Biology Dissection THE FROG

Transcription

1 Biology Dissection THE FROG Name Note: The organism you are cutting up today was once alive and vibrant, a living being. Dissection of a complex organism is not a right, but a privilege. Please be aware that this privilege is one that many students do not have.

2 Instructions: Read through the packet carefully while performing the lab. Make sure you follow the directions and locate each underlined structure before moving on to the next step. You are responsible for being able to locate and identify each of the underlined terms. You should also be able to describe their function. Part I External Anatomy Skin. The frog s skin is often described as slimy. This is because the frog s skin is also a respiratory surface and is covered with mucus which can absorb oxygen and water, so the frog does not need to drink. The mucus also prevents the frog from drying out when its out of the water and makes it slippery helping it escape predators. Eye. The frog s eye is covered by a special third membrane called the nictitating membrane which protects the eye from water when the frog is submerged and keeps it moistened when out of water. Nostrils. Called external nares, these lead directly to the mouth and give the frog its excellent sense of smell Ear. Frogs also have an ear, which is really a membrane structure which detects changes in air pressure, similar to the human ear drum. The membrane is called the tympanum. Tongue. Carefully open the frog s mouth and locate the tongue. Notice that the tongue is attached at the front of the mouth not the rear of the mouth as in humans. Why might this be the case? Legs. Notice the size of the frog s legs. These powerful legs allow the frog to jump up to six feet, the equivalent of human jumping a distance of 30 yards! Locate the cloaca, an orifice where the legs meet. Feces, urine, eggs AND sperm all leave the through this orifice, so it s difficult to determine the sex of the frog from the outside! However, a large thumb pad on the hand of the forelimb usually indicates a male. Before proceeding, record the following information, to be used in your lab report: Based on your examination of the frog s external anatomy, identify three features that you could use to recognize an amphibian. Answer only based on the dissection and your prior knowledge do not do research on the answer. One clue: other amphibians include salamanders and toads; lizards and snakes, by contrast, are reptiles. What can you tell about this animal s lifestyle? Where does it live? What sort of things does it eat? How is it adapted for its niche in the environment? Part II Internal anatomy Part A Digestive System 1. Place the frog dorsal side down and insert pins into its feet to keep the frog in place. Then make incisions with sharp scissors as seen in the picture below. Be careful not to cut to deep you may tear and damage the internal organs! The two posterior lateral (across the body) cuts need to be made long enough so that the pieces of skin and muscle fold easily aside and out of the way. Pin them down.

3 2. The fist major organ that you ll notice is the multi lobed liver. The liver will be darker in color and have three lobes: the right lobe, the left anterior lobe, and the left posterior lobe. The liver produces glycogen and a fat dissolving substance called bile. The bile is stored in the gall bladder, a small, thin sack located beneath the liver. Locate it. 3. Locate the largest of the digestive organs, the stomach. It has a half moon shape and is usually found on the left (frog s left!) side of the body cavity. The stomach is continuous with the small intestine (named for its diameter, not length!) The small intestine is coiled up and is connected with the large intestine, where the final stages of digestion occur. Waste is excreted through the cloaca, which you located earlier. 4. The small pancreas (you may not be able to see it) attached to the curve of the stomach produces acid neutralizing substances and these substances are delivered to the stomach from the gall bladder along with bile via the bile duct. 5. Locate the fingery looking structures called fat bodies, which are usually underneath the stomach. These are made out of fat cells (adipose cells) and are used to store energy. 6. Digestion really begins with at mouth. Open it and observe the tongue. Frogs do not have teeth, so food is swallowed whole! Food enters the esophagus, a tube that is continuous with the stomach. Food is moved down the esophagus by a process called peristalsis, which is coordinated muscular contraction of muscles in the esophagus. 7. Food enters the stomach and then is stored briefly in the pylorus, which looks like a lump on the stomach. The food then leaves the stomach and enters the small intestine through the duodenum. Nutrients are absorbed from the food by the small intestine. The leftovers enter the large intestine, which is shorter and fatter, where water is absorbed from the food. Waste then exits via the cloaca. 8. Locate the dark red spleen in the tissues of the mesentery, the membranous tissue that connects the segments of the small intestine. The spleen serves as a holding area for the blood.

4 9. Carefully remove the liver, stomach, and intestines IF NECESSARY; Do not to damage other organs or cut important blood vessels! Before proceeding, record the following observations to be used in your lab report: What does the frog s digestive system tell you about what it eats? What role do you think the gall bladder and pancreas play in digestion? If you could not look up the answer in a reference book, how could you find out? If you have not located each of the organs above, do not continue to the next section!

5 Part II Internal anatomy Part B The Heart and Circulatory System (Use numbered picture below) 1. The circulatory system is comprised of the heart, blood vessels, and blood. First, locate the two small pink lungs, which are on either side of the heart. Carefully remove one without damaging surrounding tissue and blood vessels. Use a dropper to inflate the lung with air (you might not be able to do this). 2. Locate the heart. The heart is a complex organ made out of one of the three classes of muscle: cardiac muscle (the others are: smooth muscle, which makes up the tubing, like blood vessels and intestines; skeletal muscle, which controls locomotion). The heart has three chambers: the right and left atria, and the ventricle(7). Locate the apex (point) of the heart, which is part of the ventricle. (FYI the mammalian heart has FOUR chambers 2 atria and 2 ventricles) 3. The right atria (5) receive oxygen poor blood via a vein (6) (veins bring blood TO the heart; arteries take blood away from the heart!). The left atrium (4) receives oxygenated blood from the lungs via a pulmonary vein(3). 4. Locate the two branches of the aorta (1, 2) (an artery), which transports blood through the body cavity and gives off many branches. Also locate the huge ventral abdominal vein (not numbered). 5. Note how the frog circulatory system has 2 loops, the Pulmocutaneous (pulmo lungs, cutaneous skin) and the Systemic loops. Heart ventral side

6 If you have not located each of the organs above, do not continue to the next section! Part II Internal anatomy Part C The Urogenital (Reproductive) System Female 1. You will have noticed immediately if you have a female frog. Locate the ovaries (a thin membrane) which contain hundreds of darkly colored eggs. 2. The eggs move through the oviducts (spaghetti-like tubes) before exiting through the cloaca. Male 1. If your frog lacks these large egg bodies look carefully for two yellow small bean shaped organs. These are the testes, where the sperm is produced. 2. The sperm pass through the vas efferentia to the kidneys and ultimately out through the cloaca. 3. (If your frog is a female, remove the eggs carefully and) locate the two dark kidneys, which are usually surrounded by blood vessels. The kidneys filter the blood of excess salts and other chemicals and send that waste via the ureters to the thin, sac-like bladder. 4. Locate the adrenal glands, which are small structures attached to or nearby the kidneys. The adrenal glands produce and release hormones that can provide the frog with a quick burst of energy in an emergency. 5. Like all organs, the kidneys are connected to the circulatory system by blood vessels. Locate the renal artery (which brings blood from the heart) and the renal vein, which takes blood back to the heart (via the lungs).

7 If you have not located each of the organs above, do not continue to the next section! If you have a female frog, be sure to look at a male, and vice versa!! Before proceeding, add the following observations to your notes, to be used in your lab report: Were you able to trace the pathway of the blood in the circulatory system? If not, what made it difficult? In what way is the real frog different from a diagram? Are all frogs the same? How do frogs reproduce? In what ways is their reproductive behavior evident from their anatomy? What does it say about their habitat?

8 Internal Anatomy continued Part D The Muscular and Skeletal Systems Introduction. An organism s muscles serve two main functions: (1) Muscles are attached to bones by tendons which pull on the bones during contraction and thus cause the bone to move (articulate), and (2) the muscles also give protection to the organs. Ligaments are not muscle tissue, but give support and stability to joints. Muscles work in antagonistic pairs. This means that when one muscle contracts, the opposite muscle of the pair relaxes to allow the resulting articulation (relative movement of limbs). Without this process, movement would be impossible. 1. Carefully remove the skin from one leg of your frog without damaging the muscles beneath. Use scissors for the task. Make sure you remove the skin all the way down to the frog s foot. 2. Look at the frog s leg from the ventral (the underside of the frog). Locate the muscles listed it the pictures below. It may be necessary to separate muscles from one another, as they are held together and covered by membrane. Use pins as in the picture below to mark the muscles you have found. (Do not poke the muscles with the pins!) Ventral Muscles of Frog thigh 1 sartorius 2 adductor longus (thin, flat) 3 adductor magnus 4 gracilis major 5 gracilis minor (hard to find) 6 semitendinosus (beneath adductor longus, also hard to find) Ventral muscles of Frog shank 1 gastrocnemius 2 tibialis posticus 3 extensor cruris (hard to find) 4 tibialis anticus longis 5 tibiofibula bone

9 3. After locating the ventral muscles, flip your frog over and locate the dorsal muscles. Frog thigh Dorsal view 1 gluteus 2 piriformis (hard to find) 3 triceps femorus 4 semimembranosus 5 biceps femoris Frog shank Dorsal view 1 gastrocnemius 2 peroneus 4. The frog s leg is made up of four major bones, listed here proximal (nearer the torso) to distal (further from the torso): femur, tibiofibula, calcaneus, and astragalis. The calcaneus and astragalis run side by side in the 3 rd segment of the frog s leg. 5. Cut neatly through the gastrocnemius and peroneus to expose the tibiofibula bone. 6. Cut neatly through all the ventral thigh muscles to expose the femur. You should also notice the femoral artery (pink) and the femoral vein (blue). Which of these two supplies the leg with blood? Which one drains the leg of blood?

10 Lab Report In your lab report, type out the answers to the questions/observations you noted during the dissection. You do not need to do additional research to answer these questions they should be based solely on your observations during dissection and your intuition and prior knowledge. Then answer the following questions, for which you might need your book (chap.18 in Honors or 26 in regular Bio) or the class web page to answer. Use these links: and the class biodiversity links at 1. Describe several key distinction features of amphibians. Name some other species of amphibians in your answer as examples. 2. The image on the left depicts the forearm bones of an amphibian ancestor. How does it compare to today s amphibians? How about to those of other vertebrates? 3. Describe the characteristics of proposal amphibian ancestors. Search for Lissamphibians and/or check out 4. How do frogs survive the winter? (see 5. In what way is frog reproduction similar to that of fish? What makes them different, in terms of reproduction, from other land vertebrates (especially reptiles)? 6. How are frogs similar to other land vertebrates? 7. Describe the pathway of amphibian circulation through the heart and through the body. Compare the pathway to a fish and a mammal. 8. In the past 10 years there have been many reports of declines in amphibian populations and discoveries of mutated frogs. Describe several possible reasons for these declines. Why are frogs especially vulnerable to environmental problems? Check out and 9. Design an experiment using frogs as a test subject. Explain in detail how you would go about conducting the experiment, using the skills you learned earlier in the year. 10. Explore endangered frogs with Google Earth. Go to this website: and open the file in Google Earth (you might need to download the free program from Summarize what you learn about at least three different endangered animals (they do not need to be amphibians) on different continents.