One practical way to see for yourself how an amphibian and reptile differ is to
compare the anatomy of two species, such as a frog and a
snake. Although you can observe the external anatomy on live ones at a pet store
or in the wild, it's hard to discover anything about their insides without doing a dissection.
Even if you don't want to dissect a frog and a snake (or a
turtle, for
another interesting reptile specimen), you might find the following description
of anatomical differences helpful. (You can also find
virtual frog
dissections online, instead of doing your own.) This is an
ideal project for junior high and high school students, although younger kids
might be interested, too.
You will need preserved frog and
snake specimens for this
project.
Frog Anatomy
Use this printable frog dissection diagram with labeled parts (.pdf) as a guide for locating organs.
- First look at the outside and mouth of the frog, observing its eyes, legs, skin, and
the round tympanic membrane (which helps it hear) on either side of its head. Open the specimen's
mouth and observe its tongue, gullet opening in the throat, vomerine teeth on
the roof of its mouth, internal nares (inside openings to nostrils), and
maxillary teeth (along the rim of the upper jaw). Male frogs will also have a
vocal sac opening in their throat.
- Use sharp scissors or a scalpel to cut the skin across the frog
specimen's belly: make an incision up the middle from just below where
the legs start to the top of the throat just under the chin. Then make two
horizontal incisions, just above the the legs and just below the arms. Be careful to cut
through the skin and muscle and not any of the
organs underneath. Pull back the sections and then make horizontal
incisions above the arms (cutting through the hard sternum) and below them, and
another vertical incision down to the cloacal opening. Pull these flaps to each
side as well, and pin them down.
- Use your fingers or a probe to inspect the organs underneath. Many of
the main organs will be easy to identify. Look for the heart, liver, gallbladder, pancreas (a
ribbon-like organ next to the stomach), stomach,
small and large intestines, kidneys (in the back of the abdominal cavity), and genitalia. Frogs have a special membrane (it looks sort of like webbing) called the
mesentery that holds the digestive tract and surrounding organs in place. You
could also open the
stomach to examine its contents and see what the frog ate.
-
Determine if your frog is male or female. Females have oviducts on both sides
of the body cavity and a mass of tiny eggs. Males have bean-shaped testes
attached to their kidneys. (This is weird but true: the frog sperm travels through the
kidneys and out the cloacal opening.)
Snake Anatomy
Click the link for a printable snake dissection diagram with labeled parts (.pdf).
Use this as a guide for locating organs.
- Inspect the external anatomy of the snake. Why do you think the
scales on the specimen's back (dorsal side) are different than the scales
on its belly (ventral side)? The ventral scales correspond to the number of
ribs that the snake has and allow greater flexibility of movement.
- Make a long incision down the center of the ventral surface, from the
cloacal opening to the throat. Carefully pull back the skin and pin it
down on either side, cutting the membrane layer underneath as necessary. Once
the snake is opened, observe how it looks inside. Then slice the
membrane away until it separates from the organs. (This membrane holds the
organs securely in place in the living snake.) This is the longest part of the
dissection process, but go slowly and cut carefully.
- Identify the major organs listed below. The heart, stomach,
gallbladder, liver, and large and small intestine will probably be the easiest
to find. These are listed in order roughly from head to tail.
Trachea: A dark-colored tube in throat that brings air to the lung.
Heart: Located just below trachea. It is a three-chambered like a frog
heart.
Right lung: Most snakes have only
one functioning lung, a long narrow sac starting near the heart. Look for it between the liver and stomach.
The specimen might also have another, smaller lung.
Liver: A long, thin orangish-colored organ on the left side (as you look at it).
Stomach: A long sac that connects to the esophagus in the throat and
small intestine lower down. Is your specimen's stomach empty or full? If full,
you might want to check out the contents to discover what the snake was eating.
Gallbladder & pancreas: The gallbladder is small and round,
usually greenish-colored from the bile for digestion stored in it. You might have to remove
some of
the yellow fat bodies to see it. (A healthy snake will have many fat
bodies.) The pancreas looks just like an extension of the
intestine, right next to the stomach.
Small & large intestine: The small intestine starts right below the stomach and forms many coils.
Gonads: These might look similar to the
intestine, but are not connected to it. Male snakes are identifiable by
testes inside and hemipenes at the cloacal opening. Females have a pair of
ovaries and might have eggs. (If both the frog and snake specimens have eggs, be
sure to compare them!)
Kidneys: These are located near the end of the
large intestine; they should be similar in color to the intestine, but if you
look closely you'll see that they are a different kind of tissue.
Print out these diagrams and fill in the labels yourself to test your
knowledge of frog and snake anatomy:
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