External Anatomy Dissection Guide

Transcription

1 External Anatomy Dissection Guide Dissection is the cutting of a dead animal or a plant into separate parts for the purpose of careful and detailed examination and study. The external anatomy is as important as the internal anatomy in learning about the organism. Follow the reading guide for the external anatomy as you begin learning about the frog s anatomy. This will include both the structure and the function of each part. Dissection Background Information Countershading is a type of camouflage used by frogs, salamander and turtles. Species with countershading have lighter colors on their stomachs and darker colors on their backs. The difference in color on their bodies helps them hide from predators on land, birds in the air, and fish and other aquatic predators. With more than 5,400 species of frogs worldwide, there are variations among structures and functions of their limbs including digging, swimming, and climbing, etc. The frog species being dissected is an aquatic frog. Thus, the forelimbs are used for movement and their hind legs are used jumping. Their webbed feet on their hind legs are used for swimming. Frog eyes come in a stunning array of colors and patterns. Most frogs see well only at a distance, but many have excellent night vision and are very sensitive to movement. An important adaptation for catching prey. The bulging eyes of frogs allow them to see in the front, sides, and partially behind them. Another amazing adaptation of frog eyes is to help them swallow! When a frog swallows, it pulls its eyes down into the roof of its mouth. The eyes bulge into the soft palate helping push the food down the frog s throat. Many species of fish, amphibians, reptiles, birds and mammals have third eyelids. This translucent or transparent membrane, known as the nictitating membrane, has various functions. The membrane extends from the frogs lower lid. It is composed of epithelial tissue similar to the frog s skin and contains blood vessels, nerves and the connective tissue collagen. The frog can raise this third eye for extended periods of time. The benefits of having this third eyelid include protection and maintaining moisture in the eyes. Humans have a vestigial nictitating membrane apparent in the corner of the eye. Amphibians have many of the same hearing structures as humans, including the tympanic membrane, or eardrum. When sound waves strike the membrane, the membrane vibrates eventually stimulating hair cells in the inner ear. These hair cells transfer the sound waves to the auditory nerve and send a signal to the brain. The membrane is large in male frogs than in female frogs. Anatomy of the Frogs Mouth Many important structures are present in the frog s mouth. The first structure to observe is the tongue. Unlike humans, frog s tongues attach to the front of their mouth instead of the back of the throat. Frog tongues are fairly large, attached to a series of strong muscles, and sticky. All which help the frog eat. Frogs don t usually actively look for food, but find a warm, moist place to hang out. When food flies by they catch the prey with their sticky, long tongues. Once the prey is caught the frog moves the prey to the back of its throat toward the gullet, which is the opening to a short esophagus.

2 Near the angles of the jaw are two openings, one on each side. These structures are known as Eustachian tubes. The Eustachian tube is a narrow passage leading from the mouth of the frog to the tympanic membrane. The Eustachian tube functions as a pressure equalizing valve. It is the middle ear in humans that causes discomfort during air travel because the air pocket inside the Eustachian tubes is vulnerable to changes in air pressure. Normally, each time you swallow, your ears make a little click or popping sound. The Eustachian tubes, although, necessary can also cause some problems. Ear infections are the most common reason parents bring their child to a doctor. This is due to the fact that Eustachian tubes are smaller and more level in children, allowing easier passage of bacteria from the throat to the middle ear. Caudal to the esophagus is a slit-like opening. This slit is the glottis and opens into the lungs. The glottis provides air to the lungs. The glottis is surrounded by two structures on either side called vocal cords. The lungs, the glottis, and the vocal cords allow the frog to make noise. Frogs have two sets of teeth. Vomerine teeth are found on the roof of the mouth between the internal nares and are important in preventing the escape of prey. The maxillary teeth are found around the edge of the mouth and are used for holding and gripping prey tightly. Nares are another word for nostrils. Frogs have two types of nares: external and internal. These two types of nostrils work together to provide oxygen to frogs when they re not in the water. In addition to nares, frogs can also breathe through their skin when submerged in water.

3 Internal Anatomy 1. Place the frog ventral side up 2. Cut along the midline of the body from the anus to the pectoral girdle 3. Make transverse (horizontal) cuts near the arms and the legs to help hold open the body cavity. 4. Pin the flaps of the body back. *If your specimen is a female, the body may be filled with eggs and an enlarged ovary. Remove these eggs to view the organs. Digestive, Circulatory and Reproductive Fat Bodies --Spaghetti shaped structures that have a bright orange or yellow color, if you have a particularly fat frog, these fat bodies may need to be removed to see the other structures. Peritoneum A spider web like membrane that covers many of the organs, you may have to carefully pick it off to get a clear view Liver--The largest structure of the body cavity. This brown colored organ is composed of three parts, or lobes. The right lobe, the left anterior lobe, and the left posterior lobe. The liver is not primarily an organ of digestion; it does secrete a digestive juice called bile. Just like in humans, bile is needed for the proper digestion of fats. Bile is emptied into the gall bladder, which then empties into the duodenum. Heart - at the top of the liver, the heart is a triangular structure. The left and right atrium can be found at the top of the heart. A single ventricle located at the bottom of the heart. The large vessel that extends out from the heart is the conus arteriosus which supplies blood to the body. Lungs - Locate the lungs by looking underneath and behind the heart and liver. They are two spongy organs. Lungs attach to the trachea via tubes called bronchi. Gall bladder--lift the lobes of the liver, there will be a small green sac under the liver. This is the gall bladder, which stores bile. (Hint: it kind of looks like a booger.) The gall bladder stores bile and then releases it into the duodenum via the bile duct. The bile duct may be too small to see. Stomach--Curving from underneath the liver is the stomach. The stomach is the first major site of chemical digestion. Frogs swallow their meals whole. Follow the stomach to where it turns into the small intestine. The pyloric sphincter valve regulates the exit of food from the stomach Pancreas This glandular organ is located within the curve of the stomach. On preserved frogs it may not be easy to find, as the gland breaks down. It secretes insulin, which is needed for the proper breakdown of sugar. Small Intestine--Leading from the stomach. The first straight portion of the small intestine is called the duodenum, the curled portion is the ileum. A membrane called the mesentery holds the ileum together. Note the blood vessels running through the mesentery; they will carry absorbed nutrients away from the intestine. Absorption of digested nutrients occurs in the small intestine. Large Intestine--As you follow the small intestine down, it will widen into the large intestine. The large intestine is also known as the cloaca in the frog. The cloaca is the last stop before wastes, sperm, or urine exit the frog's body. (The word "cloaca" means sewer.) Locate the anus. Spleen--Return to the folds of the mesentery, this dark red spherical object serves as a holding area for blood, where harmful particles can be filtered out for the immune system. Esophagus--Return to the stomach and follow it upward, where it gets smaller is the beginning of the esophagus. The esophagus is the tube that leads from the frog s mouth to the stomach. Open the frog s mouth and find the esophagus, poke your probe into it and see where it leads. STOP! If you have not located each of the organs above, do not continue!

4 Removal of the Stomach & Intestine Cut the stomach out of the frog and open it up. You may find what remains of the frog's last meal in there. Look at the texture of the stomach on the inside. Note the ridges on the walls of the stomach called rugae. Rugae help to break down food. What did you find in the stomach? Measuring the Small intestine: Remove the small intestine and stretch it out and measure it. Now measure your frog. Record the measurements below in centimeters. Frog length: cm Intestine length cm Which is longer? Removal of the Heart Carefully cut out the heart from its position above the liver. The vessel on the front of the heart is the conus arteriosis, which sends blood to the body. On the back you can find the openings for the anterior and posterior vena cava, which return blood to the heart. How many chambers does the frog heart have? Urogenital System-The frog s reproductive and excretory system is combined into one system called the urogenital system. You will need to know the structures for both the male and female frog, Kidneys flattened bean shaped organs located at the lower back of the frog, near the spine. They are often a dark color. The kidneys filter wastes from the blood. Often fat bodies are attached to the kidney. Testes in male frogs, these organs are located at the top of the kidneys, they are pale colored and round. Oviducts females do not have testes, though you may see a curly-q type structure around the outside of the kidney, these are the oviducts. Oviducts are where eggs are produced. Males can have structures that look similar, but serve no actual purpose. In males, they are called vestigial oviducts. Bladder An empty sac located at the lowest part of the body cavity. The bladder stores urine. Cloaca mentioned again as part of the urogenital system urine, sperm and eggs exit here.

5 Name: Period Post Lab Questions in your journal 1. The membrane holds the coils of the small intestine together: 2. This organ is found under the liver, it stores bile: 3. Name the 3 lobes of the liver:,, 4. The organ that is the first major site of chemical digestion: 5. Eggs, sperm, urine and wastes all empty into this structure, the sewer : 6. The small intestine leads to the: 7. Blood leaves the heart through what large vessel? 8. Yellowish structures that serve as an energy reserve: 9. The first part of the small intestine (straight part): 10. This pair of organs filters wastes from the blood: 11. A spiderweb like membrane that covers the organs: 12. Regulates the exit of partially digested food from the stomach: 13. The large intestine (cloaca) leads to the (the opening to the outside ) 14. Organ found within the mesentery that stores blood: 15. The largest organ in the body cavity: 16. Holding area for blood and also has a function in the immune system: 17. The esophagus leads to the, the glottis leads to the 18. Bile moves from the gall bladder to the duodenum through the duct. 19. The organ located near the stomach that makes insulin: 20. What structure is found above the kidneys but only in male frogs? A. B. C. D. E. F. G. H. I. J. K. L. M. N. P.

6 Genitourinary System Describe the function of the following Urinary Bladder - Kidneys- Ovaries Oviducts- Testes- Fat bodies -