Name
Date.
58
External Anatomy of the Frog
BACKGROUND
The frog is a member of the phylum Chordata because a gristly rod, the notochord, forms
along the dorsal midline of the frog embryo. .Frogs are also classified in the subphylum
Vertebrata because the adult frog has a segmented backbone, the vertebral column. Humans
are also chordates and vertebrates. Although humans belong to the class Mammalia and frogs
belong to the class Amphibia, their bodies show many similarities of structure and organization.
OBJECTIVt:S
In this activity you will observe the external anatomy of the frog.
MATERIALS
preserved frog (Rana sp)
dissecting tray
scissors
dissecting needles
forceps
hand lens
PROCEDURES AND OBSERVATIONS
Part I. External Anatomy
a. Examine the b9dy of the frog. It is mostly composed of a head and a trunk. Note that the neck is
so short that it is almost nonexistent; But the frog
can still move its head a limited amount.
1; What kind of symmetry
frog body?
is exhibited
moist and slippery with mucus secreted
mucus glands in the skin.
by
b. Observe the skin of the frog. Note the darkcolored dorsal side and the light-colored ventral
sid~~With a hand lens, look closely at the skin
.
1 ~
2. Is
the skin scaly or smooth?
3. How many nostrils are there?
by the
The frog skin has concealing coloration known
as countershading.
It tends to make the animal
blend into its surroundings. Seen from above in
dark water, the frog is hidden because its back is
dark. Seen from below, the light-colored ventral
skin blends with light coming from the sky.
The skin. of the living frog is kept. constantly(
surface.
c. Observe the large mouth extending posteriorly
on both sides of the head. Find the nostrils near
the front tip of the head~
The eyes of the frog protrude when they are
open. The frog can close its eyes only by withdrawing them into their sockets. The upper
eyelid is not movable, and lower eyelid moves
only slightly. The frog also has a third eyelid, a
transparent
nictitating
membrane
that closes
upward over the eye. It protects the eye when
the animal is on land, and allows the frog to see
underwater. In the human eye, the "third eyelid"
is a small fold of tissue in the corner of the eye
next to the nose.
d. Examinethe eyes of the frog. Try to identify the
three eyelids.
The outer part of the frog's ear is the tympanum, a flat, oval 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.
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e. Locate the tympanum, which is posterior to the
eye on each side of the head.
f. Note the two dorsal folds of skin running from
the eye to the anus, high between the hindlegs.
g. Observe the two forelegs and the two hindlegs.
4. Where are the four legs attached?
A living frog sits with its hindlegs folded and
ready to jump.
i. Examine a hindleg. Attached to the body is the
large thigh. The lower part of the limb is called
the leg. Note the long ankle. The foot is attached
.
to the ankle.
II
.'c\'
6. Describe the structure of the f'd6t' of a
hindleg.
."
The forelegs are used mainly to brace the body
as the frog lands after a jump. They are also used
for climbing, but not for swimming.
h. Examinea foreleg.
5. Describe a foot and toes of a foreleg.
/
7. Draw an external view of the. frog, dorsal
side up, with the anterior end at the top.
Label the head, trunk, mouth, nostril, eye,
tympanum, skin fold, anus, foreleg and foot,
and hindleg and foot.
Name,
Date.
Part II. Mouthparts of the Frog
The mouth cavity of the frog is lined with a
mucus membrane composed of ciliated cells.
The large mouth. surface is a part of the frog's
respiratory surface. Oxygen and carbon dioxide
are exchanged across the mouth membrane.
a. Place the frog ventral side up, anterior end toward you. With the scissors, cut the corners of
the mouth on each side where the two jaws meet,
so that you can open the mouth wi~e. Pull the
lower jaw away from the upper jaw until you can
see the back of the mouth. The back of the mouth
becomes the pharynx (throat).
The frog can flip out its tongue to catch insects
in flight. When extended, 'the tongue may be 6 to
Bcm long. A sticky mucus on the tongue helps to
trap the insect. The insect is popped into the
pharYnx where it is swallowed whole. Frogs attempt to capture 'any moving objects small
enough to be swallowed.
b. Examinethe tongue.
1. Where is the tongue attached?
.
'
2. Describe the shape of the free end of the
tongue.
point of your probe into one nostril from the outside. Watch the tip enter the mouth. Then find
the glottis, a slit in the raised area at the back of
the mouth. Feel the opening with the point of the
probe.
e. Between the openings of the nostrils in the roof of
~e mouth are paired vomerine teeth, bony ridges
attached to the skull. Feel the vomerine teeth
with the probe. Then examine them with the
hand lens. These teeth are used for holding, not
for chewing.
4. Describe the vomerine teeth.
,
f. On each side of the upper part of the mouth, near
the place where the two jaws meet, find the
openings to the Eustachian tubes. The tubes connect the mouth with the middle ear.
g. At the back of the mouth, posterior to the glottis,
is the opening to the esophagus, which Jeads to
the digestive system. Push the handle of the scalpel into the pharyn" to locate the opening. The
wrinkled membranes indicate how much the
pharynx can stretch to accomodate large pieces
of food.
'
5. Make a drawing of the inside of the frog
mouth. Label the upper jaw, lower jaw,
pharynx, tongue, maxillary teeth, opening
of nostril, glottis, vomerine teeth, opening
to Eustachian tube, and opening to
esophagus.
c. Run your finger around the inside of the upper
jaw., Use your hand lens to see the maxillary
teeth. These hold a large insect in place until it
can be swallowed. Then examine the lower jaw.
3. Are there any teeth on the lower jaw?
The frog breathes through nostrils. To inhafe,
the frog lowers the floor of its mouth. The mouth
fills with air through the nostrils. The nostrils
then close by means of valves. To fill the lungs,
the frog raises the floor of its mouth, pushing the
air,. )hrough the glottis. The air then passes
th1i)ugh the larynx, or voice box, into the lungs.
d. The openings of the nostrils are near the ant~rior
end of th~ roof of the mouth. Gently push the
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CONCLUSIONS AND APPLICATIONS
1. Describe
countershading
in animal coloration.
Of what advantage is it for an animal?
10. .
~\I.
d1<.
..1."
2. Describe
the body covering of the frog.
3. How does a frog close its eyes? Describe the eyelids of a frog.
4. Describe the structure and functions of the frog's forelegs and hindlegs.
5. Explain how a frog captures and swallows an insect.