells. This makes it less likely that eggs will be fertilized. Even if mating is successful, acid
affects the frog’s development. Embryos, if they develop at all, grow slowly in acidic water. In some locations, this means
that the pond they are in will dry up before tadpoles can become adult frogs, and the tadpoles die. Acidic water can
cause other problems. For example, embryos may develop deformed limbs. Tadpoles with such limbs do not survive for
very long.
3. ULTRAVIOLET RADIATION
The thin skin of the frog is also susceptible to ultraviolet (UV) radiation. This invisible radiation from the Sun causes
sunburns, but it has also been linked with more serious cell damage. The amount of UV radiation reaching the Earth’s
surface is increasing because of damage to the protective ozone layer surrounding our planet. Frogs at higher altitudes,
where the problem of UV is greater, seem to be the ones that are most endangered. Many highland species are used to
dealing with UV radiation (they lay black eggs and have developed a black covering that lines their internal organs for
protection), but biologists speculate that the adaptations can not keep pace with changes in the ozone layer. The frog is
not the only animal whose skin is exposed to UV radiation. Humans also have a delicate skin and are affected by the
increase in UV rays. The fact that the rate of human skin cancer is rising all over the world underscores the importance of
studying the frogs as a “bioindicator” of the health of the planet.
4. CLIMATE CHANGE
Human activities that are causing a change in climate have also been linked to the disappearance of frogs. There is
evidence of a global warming trend. One hypothesis links increasing global temperatures with the increased use of fossil
fuels such as coal, oil, and gasoline. Climate changes can cause important changes in local ecosystems. For example, if
the climate becomes drier, frogs will suffer. No frog can stay in the sun too long or completely separate itself from fresh
water.