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The word amphibian comes from the Greek word amphibiaon, which means both water and land.

Amphibians are born as newts (tadpoles) with gills, just like fish, so they can get their oxygen from the water.

They metamorphose (change) into lung breathing adults who get their oxygen from the air.

That's why amphibians were named, "both water and land."

Frogs roamed with the dinosaurs during the Jurassic Period, which was almost 200 million years ago. Evolutionary biologists Neil Shubin and Farish Jenkins
located the first evidence of a Jurassic frog in the Navajo Nation near Flagstaff, Arizona in 1982.

Humans are warm-blooded, which means that our body temperature stays pretty much the same, around 98.6 degrees. Amphibians are cold-blooded, so their
body temperature is dependent upon their surroundings.

When it is warm outside in temperate zones (where there are four seasons), frogs warm up and move around a lot. When it is cold, they move very little.
In places where it gets really cold, frogs don't move or eat until it the weather warms up. They go into a sleep-like state, called hibernation.

Desert frogs, are just the opposite. They go into hibernation by burying themselves in mud before it gets too hot. When the rainy season arrives, the mud
softens and the frogs come out of their deep sleep.

Have you ever felt a frog? Its skin feels pretty slimy... That's because membranes secrete mucus onto the skin to keep it moist. The mucus also blocks out
dirt and germs, but allows water and air to absorb through the skin. As a matter of fact, frogs drink most of their water through their skin.

Slime is one way that you can tell the difference between frogs and toads. The frog's cousin, the toad, has warty (dry and rough) skin.

Notice how a frog's back is dark and its belly is light? This type of coloring is called countershading, and it helps frogs hide from their predators because it
blends in with the environment. On land, countershading prevents the frog from casting shadows. When a frog swims, it is hard to see it because its dark
back blends in with the surface of the water. It is also hard to see from underneath the frog because its light belly blends in with the sky.

Guess how often a frog rebuilds its countershading. Approximately 48 times! This is because frogs shed their skin. Just like a snake (or someone wearing a
really tight sweater!), they wriggle their bodies and pull it over their head. Most frogs do this almost every week. Then they eat it!

The muscle is an organ also known as body tissue. The muscles control body movement by contracting (tightening and loosening). There are three types of
muscles: voluntary (striated), involuntary (smooth), and heart (cardiac) muscles.

Voluntary muscles, such as arm and leg muscles, are the types of muscles that we can control. They are referred to as striated, which means striped,
because there are stripes on the muscle fibers.

Guess how far a frog can jump. Up to twenty times its length! When the frog's back legs are stretched out, they are longer than its body. Frog's legs are also
the strongest muscles in its body. This strength and length helps the frog jump far and swim fast.

Frogs can also swim fast because they have webbed feet (skin growing between the toes). Just like humans, they have five toes on their back legs. Frogs
don't have thumbs though... They only have four toes on their front feet!

Even though the frog has many of the same bones as other vertebrates, its belly feels soft. This is because frogs do not have any rib bones. It is also why
the frog often likes to drink through the skin on its belly.

A frog's ears feel smooth and flat. That's because frogs don't have outer ears like we do. The frog's ears, called tympanum, are located just behind the frog's
eyes.

Each species (type) of frog has slightly different size ears and a slightly different placement on their heads so they can recognize the sounds of their own
kind.

Have you noticed how a frog's eyes bulge? This allows them to see in front, above, to the sides and behind, making it difficult for predators to sneak an
attack. That is, unless the predator sits still. Frogs cannot see stationary objects. They can only focus on things that move.

What is even more amazing, is that a frog can pull its eyes into its head and through its mouth. Why? ...Because the eyes help the frog push big food items
down through its throat.

When frogs hibernate in really cold climates, they often burrow under leaves or in the mud and breathe through their skin. It can still get close to freezing,
and the liver helps protect the frog by using enzymes to convert glycogen to sugary glucose. This sugary substance works just like automobile anti-freeze;
it lowers the freezing point of water. The liver sends the glucose to all of the frog's organs through the bloodstream. This means that the frog's organs won't
freeze.

The liver is a housekeeper for frog and human bodies: It extracts impurities (toxins and wastes) from the blood so they can be eliminated from the body.
It also converts food into nutrients for the body by creating Bile, an important chemical that digests food.

The flow of blood to the tissues in your body is called the circulatory system. It provides the oxygen and nutrients that tissues need to function. The
circulatory system's main components are veins, arteries, capillaries, and the heart. In a frog, the blood goes to the lungs and skin to replenish its oxygen
(In a human, it's just the lungs). This is a "closed system" because the blood stays inside the system's components.

Just like the human heart, a frog heart has valves and arteries that move blood to and from chambers and organs. Arteries carry blood away from the heart;
veins return blood to the heart. The human heart has four chambers, and the frog has three, a left and right atrium and a ventrium.

Frogs swallow their food whole so they don't chew it. The frog uses its small row of front teeth for only one thing: capturing and holding its prey until it is ready to swallow.

Have you ever seen a frog snatch a fly with its tongue? It looks really long. This is because, unlike humans, most frogs have a sticky tongue that is attached to the front of their mouth.

Did you know that some frogs do not have a tongue? Found in Panama, Africa and northern South America, tongueless frogs are aqueous (they live in the
water). They grab prey with their hands and shove it into their mouth. Some aquatic frogs live quite comfortably in the United States. The African Claw Frog,
for instance, was introduced to California in the 1960's.

Frogs have a digestive system just like other vertebrates and the Bile created in the liver is an important part of this system. Bile is a liquid so powerful that
it breaks down oils and fats in food so the intestines can absorb nutrients and vitamins. The storage tank for bile is the gall bladder.

The stomach is also part of the digestive system. It is a storage sac for food and can stretch to accommodate how much you eat. The stomach begins
digestion by breaking food into smaller particles. Stomach enzymes and acids mix the particles with liquid into what is called, chyme. Then it moves it into
the small intestines.

Stomach acid is so strong that it can eat a hole through a solid wood table. Can you guess why it doesn't eat a hole in the stomach? That's because cells in
the stomach lining ooze mucus that neutralizes the acid upon contact. The small intestine is the primary organ that absorbs nutrients into the blood stream.
It has three parts, the duodenum, jejunum, and the ileum. Each help to absorb nutrients from the chyme that was prepared in the stomach. The duodenum,
is attached to the stomach. It is like a blender making a nutrient shake. Chyme from the stomach mixes with bile and enzymes from the liver, pancreas,
gall bladder and the intestine itself. Next, is the jejunum. It pushes the nutrient shake down to the ileum, where most of the nutrients are absorbed through
the intestinal walls before the food is passed to the large intestine for disposal.

A frog gets a little air into its nose, but not much. Most of it goes through its skin where many blood vessels absorb the oxygen into the blood stream. Some
animals have skin that is so thin, that gases like oxygen can pass through it--this process is called cutaneous gas exchange. This is why the frog can
breathe through its skin as well as its lungs. It gets 80% of its oxygen this way, but it only works if the frog's skin is moist.

When a frog hibernates, it needs so little oxygen that it just breathes through its skin.

The frog gulps air by moving its throat up and down. Just like a human, the lungs are two sacs that inflate like a balloon. Lots of blood vessels located
on the inside of the lungs absorb oxygen into the blood stream.

The pancreas is a gland next to the stomach. It is important for digestion because it produces digestive enzymes that break down food in the small intestine's
duodenum. Cells inside the pancreas also produce proteins, including insulin and glucagen which regulate blood sugar. - Insulin is secreted into fat cells
that pull glucose from the blood when blood sugar levels are high and need to be lowered. - When blood sugar levels are low (like between meals), the
pancreas secretes Glucagen into the liver. There, it becomes glucose and passes into the blood stream, raising blood sugar. - Glucose is how we get the
calories we need for energy.

Frogs have no sex organs outside the body. Both have an opening called a cloaca, which allows passage of wastes, sperm and eggs.

You can usually identify male frogs because they croak. They do this to attract females and scare away other males.

You can't tell the difference between males and females by looking at them unless it is breeding season when the male gets dark spots and the female gets
rough skin.

The spleen is a filter and a storage tank for blood. It works as a filter by destroying worn-out red blood cells and platelets. It also produces lymphatic fluid,
a clear liquid containing white blood cells, which passes through tissues to wash impurities out of them. The spleen stores extra red blood cells and 20 to
30 % of all blood cells.

Despite all of its important functions, the frog and humans can survive without a spleen.

The kidneys are also a blood filter. They filter blood by helping the body remove water, acid, and nutrients that it doesn't need and reabsorbing them when it
doesn't have enough. Another important kidney function is regulating blood pressure by removing excess salt from the blood stream.

So how do the kidneys remove excess water and nutrients? They create urine by filtering the extra fluids out of blood. Urine empties into the bladder where
the body can remove it through the archinephric ducts and the cloaca.

The difference between frog and human kidneys is the frog's ability to reabsorb water and nutrients from the bladder when it has been on land for a while.

The kidneys must filter a lot of blood to create urine. Did you know that, in a human, it takes 1000 liters of blood to create one liter of urine?

You can always tell when frog mating season starts because a chorus of males will make large croaking sounds in their throats. Most male frogs start
courting females in the pond when they come out of hibernation in the spring. The sound of the frogs is so loud, that it attracts females to the breeding site.

The frog males will grab the first females to arrive. They have nuptial pads, rough areas on the thumb, to aid in mounting and holding the female. In just a
few seconds, the female releases her eggs and the male deposits sperm to fertilize them. If a male accidentally grabs another male, it will signal by making
a loud grunt so the frog will let go. The female makes a similar sound after she has spawned her eggs. It signals the mating frog to release her.

The two testes of the frog produce the sperm, and it travels through the same duct as wastes. It exits the body through the cloaca.

The female's ovaries are very large organs that can fill most of the frog's body when she is ovulating (producing eggs). She can lay eggs in less than five
seconds by squeezing her belly with her front feet and pushing them through the oviducts and out the cloaca.

Depending on the frog species, a female will deposit between just a few to thousands of eggs. Very few will survive to adulthood.

When an egg is first deposited in the water, it is tiny, only 2 or 3 millimeters. It stays this way just long enough for the male's sperm to fertilize the egg and
then jelly around its surface swells. This creates a protective layer around the egg so water cannot get inside.

A tadpole forms in just a couple of weeks. Its first meal is close at hand; it's the jelly around the egg, which it dissolves with a secretion. The tadpole lives like
a fish during its first five weeks. It forms gills, eyes and a mouth and lives on algae. At five weeks, it loses its gills and develops hind legs and lungs. Once
this happens, the tadpole has to breathe air. At around ten weeks, the tadpole becomes a froglet, developing fore legs and losing its tail. At fourteen weeks,
its tail is gone and it hangs out on land, eating all of the things that frogs love such as insects, worms and slugs. This whole process is called,
metamorphosis.

According to the latest research from University of California Berkeley, over 88% of known amphibians in the world are frogs or toads. The latest number
identified was 4,738. The most common frog is the rain frog.

Frogs are important to the environment because they eat a lot, even millions, of insects and other small invertebrates. This helps keeps the populations of
pests balanced in nature.

Many scientists believe that declining frog populations are indications of unhealthy environments. Some theories include pollutants that cause unhealthy
fungi to grow on amphibian skin and changing temperatures, which affect frog metamorphosis.

Makoto Asashima is a biologist at Tokyo University in Japan. He led a team of researchers who figured out how to grow frog kidneys, eyes and ears from
frog embryo (fertilized egg) stem cells . They have been experimenting with transplanting the kidneys and hope they can learn to clone and transplant
organs in human beings one day.
     
 
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