Name
Date
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EXPLORATION
Amphibians were probably the first animals to possess four limbs,
and they demonstrate a greater complexity over fishes. Most of the
changes made life on land possible. By examining the anatomy of
amphibians, one can see many structures that are basic to both
amphibians and all the more advanced vertebrates.
OBJECTIVES
Draw and label various features of a frog.
Give a function of each major organ of a frog.
Identify the external organs of a preserved frog.
Perform procedures to identify the internal organs
of a frog.
MATERIALS
c!!lhrnrn
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frog (preserved)
scissors
dissecting pins (6-10)
dissecting pan
stereomicroscope
dissecting probe
forceps
microscope slides (2)
coverslip
microscope
laboratory apron
PROCEDURE
Part A. External Anatomy
Refer to Figure 1 for this part of the Procedure.
1. Put on a laboratory apron. Rinse a preserved
frog well with water. Place It ventral surface
down in your dissecting pan.
2. Note the arrangement ofthe spots and the
coloration of the frog. The color of the frog is
caused by scattered granules in the epidermis
and chromatophore cells in the dermis.
Chromatophore cells are cells that contain
pigments.
3. Remove a 1 cm x 1 cm section of the dorsal
skin containing one ofthe frog's spots. Make a
wet mount of this piece of skin.
4. Place the wet mount on the microscope under
low and then high power. Chrornatophores are
usually star shaped. Dispersal of the pigment
into the rays makes the skin darker. When the
pigment is concentrated in the center of the
chromatophore, the skin is lighter.
5. Make a drawing of the chromatophore in the
space provided in Data and Observations.
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6. Locate the thin membrane that covers the eye
from below. This is the nictitating membrane.
7.
8.
9.
10.
Rgure 2.
This membrane covers the eye when the frog is
under water.
Notice the large tympanic membranes behind
the eyes. These membranes function as
eardrums t o receive sound waves.
Examine the forelegs and hindlegs of the frog,
noting the number of toes. If your frog is a
male, it will have roughened pads near the
thumbs. These are used t o hold the female
during mating.
Answer questions 1. 2, 3, and 4 in the Analysis.
Find in Figure 1 the structures that are in bold
print in Part A.
Vomorinm
tooth
Nostril
Part B. Oral Cavity
'
tonguo
Refer t o Figure 2 for this part of the Procedure.
1. Open the mouth by cutting the jaws with the
scissors. Locate the maxillary teeth around the
edge of the jaw. They hold food but are not
used for chewing.
2. Locate the slit-like glottis at the back of the
throat. This opening leads to the respiratory
system. Above the glottis is the opening to the
esophagus.
3. Find the eustachian tub- at the posterior
corners of the upper jaw. Probe with your
dissecting probe t o find out where they lead.
These tubes equalize pressure within the ear.
4. Locate in a male frog the opening that leads t o
the vocal sacs at the widest corner of the lower
jaw. They amplify the male's mating call.
5. Notice the shape of the tongue and where it is
attached. It can be flipped forward to catch
Prey.
6. Locate t h e nostril openingr in the roof of the
mouth. Between the nostril openings are two
vomerlne teeth. Feel these teeth and t h e
maxillary teeth with your fingers.
7. Locate in Figure 1 the external narea on t h e
dorsal surface of the frog's head.
8. Answer questions 5, 6, and 7 in the Analysis.
9. Find in Figure 2 the structures that are in bold
print in Part B.
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Part C. Digestive System
2. Insert the point of your sclssors just through
the muscle above t h e anal opening and make a
cut extending t o t h e lower jaw. Cut sideways at
Refer t o Figures 3 and 4 for this part of t h e
Procedure.
1. Place your frog dorsal surface down in the
dissecting pan. Open the frog by cutting the
skin around the abdomen in the manner shown
in Figure 3.
Oral cavlty
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both the forelegs and hindlegs as shown in
Figure 3. Pin t h e muscles down to t h e
dissecting pan. NOTE: If your frog is female and
contalns black eggs, they must be removed
carefully before the internal organs can be
observed.
Date
Name
3. Locate the esophagus as shown in Figure 4.
Pass a probe into the stomach. Note that the
lower end of the stomach is constricted. This
constriction is the pyloric sphincter. it
regulates the amount of food that enters the
small intestine.
4. Cut open the stomach and observe its lining, If
food is present in the stomach, can it be
identified? Usually insect body parts will be
present.
5. Follow the digestive tract beyond the pyloric
sphincter to the coiled small intestine. The first
portion that usually runs parallel to the
stomach is the duodenum.
6. Cut open the lower part of the small intestine.
Place a piece of the small intestine on a clean
glass slide with the inside surface up. Observe
the surface of the small intestine under a
stereomicroscope. Note the villi, the many
small folds in the lining that increase the
absorption of nutrients into the bloodstream.
7. Follow the digestive tract below the small
intestine where it widens into the large
intestine or colon. The colon ends in the
rectum, which in turn opens into the cloaca
The cloaca opens to the outside of the frog.
The digestive, reproductive, and excretory
systems all open into the cloaca.
8. Note the large brown liver. Lift the lobes of the
liver to locate the green gallbladder. The
gallbladder stores bile that is secreted by the
liver. Bile aids in the digestion of fats.
9. Locate the pancreas, a soft, irregular pinkish
organ that produces digestive enzymes, found
lying in a membrane between the stomach and
duodenum.
10. Answer questions 8, 9, 10, and 11 In the
Analysis.
11. Find in Figure 4 the structures that are in bold
print in Part C.
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Part D. Respiratory and Circulatory
1. Probe the glottis to see where it leads. Locate
2.
3.
4.
5.
6.
the trachea, the passageway betwecn the glottis
and the lungs.
Locate the pinkish-gray lungs.
Notice the threechambered heart between the
lungs and posterior to the trachea. The pointed
ventricle is lighter in color than the rest of the
heart. The two thin-walled atria are darker
colored.
Lift the stomach and find the spleen, a round
red organ. The spleen filters the blood, taking
out improperly functioning red blood cells.
Answer question 12 in the Analysis.
Find in Figure 4 the structures that are in bold
print in Part D.
Part E. Excretory and Reproductive
Systems
Refer to Figure 4 for this part of the Procedure.
I. Examine the kidneys that lie against the dorsal
body wall in the posterior region of the body
cavity. Each kidney has a yellow stripe, known
as the adrenal body that secretes hormones.
The kidneys filter the blood and urine that
drains into the urinary bladder, a thin-walled
bag that attaches to the cloaca.
2. Locate in a female frog, two lobed, grayish
ovaries that lie close to the kidneys. In a
mature female, the two ovaries might be filled
with black and white eggs.
3. Locate in a male frog, the white testes that can
be found close to the kidneys. Look at the
reproductive organs of both sexes.
4. Examine the yellow, finger-like fat bodla
attached near the kidneys. Compare their size
with these in a frog of the opposite sex The fat
bodies provide nourishment for the gametes.
5. Answer question 13 in the Analysis.
6. Find in Figure 4 the structures that are in bold
print in Part E.
Systems
Refer to Flgure 4 for this part of the Procedure.
Part F. The Brain
Air is drawn into the mouth by expansion of t h e
throat. The external nares close, then the throat
muscles contract and air is forced into the lungs
through the glottis. Air is expelled as the nares
remain closed, the throat expands, and air enters
the mouth again from the lungs. The glottis closes,
the nares open, and the throat contracts, forcing
the air out through the nares. This method of
breathing differs from that of more advanced
vertebrates because the frog lacks a diaphragm.
Refer to Figures 5 and 6 for this part of the
Procedure.
1. Turn over your frog so that the dorsal side
once again faces up.
2. Insert the point of your scissors through the
skin at the base of the head and remove the
skin from the head'area.
3. Bend the frog to determlne the approximate
region of the "neck."
= a
'lgure 4.
1
0-
Name
Date
Rgurt 6.
Olkctoy lob.
Corobrum
optic lobe
C~robollum
Mdulla oblongata
Spinal cord
4. Insert your scissors and clip across the upper
spinal cord in the region of the neck,
5. Locate the white spinal cord enclosed within
the vertebrae.
6. Use your forceps t o remove the bone above the
spinal cord, working forward until you have
.
reached the nostril area. You will be exposing
the brain, as shown in Figure 6.
7. Locate the olfactory lobes, cerebrum, optic
lobe, cerebellum, and medulla oblongata of
the brain and the spinal cord, using Figure 6 as
a guide.
DATA AND OBSERVATIONS
Chromrtophores
ANALYSIS
1. What is the function of the nictitating membrane?
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2. a;' How are the eyes positioned in the frog's head in comparison to the positioning of your eyes?
b. How is this positioning of the eyes an adaptive advantage for a frog?
3. Compare the colors of the dorsal and ventral surfaces of the frog. Of what adaptive value to the frog is
each of these colorations to its survival?
4. Is the dispersal of the pigment in all of the chromatophores uniform?
5. How is the tongue attached?
6. Where does the glottis lead?
7. To what structures do the eustachian tubes lead?
,
8. How does the lining of the stomach compare with the lining of the small intestine?
9. What is the function of the villi?
10. How many lobes does the frog's liver have?
11. What is the function of the pyloric sphincter?
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12. Sequence the passage of air into and out of a frog.
13. Are the fat bodies larger in male or female frogs?
Why Is this so?
14. The largest parts of the frog's brain are the olfactory lobes and optic lobes, the centers of smell and
vision. How is this adaptation an advantage for the frog's lifestyle?
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