Frog Dissection
Background Information
Frogs are members of the class amphibia because they live both in and out of water. There are approximately 4,740 different species of frogs throughout the world, 90 of which are found in the U.S. They are found in the highest concentration in warmer tropical climates. Frogs eat insects along with some types of small animals, including earthworms and minnows.
The way frogs breathe can be considered unique. When they are tadpoles, they breathe through internal gills and their skin. Through development, they develop the lungs necessary to respiration on land, and their gills are lost. Like humans, frogs take in air through their nostrils with their mouths closed before exhaling with body contractions. Tiny blood vessels, capillaries, under the outer skin layers also help frogs breathe through their skin.
Frogs range widely in appearance and size. Some of the tiniest frogs are smaller than a dime, while the largest frogs can grow to be longer than a foot. A species called the “Glass Frog” has skin so translucent an observer can see its heart beating.
Major Internal and External Anatomy
External nares: anterior openings for the exit or entry of air
Eye: allows the frog to see
Nictitating membrane: a transparent part of a frog's lower eyelid that moves over the eye to clean it and protect it
Glottis: opening through which air enters the trachea
Eustachian tubes: openings in the mouth that lead to tubes that connect to the middle ear to equalize air pressure
Tongue: the muscular structure attached to the front of the mouth which is extended to catch insects
Pharynx: a tubelike structure that serves as the passageway for food, liquid, and air
Esophagus: a muscular tube that connects the mouth to the stomach
Maxillary teeth: sharp teeth in the maxilla of a frog's mouth that function in holding captured prey
Internal nares: openings for the intake of air
Vomerine teeth: small projections in the top of a frog's mouth that function in holding prey
Liver: secretes bile and processes digested food molecules
Fat bodies: masses of fat in the body cavities of frogs which are needed for hibernating and mating
Sperm ducts: site of sperm secretion in male reproductive system
Testes: male sex organs which produce sperm
Kidney: two long excretory organs that lie along the back; filter wastes from the blood and excrete them in the urine
Small intestine: the principal organ of digestion and absorption of digested food
Large intestine: posterior organ of the digestive system which stores undigested food
Pancreas: gland which secretes digestive enzymes into the next part of the digestive system
Spleen: organ through which the products of the frog's digestive and urogenital system pass when discharged from the body
Lung: receives air coming through the windpipe and serves as the site of oxygen absorption
Heart: the organ of the circulatory system which pumps blood throughout the frog
Stomach: stores food and mixes it with enzymes to begin digestion
Incision Guide
First, cut the hinges of the frog's mouth to open it wider and to see the back of the mouth as indicated by the purple portions below.
Cut along the medial orientation. Then, near the cranial end, cut from the medial line down under the frog's axilla. Repeat with the caudal end, cutting from the medial line above its hind legs. Pin each skin flap so it is out of the way and so the liver is exposed.
Pull the liver aside, revealing the stomach and the small intestine.
If not already exposed, cut deeper into the cranial end to pull the skin back to reveal the heart and the esophagus. The lungs are found visceral to the heart.
In order to locate the kidneys, pancreas, and large intestine, the stomach and the small intestine must be moved. Our frog was male, so there were sperm ducts and testes on the kidneys seen by moving the intestines. To do this, pick the small intestine up with tweezers and let the mesenteries rip from the intestines.
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