Purpose: To
observe the characteristics that identify a frog as
an amphibian. To relate
the structures of the frog to their functions.
Procedure: Place
the frog on the dissecting pan, ventral side down. Locate the structures identified in the data section that follows.
Data: General External Features
The
body of a frog can be divided into a head, which
extends posteriorly to the shoulder region, and a trunk.
Notice that a distinct neck is absent. This is a retention of a characteristic of
fishes, for which an independent motion of the head
and trunk would be disadvantageous during swimming.
The appendages, forearms and hind legs, allow
the frog to move about on land.
The large webbed hind feet aid in swimming.
Label the head, trunk and appendages
on the diagram below.
Head Section
A
large mouth,
a pair of nostrils, or external nares
(singular naris), and the eyes will be recognized on the head. The upper eyelid is a
simple fold of skin, but the lower lid is a transparent
membrane which can be drawn across the surface of the
eyeball. This
is the nictitating
membrane, which protects the eye when the
frog is under water and keeps it moist when the frog
is on land. The
disc-shaped area posterior to each eye is the ear drum,
or tympanic
membrane. Look carefully at the top of the
head between the eyes, and you may see a small, light
brow
spot, which has about the diameter of a pin.
This is a remnant of a median, light-sensitive
eye that characterized primitive group of fishes and
amphibians.
Label the structures,
in blue print, on your drawing below.
Trunk Section
The
combined orifice of the digestive and urogenital tracts,
the cloacal aperture,
is at the posterior end of the trunk just dorsal to
the junction of the hind legs. The anus of human beings and other mammals,
since it is the opening of just the digestive tract,
is only partly comparable to the cloacal aperture.
Frogs’ skin is exceptionally thin as well as
vascular, and it is kept moist by the secretions of
numerous microscopic mucous glands.
These attributes make it an effective respiratory
membrane. About
one-third of the environment of gases between the blood
and environment takes place through the skin.
This is referred to as cutaneous respiration.
However, the characteristics of the skin also
permit considerable exchange of water across it-a loss
of body water when on land, a gain of water when in
an aquatic environment.
Most of the water that the frog needs is absorbed
by osmosis through the skin. 
The
small, branching, dark spots which you see with a microscope
are the pigment cells, or chromatophores. These particular cells contain granules of
a dark pigment known as melanin.
The granules can migrate out to the processes of the
chromatophores, making the frog darker, or they can
concentrate near the center of the cells, giving the
frog a lighter color.
Other pigment cells contain a yellow pigment,
and others contain refractive crystals of guanine which
disperse light to give a blue effect. The combination of yellow and blue causes the
green colors.
Label the structures,
in blue print, on your drawing below.
Color the frog diagram to represent how the pattern
conceals the frog in its natural habitat.
Appendages
The
legs of a frog have the same parts as our own.
In the front leg, or pectoral
appendage, notice the upper
arm, the elbow,
the forearm, the wrist
joint, and the hand. How many fingers are on the front hand?. Are they webbed? Although the finger closest
to the body is comparable to our second digit, it is
often called the thumb because it is stouter than the
others. During
the breeding season, the thumb is particularly stout
and darkly pigmented - an example of sexual dimorphism.
In the hind limb, or pelvic
appendage, observe and label the thigh,
the knee, the
shank, the ankle
joint, and the very long foot. Two elongated ankle bones lie within the proximal
part of the foot. Describe the foot. How many toes are
on the foot? Are
the toes webbed? The first toe is the smallest and the
one closest to the body, and it is comparable to our
great toe. Technically
it is called the hallux,
and the small spur at its base is the prehallux. The prehallux is much larger in toads, which
use their hind feet in burrowing.
Label the structures, in blue print, on your drawing
below.
Frog,s Sex:
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. Male frogs also
have a larger tympanic membrane than females. Observe
several frogs to see the difference between males and
females.
Analysis Questions:
