By Kelsey and Ellie


Scientific classification:
Kingdom: Animalia
Phylum: Chordata
Class: Amphibia
Order: Anura

General Description

The frog is an amphibian under the order Anura (Greek meaning “Tail-Less”). Adult frogs are characterized by long hind legs, a short body, webbed digits no tail and protruding eyes. Most frost have a semi-aquatic lifestyle but move freely on land by jumping or climbing. Because a frog is an amphibian, it is a cold blooded animal and has thin moist skin. Frogs at an early stage are called Tadpoles, are aquatic, and use gills for breathing. When a frog develops into an adult, it uses lungs. Frogs can take oxygen through lungs and through their skin. There are many different kinds of frogs found all over.

Where are they found?  Frogs can be found almost anywhere except the Antarctica, however, most species are found in tropical regions. You can find frogs in water or near places that have water like ponds and streams.
However, some frogs will never enter the water. They live mainly on land and go to the water only to mate. There are some kinds of frogs that live in trees. These frogs have tiny sticky pads on their fingers and toes to help them cling to the tree trunk as they climb.
Some frogs are burrowers. Burrower frogs live on land, have short hind legs, and cannot hop. Frogs that live in cold winter places, hibernate during this time. They either hibernate in burrows or buried in mud bottom of ponds.


Population? Frogs are the most diverse groups of vertebrates. There are more than 5,000 species. However, populations of certain frog species are significantly declining

Diet? Adult frogs follow a carnivorous diet of mostly of arthropods, annelids and gastropods. Frogs catch their food with their long, sticky tongue.

Protection?There are many different techniques used by frogs for their protection. Some frogs use Camouflage for protection. Most camouflaged frogs are nocturnal, which adds to their ability to hide. Camouflage helps to blend into the environment to hide from their enemies. An example of this is The Tree-frog. Some frogs have the ability to change color, but this is usually restricted to shades of one or two colors. Some will be very brightly colored body (especially red and yellow) to warn enemies that it taste bad or are poisonous, so stay away. Another form of protection is to poison potential predators; many frogs contain mild toxins that make them distasteful or harmful. Some frogs have poison glands in their skin; the skin will secret and cover the body with the poison and this will deter the enemy from eating them. Some will try to fool the enemy by puffing themselves up with air to make it look too big to be swallowed. An example is the Tomato frog. Some will give out a loud scream to startle the enemy to drop them. Some will urinate as they are jumping away to give off a bad taste as well as cover their scent. 

Life Cycle?There are four main stages of the frog's life cycle: egg, tadpole, metamorphosis and adult. The eggs will hatch within 3-25 days depending on the species and water temperature. If it is warmer, it will be faster. Colder temperature will slow down the hatching. Eggs vary in size, color and shape with different species. The eggs are usually covered with a jellylike substance, which acts as a protective coating. Even the jelly differs depending on species. The eggs hatch into tadpoles that swim in fresh water and breathe with gills. Tadpoles do not look like frogs yet; they have a tail and have external gills, which they breathe through. Tadpoles eat plants and decaying animal matter. As a tadpole grows, it develops legs and loses its tail. When this process is complete, the froglet breathes with lungs and lives on land. Not all eggs or tadpoles will make it to the adult stage. There are many threats like ducks, fish, insects and other water creatures that will eat the eggs. When they are tadpoles, they still face dangers like being eaten by larger water animals or die when the pond dries up.


back to top


The majority of frogs prefer moist regions. Frogs live in a variety of different habitats; ranging anywhere from tropic to sub-artic. Most frogs can be found in the tropical rainforest. Frogs live both an aquatic and terrestrial lifestyle; frogs can be found living in ponds, creeks and trees. It is in ponds or lakes where frogs lay their eggs and where Tadpoles develop.

back to top

General Anatomy


Feet and Legs: The structure of the feet and legs varies depending on the frog species. This usually is determined on whether they live primarily on the ground, in water, in trees, or in burrows. Frogs must be able to move quickly through their environment to catch prey and escape predators, and numerous adaptations help them do so. Many frogs, especially those that live in water, have webbed toes. Almost all frogs have long back legs and short front legs.

Skin: The frog’s skin is thin, smooth and loose. The skin of a frog lacks scales, hair, or other protective features. Glands in the skin secrete mucus to help keep the skin moist.

Coloring: They may vary in color. Most frogs are green or brown, but some have colorful markings.

Tongue: Most frogs have a sticky tongue attached to the front part of the mouth. Their tongue is highly specialized. It is flicked out to grasp at prey.

Body: Most frogs do not have a neck. Adult Frogs do not have a tail.

Head: Frogs generally have a flat and point head.

Eyes: They have bulging eyes, providing them with good depth perception, which helps them when capturing prey. Frogs have something called the Nictitating Membrane witch is the frog's third eyelid; this is used to cover frogs eyes when they are underwater.

Teeth: Frogs have very small teeth on their upper jaw. These are practically useless for chewing.

Ears: Although frogs have a sense of hearing, they have no external ear. Their ear canal is covered by round, flat skin similar to the head of a drum and is known as the tympanum.



Stomach: The Stomach is a large, firm, sac-like organ on the left side of the frog

Small Intestine: The small intestine is a long, folded, tube-like organ that is posterior to the stomach. It is similar in color to the stomach, but smaller in diameter.

Pancreases: The pancreas is a thin, yellowish ribbon. As you lift the small intestine you will see it between the small intestine and the stomach.

Lungs: The lungs are at the anterior end of the body cavity on either side of the heart. In a preserved frog, they are small, reddish-pink organs.

Liver: The liver is a large, brownish colored organ covering most of the body cavity.

Heart: The heart is a small triangular shaped organ between the front legs and anterior to the liver.

Gall Bladder: When you raise the liver up you will see a small, greenish sac underneath it. This is the gall bladder.

Male Testes: They are tan-colored, bean-shaped organs near the anterior end of each kidney.

Male Spleen: It is a small, round reddish organ

Male Kidney: They are elongated, brownish-colored organs found in the lower part of the frog's abdomen. They are situated on either side of the middle of the frog.

Female Ovaries: They are dark organs which may fill most of the frog's body cavity, depending on the time of year that the frog was collected.

Female Oviducts: They are yellowish, coiled tubes near the back surface of the ovaries. They are found on either side of the body cavity.

Female Spleen: It is a small, round reddish organ

Female Kidney: They are elongated, brownish-colored organs found in the lower part of the frog's abdomen. They are situated on either side of the middle of the frog.


back to top

Reproductive System

The frog is an organism that reproduces by external fertilization. External Fertilization is when the egg is fertilized outside the mother’s body. External fertilization is more common among animals that breathe in the water. There are hazards for any animal who reproduces externally: (1.) it is harder for the egg and sperm to meet; (2) eggs are not protected by their mother, there are many external threats to the egg; eggs are more vulnerable to their environment and other predators, in addition the mother can not give any extra nourishment for the egg. To overcome such hazards Frogs have special behaviors and strategies to increase chances of successful fertilization. Frogs use a mating behavior called amplexus where the female alerts the male when it is best to release sperm. Male and Female frogs also send out millions of sperm and eggs. This insures that the eggs and sperm are released at the same time and place to increase chances of fertilization. Female Frogs lay millions of eggs so that enough will survive. Frogs are Oviparous meaning they lay eggs; fertilized eggs develop and hatch outside body. Inside every egg, there is a yolk sac to provide nourishment.

Female Frogs: Female frogs have two ovaries. The ovary is the reproductive organ of the female that produces eggs along with female sex hormones. The oviduct is a long whitish tubs where the egg travels down to reach the cloacae. The Oviduct is similar to the fallopian tube found in female humans. Female frogs also have fat bodies witch is a cluster of fatty tissues that provides nutrients for the gametes. Female frogs have a uterus for storing eggs. Eggs leave the frog from the cloacae.

Male Frogs:Male frogs have testes witch is the male reproductive organ that produces sperm. Found in the testes; the exocrine is where sperm is produced. Male Frogs have sperm ducts to bring sperm out and carry sperm to the cloaca. Sperm travels through the urethra and leaves the frog's body cavity through the cloaca.





Back to top

Circulatory System


The Circulatory system of a frog consists of the heart, blood vessels, and blood. Frogs have a closed double loop circulatory system. A closed circulatory system means that the frog’s blood is contained with in blood vessels and the heart. The double loop system means that the blood goes back to the heart two times before the blood goes through the whole system of the frog’s body. There are three chambers consisting of one ventricle and two atria. The ventricle pumps blood through a single artery (truncus arteriosus) witch distributes blood thought the body. Oxygenated blood coming back to the heart from lungs enters the heart into different atria than the de-oxygenated blood does coming from the body capillaries. The valve helps to keep blood from only mixing as minimally as possible. The valve directs the oxygenated blood to the aorta, which will leave heart and go to the body capillaries and the deoxygenated blood out the pulmonary vein and to the lungs. Oxygenated and non-oxygenated bloods still mix, because Frogs lack a septum that would separate blood. The Frog's circulatory system works is because frogs do not need as much oxygen rich blood in their body because they are getting oxygen from two sources, their lungs, and through their skin. Both oxygen and carbon dioxide are able to transfer through the skin of frogs. Another reason why their three chambered heart works for frogs is because frogs are ectoderm's which means the maintain their body temperature externally and this requires a lot less oxygen.


back to top

Dissection Images

frog The frog before dissection


Cutting open the frog


openfrogiea Peeling back the skin of the frog


openedfrogieb The opened frog


Froglivera The frog's liver


froghearta The frog's heart


oviduct The frog's oviduct

back to top




Quiz 1:


Quiz 2:


back to top

Study Guide and Diagrams

Life Cycle
  • what are threats to eggs and tadpoles
  • 1) eggs
    • large amount laid
    • in water
    • how long till it will hatch
    • how eggs look
  • 2) tadpoles
    • aquatic - freshwater
    • gills
    • tail
    • diet
  • 3) frogs
    • lungs
    • terrestrial
    • change of features

Where are Frogs Found?



External Anatomy

Internal Anatomy

Reproductive System
  • Oviparous
  • External Fertilization
  • Hazards with external fertilization
  • adaptations/behaviors

Female Frog

(know parts\functions)

  • Ovaries
  • Uterus
  • Oviduct
  • Fat bodies
  • Cloacae

Male Frog

(know parts\functions)

  • Testes
  • Exocrine
  • Urethra
  • Cloacae
Circulatory System
  • closed double loop
  • three chamber
  • o2 rich and o2 poor mixed
  • frog adaptations for mixed blood
    • oxygen sources
      • lungs
      • skin
    • ectodermic


back to top





“ANATOMY OF THE FROG." Britannica Student Encyclopedia. 2007. Encyclopedia Britannica Online. 5 May 2007  



Canelli, Robert. “The Circulatory System”. Yale-New Haven Teachers Institute. 2007. 4 May 2007.



Farabee, Michael J. “The Circulatory System ”. On-Line Biology Book. 2007. 6 May 2007.



“Frogs”. Queensland Government. 03 January 2007. The State of Queensland (Environmental Protection Agency) 4 May 2007.



"Frog." Wikipedia, The Free Encyclopedia. 1 May 2007, 10:18 UTC. Wikimedia Foundation, Inc. 4 May 2007



“Reproductive System”. Marymount School Middle School Science and Technology Department. May 4 2007..



Ritchison, Gary. “Lecture Notes 9 -Circulatory System”. Bio 432 Comparative Verbrate Anatomy. 6 May 2007.