Frog DissectionPictures:  Modern Biology, Holt

Background:As members of the class Amphibia, frogs may live some of their adult lives on land, but they must return to water to reproduce. Eggs are laid and fertilized in water. On the outside of the frog’s head are two external nares, or nostrils; two tympani, or eardrums; and two eyes, each of which has three lids. The third lid, called the nictitating membrane, is transparent. Inside the mouth are two internal nares, or openings into the nostrils; two vomerine teeth in the middle of the roof of the mouth; and two maxillary teeth at the sides of the mouth. Also inside the mouth behind the tongue is the pharynx, or throat. 

In the pharynx, there are several openings: one into the esophagus, the tube into which food is swallowed; one into the glottis, through which air enters the larynx, or voice box; and two into the Eustachian tubes, which connect the pharynx to the ear. The digestive system consists of the organs of the digestive tract, or food tube, and the digestive glands. From the esophagus, swallowed food moves into the stomach and then into the small intestine. Bile is a digestive juice made by the liver and stored in the gallbladder. Bile flows into a tube called the common bile duct, into which pancreatic juice, a digestive juice from the pancreas, also flows. The contents of the common bile duct flow into the small intestine, where most of the digestion and absorption of food into the bloodstream takes place.

 Indigestible materials pass through the large intestine and then into the cloaca, the common exit chamber of the digestive, excretory, and reproductive systems. The respiratory system consists of the nostrils and the larynx, which opens into two lungs, hollow sacs with thin walls. The walls of the lungs are filled with capillaries, which are microscopic blood vessels through which materials pass into and out of the blood. The circulatory system consists of the heart, blood vessels, and blood. The heart has two receiving

chambers, or atria, and one sending chamber, or ventricle. Blood is carried to the heart in vessels called veins. Veins from different parts of the body enter the right and left atria. Blood from both atria goes into the ventricle and then is pumped into the arteries, which are blood vessels that carry blood away from the heart. 

The urinary system consists of the frog’s kidneys, ureters, bladder, and cloaca. The kidneys are organs that excrete urine. Connected to each kidney is a ureter, a tube through which urine passes into the urinary bladder, a sac that stores urine until it passes out of the body through the cloaca. The organs of the male reproductive system are the testes, sperm ducts, and cloaca. Those of the female system are the ovaries, oviducts, uteri, and cloaca. The testes produce sperm, or male sex cells, which move through sperm ducts, tubes that carry sperm into the cloaca, from which the sperm move outside the body. The ovaries produce eggs, or female sex cells, which move through oviducts into the uteri, then through the cloaca outside the body.

The central nervous system of the frog consists of  the brain, which is enclosed in the skull, and the spinal cord, which is enclosed in the backbone. Nerves branch out from the spinal cord. The frog’s skeletal and muscular systems consist of its framework of bones and joints, to which nearly all the voluntary muscles of the body  are attached. Voluntary muscles, which are those over which the frog has control, occur in pairs of flexors and extensors. When a flexor of a leg or other body part contracts, that  part is bent. When the extensor of that body part contracts, the part straightens. 

Objectives:• Describe the appearance of various organs found in the frog.• Name the organs that make up various systems of the frog.

Purpose:In this lab, you will dissect a frog in order to observe the external and internal structures of frog anatomy.

Materials:• safety goggles, gloves, and a lab apron• forceps• preserved frog• dissecting pins (6–10)• dissecting tray and paper towels• plastic storage bag and twist tie• scissors• marking pen• dissecting needle

Procedure: 1. Put on safety goggles, gloves, and a lab apron.      2. Place a frog on a dissection tray. 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. Observe several frogs to see the difference between males and females.

3.  Use the diagram below to locate and identify the external features of the head. Find the mouth, external nares, tympani, eyes, and nictitating membranes.

4. Turn the frog on its back and pin down the legs. Cut the hinges of the mouth and open it wide. Use the diagram below to locate and identify the structures inside the mouth. Use a probe to help find each part: the vomerine teeth, the maxillary teeth, the internal nares, the tongue, the openings to the Eustachian tubes, the esophagus, the pharynx, and the slit-like glottis.

5. Look for the opening to the frog’s cloaca, located between the hind legs. Use forceps to lift the skin and use scissors to cut along the center of the body from the cloaca to the lip. Turn back the skin, cut toward the side at each leg, and pin the skin flat. The diagram above shows how to make these cuts

6. Lift and cut through the muscles and breast bone to open up the body cavity. If your frog is a female, the abdominal cavity may be filled with dark-colored eggs. If so, remove the eggs on one side so you can see the organs underlying them.

7. Use the diagram below to locate and identify the organs of the digestive system: esophagus, stomach, small intestine, large intestine, cloaca, liver, gallbladder, and pancreas.

 

8. Again refer to the diagram below to identify the parts of the circulatory and respiratory systems that are in the chest cavity. Find the left atrium, right atrium, and ventricle of the heart. Find an artery attached to the heart and another artery near the backbone. Find a vein near one of the shoulders. Find the two lungs.

9. Use a probe and scissors to lift and remove the intestines and liver. Use the diagram on the next page to identify the parts of the urinary and reproductive systems. Remove the peritoneal

membrane, which is connective tissue that lies on top of the red kidneys. Observe the yellow fat bodies that are attached to the kidneys. Find the ureters; the urinary bladder; the testes and sperm ducts in the male; and the ovaries, oviducts, and uteri in the female.

10. Remove the kidneys and look for threadlike spinal nerves that extend from the spinal cord. Dissect a thigh, and trace one nerve into a leg muscle. Note the size and texture of the leg muscles.

11. Dispose of your materials according to the directions from your teacher.

12. Clean up your work area and wash your hands before leaving the lab.

Frog Dissection Worksheet

1. What do you think is the function of the nictitating membrane, and why?

2. A frog does not chew its food. What do the positions of its teeth suggest about how the frog uses them?

3. Trace the path of food through the digestive tract.4. Trace the path of blood through the circulatory system,

starting at the right atrium.5. Trace the path of air through the respiratory system.6. Trace the paths of sperm in a male and eggs in a female.7. Trace the path of urine in both sexes.

8. Which parts of the frog’s nervous system can be observed in its abdominal cavity and hind leg?

9. Suppose in a living frog the spinal nerve extending to the leg muscle were cut. What ability would the frog lose? Why?

10. The abdominal cavity of a frog at the end of hibernation season would contain very small fat bodies or none at all. What is the function of the fat bodies?

11.  Structures of an animal’s body that fit it for its environment are adaptations. How do the frog’s powerful hind legs help it to fit into a life both in water and on land?

12. During one mating of frogs, the female lays some 2,000 to 3,000 eggs in water as the male sheds millions of sperm over them. How do these large numbers relate to the frog’s fitness for life in water?

Anatomy of a frog

ogs are amphibians, living both on land and in water. Their anatomy is very unique. Their bodies are similar to humans in that they have skin, bones, muscles, and organs. The body of a frog can be divided into a head, a short neck, and a trunk. The head contains the brain, mouth, eyes, ears and nose. The frog's head movement is limited due to the short, almost rigid neck. The trunk of a frog forms walls for a single body cavity known as the coelom. The coelom holds all of the frog's internal organs. Frogs have the same kinds of organs as humans and the same organ systems. For example, frogs have a long, sticky tongue which they use to capture food. They also have teeth, which unfortunately are very weak and rather useless. Humans have tongues and teeth as well (and a mouth of course).

If you closely examine the head of a frog, you will find the following: eye sockets, eyes, mouth, tongue, vomerine teeth, maxillary teeth, gullet teeth, external nostrils, internal nostrils, the glottis opening, eustachian tube openings, the tympanic membranes and the esophagus. The eyes, the mouth and the nostrils are all examples of a frog's external structures. In addition, a frog's external structures also include the webbed feet and the cloaca opening. The tympanic membranes or eardrums are exposed, but a frog does not have external ears. The internal structures of a frog include: the heart, the lungs, the kidneys, the stomach, the liver, the small intestine, the large intestine, the spleen, the pancreas, the gall bladder, the urinary bladder, the cloaca, the ureter, the oviducts, the testes, the ovaries and fat bodies. Again, the frog has organs that are similar to those of humans. For example, a frog has a brain, kidneys, lungs, eyes, a stomach, intestines and a heart. The one major difference between the anatomy of a frog and that of humans is that the anatomy of a frog is simpler than the anatomy of a man. Frogs don't have ribs or a diaphragm. Humans have both and a diaphragm (thoracic diaphragm) plays an important function in breathing and respiration. Breathing takes oxygen in and carbon dioxide out of the body.

Respiration is the process by which our cells are provided with oxygen for metabolism and carbon dioxide, which is produced as a waste gas, is removed.

A frog uses its tongue for grabbing prey. The vomarine and maxillary teeth are used for holding the prey. The internal nostrils are used by the frog for breathing. The tympanic membrane is the eardrum. It is located behind the frog's eyes. The eustachian tubes equalize the pressure in the frog's inner ear. The glottis is a tube, which leads to the lungs, while the esophagus is a tube which leads to the frog's stomach. The stomach helps the frog break down food and the liver also helps with digestion (it makes bile). Bile (also known as gall) is a fluid secreted by hepatocytes from the liver of most vertebrates (humans and frogs are vertebrates). Hepatocytes are cells present in the liver, and they initiate the formation and secretion of bile. In many species, bile is stored in the gall bladder between meals. When eating, the bile is discharged into the duodenum. Bile, therefore helps with digestion. The duodenum, which is the first and shortest part of the small intestine, is responsible for the breakdown of food in the small intestine. Most chemical digestion takes place in the duodenum. The small intestine absorbs nutrients from food. The large intestine absorbs water. It also collects waste. You can also think of the cloaca as storing waste, as this part of the frog collects eggs, sperm, urine and feces. The cloaca (opening) is also where sperm, eggs, urine, and feces exit the frog's body. The spleen stores blood, while the kidneys filter the blood. The ureters carry urine from the kidneys to the bladder. The (urinary) bladder stores urine. The testes make sperm, while the ovaries makes eggs and the eggs travel through the oviducts.

A frog's skin is always moist. It is made up of two layers, an outer epidermis and an inner dermis. In addition to protecting the frog, the skin also helps the frog breathe. A frog will take in oxygen from the water through their skin. The oxygen in the water passes through their skin and goes directly to their blood. Frogs also have a pair of lungs which allows them to breathe when on land. A frog has very few bones. They make up the skeleton of the frog. The skull (head bone) is large and flat. The legs are long for jumping. In addition to being specialized for jumping, the bones in their upper and hind legs are also specialized for leaping. The muscles move the skeleton of the frog. The muscles help the frog jump and swim.

Frogs have other systems similar to humans that are a part of their bodies. For example, a frog has a nervous system (all of the nerves and the spine), a circulatory system (the heart and blood going through the body) and a digestive system (the food going through the mouth to the intestines). The heart (which is part of the circulatory system) is three-chambered. There are two atria and one ventricle. There is a valve within the frog's heart known as the spiral valve. It directs the flow of blood to prevent oxygenated and de-oxygenated blood from mixing. A frog's sense of hearing (which is part of the nervous system) is highly developed. Frogs can detect high-pitched sounds with their ears. In addition to breathing oxygen through their skin when in water, frogs can also detect low-pitched sounds through

their skin. Another highly developed system is a frog's sense of sight and smell (which again are both part of the nervous system). Frogs detect predators and prey using their large eyes. Their eyes however have poorly developed eyelids, which do not close. In order to close its eye, a frog has to draw the eye into its socket. There is a third eyelid, known as the nictitating membrane. It can be drawn over the pulled in eye (eyeball). Frogs also use their sense of smell to detect chemical signals. These signals help them to identify potential food.

Now that we know the basics of frog anatomy, let's move onto the Activities and Extended Learning.

Activities

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Extended Learning

Write some of the things you have learned about frogs from the Study Guide. Your answer should be in short essay format, 3-5 paragraphs. As you write the essay, answer the following questions in complete sentences, using your own words:

1. How are frogs adapted for aquatic life?

2. How are frogs adapted for terrestrial life?

3. If frogs have lungs, why do they also breathe through their skin?

Get a book or search the web for the human body. See if you can find the similarities and differences between a frog's anatomy and a human's anatomy. You should be able find some of the information in the Study Guide. You can also use the links provided in the Resources.

How are frogs and humans alike?

How are frogs and humans different?

Diffrences and similarities of frog and a toad

Both the frog and the toad are amphibians that live in both land and water. The word amphibian comes from the Greek words amphi and bios, which mean "double life". They both have long life spans of thirty to forty years. The similarities between the two are many, right from the way their eggs look and hatch to the time taken for a tadpole to become a frog or a toad.

Most of us confuse frogs and toads because they look very similar. If only we cared to take a closer look, we can see there are some differences between the two.

A midwife toad carrying eggs

Picture of a leopard frog

A midwife toad carrying eggs

A Leopard Frog

Some differences

Common frogs are a light brownish green color, which helps them to stay camouflaged at the bottom of a muddy pond, or in a pile of damp leaves. Toads that live in temperate regions are usually brown and olive in color, while those in the tropics are brighter in color.

Frogs have smooth, moist skins, whilst toads are dry and warty-skinned. Secondly, frogs have long powerful hind legs, which they use for swimming and jumping. Toads, on the other hand, have shorter legs and can only crawl along.

A third difference, you might not be able to see easily is that while most frogs have teeth, most toads have none. Also toads lay lesser number of eggs than frogs. A toad lays between 4,000 and 12,000 eggs every year, while a female bullfrog may lay anywhere between 18,000 and 20,000 in a year.

A frog spawn (mass of eggs laid in still water) is quite easy to spot as it settles near the surface of the water in large round clumps, while a toad spawn is laid in long strings.

The two tadpoles:

As we have mentioned earlier, similarities between the two, which are perhaps more in number than dissimilarities, begin right at the egg laying stage (except, of course, the number of eggs laid). The eggs of both look like specks of dust floating on top of the water in a jelly-like substance. After one or two weeks, tadpoles begin to emerge from the jelly-like spawn, which they feed on during their first few days before moving on to nibble at algae with their rasping teeth. At this stage, the tadpoles look more like fish, with gills and a long swimming tail, than frog or toads. They breathe through the gills like a fish.

It takes around twelve to fourteen weeks for tadpoles to develop further and the process of change is known as 'metamorphosis’. At the end of this stage, the tadpoles drop their gills and tail and develop legs and lungs. The last stage, after which the tadpole becomes a frog or toad could take up to one year!