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STUDENT LABORATORY PACKET
Lab 9: Bull Frog: External Examination
Student’s Name ___________________________________Modified from Prentice Hall Lab. Manual
Lab Instructor ___________________________________ Date ______________ Points_____
Objective: 1. To observe the external anatomy of the frog
2. To identify examples of how form fits function in a frog
-------------------------------------------------Pre-Lab (Do Before Coming to Lab)-----------------------------------------Background: Dragonfly pp. 784 – 788, Parrot Textbook p 52
1. Fill in the chart:
Classification
(Taxonomic
Categories)
Taxonomic Category of the
Frog
What are the defining characteristics of
organisms classified in each category?
Domain
Kingdom
Subphylum
Phylum
2. Define bilateral organism and give three examples.
3. Describe each anatomical direction AND label them on the frog, an organism with bilateral
symmetry:
Dorsal:______________________________________________
Ventral:_____________________________________________
Anterior:____________________________________________
Posterior:____________________________________________
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--------------------------------------------------------------Laboratory Exercise-------------------------------------------------
Materials: a preserved single injected frog, dissecting tray, hand lens, latex gloves, scissors, forceps,
and a blunt probe.
Note: Please follow your instructor’s directions on how to handle your frog. When
finished with the frog, return to appropriate moist container. All students should wear gloves. If you have an
Procedure:
allergy to latex, wear plastic gloves. Handle the frogs carefully, since you will be using the same frogs for two
weeks.
1. Legs: Notice the size of the frog’s legs. These powerful legs allow the frog to jump up to six feet. Locate
the cloaca, an orifice (opening) 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. A male frog, however,
usually has a thick pad on its “thumb” (see image below). Generally, males are smaller than females in
overall body size.
a. Look at the hand digits, or fingers, on the frog’s forelegs. Compare the size of your frog to
others by leaving the frog in the tray and glancing at the frog in the group next to yours.
b. What is the gender of your frog? Give evidence to support this.
2. Skin: The frog’s skin functions as a respiratory surface when the frog is completely submerged in water.
The skin is composed of membranous tissue that is permeable to water and contains a large network of
blood vessels. When the frog is not submerged in water, mucus is produced which keeps the frog moist
and helps it to absorb oxygen in the air. The mucus makes the frog slippery – helping it to escape
predators.
a. Examine the skin of the frog. Note the dark-colored dorsal side and the light colored
ventral side (refer to the pre-lab if necessary).
b. Knowing that the frog lives in water, explain why you think the dorsal side is dark and the ventral
side is light.
c. Use tweezers to lift a layer of skin from the underlying muscle on the ventral side of the
frog. Cut a small ~1 inch square piece of skin from the belly; do not cut the underlying
muscle wall.
d. What do you observe about the thickness of the frog skin? How do you think this aids in gas
exchange?
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3. Eye: The frog has an upper eyelid that is not movable and a lower eyelid that moves only slightly. The
frog’s eye is also covered by a special third membrane called the nictitating membrane which is
transparent.
a. Examine the eyes and try to identify the three eyelids. Use the blunt probe to separate the
nictitating membrane from the lower eyelid. (see image below)
b. What do you think the function of the nictitating membrane (third eyelid) is?
4. Ear: Frogs have ear-like structures called the tympanic membrane. Sound waves cause the tympanum to
vibrate. The vibrations are transferred to a small bone inside the ear, which in turn stimulates impulses in
certain nerve endings, allowing the frog to hear. (see above image)
a. Identify the tympanic membranes on your frog. They are circular structures located
posterior to the eyes.
b. How is a frog’s tympanic membrane different than a human ear?
5. Nostrils: The nostrils, called external nares, lead directly to the mouth and give the frog its excellent
sense of smell. A frog can take in air in through its nostrils and down into its lungs.
a. Identify the external nares on the frog. (see image below)
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6. Mouth: To inhale oxygen, the frog lowers the floor of its mouth. The nostrils open, allowing air to enter
the mouth. The nostrils then close and the air in the mouth is forced through the glottis into the lungs by
contraction of the lower mouth. To eliminate carbon dioxide in the lungs, the floor of the mouth moves
down, drawing the air out of the lungs and into the mouth. The nostrils open and the floor of the mouth
moves up, pushing the air out of the nostrils. The lining of the frog’s mouth also serves as a respiratory
surface, where oxygen and carbon dioxide are exchanged.
a. Identify the frog’s mouth and open it. If necessary, cut the corners of the jaws carefully.
Pull the lower jaw away from the upper jaw until you can see the back of the mouth. With
the mouth open, stick a probe from the external nares (see image on previous page) through
the nostril and into the mouth to observe where air is inhaled.
b. Find the glottis, a slit in the raised area at the back of the mouth. Feel the opening with the
point of the probe (see image below).
c. The mechanism of a frog taking air into the lungs is different than in humans. Frogs do not have
ribs or a diaphragm, which in humans helps serve to expand the chest thereby decreasing the
pressure in the lung, allowing outside air to flow in. Describe how the mouth of a frog functions
similarly to the diaphragm of a human.
7. Pharynx: At the back of the mouth, posterior to the glottis, is the opening to the esophagus, which leads
to the digestive system.
a. Push the handle of the probe into the pharynx to locate the opening. The wrinkled
membranes indicate how much the pharynx can stretch to accommodate a large piece of
food.
b. Distinguish between the type of materials that will enter through the glottis and the esophagus.
Where will these materials travel to in each case?
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8. Tongue: The frog uses its tongue to catch insects in flight. When the tongue is extended it may be 6 to 7
cm long. A sticky mucus on the tongue helps to trap the insect. The insect is popped into the pharynx where
it is swallowed whole.
a. 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.
b. How does the position of the frog’s tongue aid in catching bugs?
9. Teeth: Frogs have two types of teeth. Vomerine teeth are located between the internal nares. They are
small projections in the top of a frog’s mouth. Maxillary teeth are very small, sharp, cone teeth around the
edge of the upper jaw of a frog's mouth (see image below).
a. Examine both types of teeth with your hand lens.
b. What do you think is the function of the vomerine teeth? Maxillary teeth? How do their shapes
aid in their function?
Copy your lab onto the “Student Answer Packet”. Only these sheets will be collected and graded.
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Student Answer Packet
LAB: Bull Frog: External Examination
Student’s Full Name __________________________________
Lab Instructor __________________________________
Date ___________
FINAL GRADE
Pre-Lab
Completed as homework:
Max. value
Procedure/Observations:
Max. value
1pt.
9 pts.
Points_______
___________
____ pt.
____ pts.
Directions: Answer the questions in complete sentences. The number of each question corresponds to the
procedure in the lab. Use the internet to look up the correct answers to questions you hypothesized about in the
lab.
1. What is the gender of your frog? Give evidence to support this.
2. Knowing that the frog lives in water, explain why you think the dorsal side is dark and the ventral
side is light.
What do you observe about the thickness of the frog skin? How do you think this aids in gas
exchange?
3. What do you think the function of the nictitating membrane (third eyelid) is?
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4. How is a frog’s tympanic membrane different than a human ear?
6. The mechanism of a frog taking air into the lungs is different than in humans. Frogs do not have
ribs or a diaphragm, which in humans helps serve to expand the chest thereby decreasing the
pressure in the lung, allowing outside air to flow in. Describe how the mouth of a frog functions
similarly to the diaphragm of a human.
7. Distinguish between the type of materials that will enter through the glottis and the esophagus.
Where will these materials travel to in each case?
8. How does the position of the frog’s tongue aid in catching bugs?
9. What do you think is the function of the vomerine teeth? Maxillary teeth? How do their shapes aid
in their function?