Biol 212 Zoology
Lab 13: The Amphibians (10 points)
The subphylum Vertebrata of the phylum Chordata is divided into the superclass
Cyclostomata, which includes the classes Myxini and Petromyzontida, and the superclass
Gnathostomata, which includes the remainder of the vertebrate classes. A major definitive
characteristic of the Cyclostomata is the lack of jaws, and that of the Gnathostomata is the
presence of jaws, as we saw in Lab 12.
The superclass Gnathostomata is divided into the jawed fishes (classes Chondrichthyes,
Actinopterygii, Sarcoptergyii and Dipnoi) and the tetrapods. The tetrapods include the classes
Amphibia, mammalian and Reptilia (birds are now considered to be part of the class Reptilia,
and are very closely related to velociraptors!). The most obvious synapomorphy of the clade
Tetrapoda is that they have four limbs.
Amphibians have fish-like eggs without egg shells that dehydrate easily, thus must be
deposited in water or into moist environments. The remainder of the tetrapod group, classes
Mammalia and Reptilia, are called amniotes. They have eggs with egg shells that are resistant to
desiccation, drying out), thus freeing these groups from living in or near water; they also have
four internal membrane layers, one of which is called the amnion, thus the clade’s name, that
surrounds the developing embryo, protecting it from shock. Apomorphic characteristics of the
mammals include hair and having mammaries to suckle their (our) young. Synapomorphic
characteristics of reptiles include an integument with highly-keratinized, water-proof scales that
retard desiccation, that may be modified into feathers.
The class Amphibia is a transition or “missing link” between the class Actinopterygii, the
ray-finned fishes, and the class Reptilia. Amphibians, which include the salamanders, frogs and
toads, and a legless group called the caecilians, exhibit fish-like larval forms and terrestrial adult
forms. The larval stages, called tadpoles or pollywogs, possess gills, a lateral-line system, a postanal tail and are herbivores. Adult amphibians breathe using lungs, do not have a lateral line,
rather have tympanic membranes for hearing, have four legs instead of a tail and are carnivores.
The integument of both adults and larval amphibians is thin, without scales and without keratin,
meaning it is not resistant to desiccation; thus most amphibians are restricted to aquatic or moist
habitats. Also, since amphibian skin does not contain keratin, amphibians may respire through
their skin, something mammals and reptiles cannot do.
The purpose of this lab is to introduce you to the anatomy of a typical tetrapod, the frog,
and to familiarize you with the general characteristics and relationships of the tetrapods.
For the Lab Report:
*On the upper, right-hand corner of your lab report, print your name, Biol 212, Lab 13: The
Amphibians, and the date(s) you completed this lab.
Introductory Objectives
Objective 1: List the terrestrial and aquatic characteristics of adult amphibians, amphibian eggs
and larval amphibians.
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Objective 2: Give the relationships, in outline form, of the following taxa:
class Actinopterygii
class Mammalia
clade Amniota
class Myxini
class Amphibia
class Petromyzontida
class Chondrichthyes
class Reptilia
superclass Cyclostomata
class Sarcopterygii
class Dipnoi
clade Tetrapoda
superclass Gnathostomata
Objective 3: Give the definitive (apomorphic) characteristics of the following taxa:
clade Amniota
class Mammalia
class Amphibia
class Reptilia
superclass Cyclostomata
clade Tetrapoda
superclass Gnathostomata
For the Lab Report:
Write out these questions then answer them:
1. Outline the relationships of the following taxa:
class Actinopterygii
clade Amniota
class Amphibia
class Chondrichthyes
superclass Cyclostomata
class Dipnoi
superclass Gnathostomata
class Mammalia
class Myxini
class Petromyzontida
class Reptilia
class Sarcopterygii
clade Tetrapoda
2. List the apomorphic characteristics of the following taxa:
clade Amniota
class Mammalia
class Amphibia
class Reptilia
superclass Cyclostomata
class Tetrapoda
superclass Gnathostomata
3. What characteristics of adult amphibians, amphibian eggs and larval amphibians make
amphibians a transitional group between aquatic and terrestrial animals?
Exercise 13.1: Frog External Anatomy
Introduction
Frogs have a distinct head and body, region, but do not have a neck, nor do adult frogs
have a post-anal tail. The mouth and two external nares (nostrils) are located on the head, to the
anterior. The external nares are used for respiration. The eyes are large and lateral. The eyes have
an upper eyelid, a lower eyelid and a nictating membrane. The nictating membrane is a third
eyelid capable of moving up and covering the eye for mechanical protection and protection
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against desiccation. Posterior to the eye and lateral is the large, round tympanic membrane,
used for hearing in the adult amphibian. The frog has four limbs with webbing between the digits
of the limbs, allowing the frog to be an effective swimmer; further, the hind limbs have
adaptations that allow jumping on land.
Within the mouth of a frog, along the lower jaw or mandible, is the tongue. In males,
lateral to the tongue, are openings to the vocal sacs. The posterior portion of the mouth is called
the pharynx. Posterior to the pharynx on the floor of the mouth is a slit, the glottis, serving as
the opening into the larynx or voice box. Along the edges of the upper part of the jaw, the
maxilla, the maxillary teeth can be felt. Medially and anterior, on the roof of the mouth, are the
vomerine teeth. Lateral to the vomerine teeth are the internal nares which connect to the
external nares. Posterior and lateral in the roof of the mouth are the openings into the eustachian
tubes, which connect to the middle ear, allowing for pressure equalization between the external
environment and middle ear—this
is necessary for optimal functioning
of the tympanic membrane.
Also visible in the external
anatomy of the frog is the cloaca,
which is a common urogenital/anal
opening. It is located on the ventral
surface, between the legs.
Objectives
Objective 4: On a preserved frog,
identify the mouth, external naris,
upper eyelid, lower eyelid, nictating
membrane, tympanic membrane,
cloaca, internal naris, Eustachian
tube, vomerine teeth, maxillary
teeth, glottis, tongue, mandible,
pharynx, opening to esophagus,
opening to vocal sac, cloaca.
Objective 5: State the
characteristics observed in a
preserved frog that allow it to live in
water and
those characteristics that allow it to live
on land.
Fig. 13.1: Frog mouth, maxillary region (top),
mandibular region (bottom). Modified from
Underhill.
Materials and Methods
*Preserved frogs (Rana pipiens or other)
-Dissection pans and tools (needle probes, maul probes, forceps, scalpels or straight-edged razor
blades, dissection pins)
-Latex gloves
-Dissection microscopes
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1. Observe and draw the external anatomy of the frog. Label it as per the For the Lab Report
box below.
For the Lab Report:
4. Write out, “4. Phylum Chordata, subphylum Gnathostomata, class Amphibia, Rana pipiens,
external anatomy.” Sketch a lateral view of the external anatomy of your frog. Identify and
label the mouth, external naris, upper eyelid, lower eyelid, nictating membrane and tympanic
membrane. Include an accurate size rule next to your drawing. No credit for drawings
without accurate size rules. To the right of your drawing, state how big the size rule is. Also,
include any notes that might help you to identify the organism on the lab practical! Have
your instructor check and initial your drawings for credit; all drawings must be completed in
lab and signed by your instructor for credit!
2. Open the mouth of your frog wide, being careful not to break the mandible. Draw and label
the maxillary region as per the For the Lab Report box below.
For the Lab Report:
5. Write out, “5. Phylum Chordata, subphylum Gnathostomata, class Amphibia, Rana pipiens,
anatomy of mouth, maxillary region.” Sketch the structures of the upper jaw. Label the
maxilla, internal naris, vomerine teeth, maxillary teeth, Eustachian tube opening, pharynx
and opening to esophagus. Include an accurate size rule next to your drawing. No credit for
drawings without accurate size rules. To the right of your drawing, state how big the size rule
is. Also, include any notes that might help you to identify the organism on the lab practical!
Have your instructor check and initial your drawings for credit; all drawings must be
completed in lab and signed by your instructor for credit!
3. Examine the mandibular region of the open jaw. Draw and label the mandibular region as per
the For the Lab Report box below.
For the Lab Report:
6. Write out, “6. Phylum Chordata, subphylum Gnathostomata, class Amphibia, Rana pipiens,
anatomy of mouth, mandibular region.” Sketch the structures of the lower jaw. Label the
mandible, tongue, opening to vocal sac (if frog is male) and glottis. Include an accurate size
rule next to your drawing. No credit for drawings without accurate size rules. To the right of
your drawing, state how big the size rule is. Also, include any notes that might help you to
identify the organism on the lab practical! Have your instructor check and initial your
drawings for credit; all drawings must be completed in lab and signed by your instructor for
credit!
For the Lab Report:
Write out these questions then answer them:
7. What do you think the function of the maxillary teeth is? Why do you think so?
8. What do you think the function of the vomerine teeth is? Why do you think so?
9. Can frogs breathe with their mouth closed? How do you know?
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Exercise 13.2: Frog Integument
Introduction
The integument of land vertebrates consists of an outer epidermis and an inner dermis.
The outer layer of the epidermis is a dead layer of cells called the stratum corneum; it is
produced by the underlying living layer, the stratum germinitivum. The stratum corneum is
constantly being rubbed off, protecting the other layers of the integument from damage.
Together, the stratum corneum and stratum germinitivum make up the epidermis.
The dermis is attached to connective tissue that, in turn, attaches to the underlying muscle
layer. Within the dermis are located specialized, pigmented cells called chromatophores. They
are under limited muscle control and allow the frog to change colors. Some vertebrates, such as
chameleons, a type of lizard, can change colors rapidly by adjusting the shape of their
chromatophores.
Objectives
Objective 6: State the relationship between integument, epidermis, dermis, connective tissue and
muscles in land vertebrates.
Objective 7: Give the two layers of the epidermis in the frog, along with their function.
Objective 8: Give the location and function of chromatophores.
Materials and Methods
-Microscope slide and coverslip
-Compound microscope
1. Since you’ll need to examine a small sample of skin under the compound microscope, and
preparatory to studying the musculature of the frog, you will have to skin your frog. To do
this, obtain a fine-point pair of scissors. Insert the point of the scissors just barely under the
skin in the region of the cloaca. Make a very shallow incision on the mid-ventral surface of
the frog, from the cloaca to the chin, being careful not to cut or damage underlying muscles.
Now, follow the curvature of the jaw to the dorsal surface, cutting posterior to the tympanic
membrane, so that an incision has been made completely around the head region. Next, make
a shallow incision along the vertebrae, from head region to the cloaca. Finally, make
incisions completely around both wrists and completely around the beginnings of the feet
(lower ankle region). Using a blunt probe, loosen the skin from the body and peel it off,
inside out, over the upper limbs and lower limbs.
2. Cut off a small square of skin, maybe two to three mm square. Put this on a microscope slide
with a drop of water. Apply a coverslip. Examine the skin under a compound microscope
under low power. Find and draw a chromatophore as per the For the Lab Report box below.
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For the Lab Report:
10. Write out, “10. Phylum Chordata, subphylum Gnathostomata, class Amphibia, Rana pipiens,
anatomy of integument, chromatophore.” Sketch a chromatophore. Include an accurate size
rule next to your drawing. No credit for drawings without accurate size rules. To the right of
your drawing, state how big the size rule is, in m. Also, include any notes that might help
you to identify the organism on the lab practical! Have your instructor check and initial your
drawings for credit; all drawings must be completed in lab and signed by your instructor for
credit!
For the Lab Report:
Write out these questions then answer them:
11. The integument of land vertebrates is made of what two layers?
12. What is the function of the stratum corneum?
13. What is the function of the stratum germinitivum?
14. What is the location and function of chromatophores?
Exercise 13.3: The Skeletal Muscles
Introduction
The major superficial muscles of the frog are given in Fig. 13.2 below.
Objectives
Objective 9: Identify the following muscles and structures of the torso on a preserved frog:
dentoid, pectoralis major, cutaneous pectoralis, rectus abdominus, linea alba, external oblique,
temporalis, latissimus dorsi, longissimus dorsi, coccygeoiliacus and gluteus.
Objective 10: Identify the following muscles of the upper extremity on a preserved frog: extensor
carpi radialis, flexor carpi radialis, flexor carpi ulnaris, extensor carpi ulnaris, extensor digitorum
communis.
Objective 11: Identify the following muscles of the lower extremity on a preserved frog:
sartorius, adductor longus, adductor magnus, gracilis major, gracilis minor, semimembranosus,
biceps femoris, triceps femoris, gastrocnemius, Achilles’ tendon, peroneus.
Objective 12: State the function of the gastrocnemius and peroneus muscles.
Materials and Methods
1. You should have skinned your frog in Exercise 13.2; if you have not, return to that exercise
and follow the instructions.
2. Examine the ventral muscles of the frog. Accurately draw and label the major muscles of the
ventral region in at least half of the frog as per the For the Lab Report box below.
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Fig. 13.2: Frog superficial muscles; dorsal muscles
(left), ventral muscles (right).
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For the Lab Report:
15. Write out, “15. Phylum Chordata, subphylum Gnathostomata, class Amphibia, Rana pipiens,
ventral muscles.” Accurately draw the ventral musculature of the frog; you need only draw
the muscles of the right or left appendages. Label the muscles indicated in Objectives 9, 10
and 11 that you can see on the ventral surface. Include an accurate size rule next to your
drawing. No credit for drawings without accurate size rules. To the right of your drawing,
state how big the size rule is. Also, include any notes that might help you to identify the
organism on the lab practical! Have your instructor check and initial your drawings for
credit; all drawings must be completed in lab and signed by your instructor for credit!
3. Examine the dorsal muscles of the frog. Accurately draw and label the major muscles of the
dorsal region as per the For the Lab Report box below.
For the Lab Report:
16. Write out, “16. Phylum Chordata, subphylum Gnathostomata, class Amphibia, Rana pipiens,
dorsal muscles.” Accurately draw the dorsal musculature of the frog; you need only draw the
muscles of the right or left appendages. Label the muscles indicated in Objectives 9, 10 and
11 that you can see on the dorsal surface. Include an accurate size rule next to your drawing.
No credit for drawings without accurate size rules. To the right of your drawing, state how
big the size rule is. Also, include any notes that might help you to identify the organism on
the lab practical! Have your instructor check and initial your drawings for credit; all drawings
must be completed in lab and signed by your instructor for credit!
4. Examine the gastrocnemius and peroneus muscles. Answer the questions below!
For the Lab Report:
Write out these questions then answer them:
17. What is the function of the gastrocnemius muscle?
18. What is the function of the peroneus muscle?
Exercise 13.4: The Digestive and Respiratory Systems
Introduction
All tetrapods, including amphibians, have a complete digestive system, with a mouth,
esophagus, stomach, intestine, liver, rectum and anus. Further, air-breathing tetrapods have
lungs. Fig. 13.3 presents diagrams of a frog dissection.
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Fig. 14.3: Frog dissection. Key:
A: Fat body
B: Liver
C. Small intestine
D. Large intestine
E. Oviduct
F: Gall bladder
G: Stomach
H: Pancreas
I: Rectum
J: Heart
K: Conus arteriosus
L: Larynx
M: Lungs
N: Postcaval vein
O: Ovary
P: Spleen
Q: Kidney
R: Adrenal gland
S: Testes
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Objectives
Objective 13: In a dissected frog, identify the parietal peritoneum and the coelomic cavity.
Objective 14: In a dissected frog, identify the following structures: esophagus, heart, stomach,
small intestine, large intestine (colon), liver, gallbladder, pancreas and cloaca.
Objective 15: In a dissected frog, identify the bronchi, lungs and alveoli.
Materials and Methods
1. Insert the tip of your fine-point scissors through the muscle layer at the region of the cloaca.
Lift up as you cut so as to not damage underlying organs as you make the mid-ventral
incision from the cloaca to the jaw. Make lateral incisions posterior to the arms and anterior
to the legs. Pin back the body walls to reveal the internal organs.
2. Draw and identify the internal organs initially exposed as per the For the Lab Report box
below.
For the Lab Report:
19. Write out, “19. Phylum Chordata, subphylum Gnathostomata, class Amphibia, Rana pipiens,
internal organs.” Sketch the initial dissection of the frog. Label the parietal peritoneum,
coelomic cavity, esophagus, heart, stomach, small intestine, large intestine (colon), liver,
gallbladder, pancreas, anus and cloaca. Include an accurate size rule next to your drawing.
No credit for drawings without accurate size rules. To the right of your drawing, state how
big the size rule is. Also, include any notes that might help you to identify the organism on
the lab practical! Have your instructor check and initial your drawings for credit; all drawings
must be completed in lab and signed by your instructor for credit!
For the Lab Report:
Write out these questions then answer them:
20. Is the digestive tract of the adult frog relatively long or short? Why?
21. Do you think the digestive tract of a larval frog (tadpole) is relatively long or short? Why?
3. Remove one of the lungs. Make a longitudinal section then observe the section under the
dissection (or compound) microscope. Draw a section of the lungs and label the alveoli, the
microscopic, bubble-like air sacs in the lungs. Keep a lookout for parasites! Frogs commonly
carry nematodes and flukes in their lungs. If you do see parasites, call you instructor over.
We’ll keep a list on the board.
For the Lab Report:
22. Write out, “22. Phylum Chordata, subphylum Gnathostomata, class Amphibia, Rana pipiens,
lung section.” Sketch the lung section. Label the bronchi and alveoli. Include an accurate size
rule next to your drawing. No credit for drawings without accurate size rules. To the right of
your drawing, state how big the size rule is. Also, include any notes that might help you to
identify the organism on the lab practical! Have your instructor check and initial your
drawings for credit; all drawings must be completed in lab and signed by your instructor for
credit!
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For the Lab Report:
Write out these questions then answer them:
23. What is the function of the alveoli?
24. What parasites (if any) were reported by the class in the frog lungs?
Exercise 13.5: The Urogenital System
Introduction
The urogenital system of male and female frogs is diagrammed below.
Fig 13.4: Male (left) and female (right) urogenital systems of frog.
Objectives
Objective 16: In a dissected male frog, identify the following structures: kidneys, adrenal glands,
testes, mesorchium, ureters (mesonephric ducts), urinary bladder, fat bodies and cloaca.
Objective 17: In a dissected female frog, identify the following structures: kidneys, adrenal
glands, ovary, mesovarium, oviduct, mesotubarium, ovisac, Wolffian duct, ureters (mesonephric
ducts), urinary bladder, fat bodies and cloaca.
Materials and Methods
1. Expose the cloaca by carefully cutting laterally through both sides of the ischiopubic
symphysis, the bone joining the front of the groin area. Snip the large intestine near the
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rectum and lift the digestive organs aside.
2. If you have a male frog, draw and label the structures as per the For the Lab Report box
below; if you have a female frog, draw and label the structures as per the following for the
Lab Report box. Before going on to the next step, share your frog dissection with any lab
group that needs to draw and study a frog of its sex.
For the Lab Report:
25. Write out, “25. Phylum Chordata, subphylum Gnathostomata, class Amphibia, Rana pipiens,
male anatomy.” Draw and label the kidneys, adrenal glands, testes, mesorchium, ureters
(mesonephric ducts), urinary bladder, fat bodies, dorsal aorta, posterior vena cava and cloaca.
Include an accurate size rule. No credit for drawings without accurate size rules. To the right
of your drawing, state how big the size rule is. Also, include any notes that might help you to
identify the organism on the lab practical! Have your instructor check and initial your
drawings for credit; all drawings must be completed in lab and signed by your instructor for
credit!
For the Lab Report:
26. Write out, “26. Phylum Chordata, subphylum Gnathostomata, class Amphibia, Rana pipiens,
female anatomy.” Draw and label the kidneys, adrenal glands, ovary, mesovarium, oviduct,
mesotubarium, ovisac, Wolffian duct, ureters (mesonephric ducts), urinary bladder, fat
bodies, dorsal aorta, posterior vena cava and cloaca. Include an accurate size rule. No credit
for drawings without accurate size rules. To the right of your drawing, state how big the size
rule is. Also, include any notes that might help you to identify the organism on the lab
practical! Have your instructor check and initial your drawings for credit; all drawings must
be completed in lab and signed by your instructor for credit!
For the Lab Report:
Write out these questions then answer them:
27. Why isn’t there a gonoduct (spermatic duct) in male frogs?
28. What is the function of the mesorchium, mesotubarium and mesovarium?
29. Do frogs use external or internal fertilization? How do you know by looking at the anatomy
of the sexual organs?
~When you’re finished, help clean up!
1. Is your lab bench clean and wiped down with antiseptic solution?
2. Are all materials returned to their proper place?
3. Is the oil immersion objective of your microscope clean?
4. Is the lowest-power objective of your microscope positioned down?
5. Is the power cord draped loosely about one of the oculars?
6. Is your microscope put away?
7. Is all refuse disposed of properly?
8. Is the lab generally in order?
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