FROG DISSECTION GUIDE

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Randall Martin
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FROG DISSECTION GUIDE I. Introduction Frogs belong to the class amphibian. Although many differences exist between humans and frogs, the basic body plans are similar.

1 FROG DISSECTION GUIDE I. Introduction Frogs belong to the class amphibian. Although many differences exist between humans and frogs, the basic body plans are similar. Humans and frogs both belong to the phylum Chordata. This guide is intended to lead you through the anatomy of the frog dissection and to make connections to the functional significance of the structures that you will locate. By studying the anatomy of the frog, you will better understand human body systems. II. Materials - preserved frog - disposable gloves - dissecting instruments: scissors, scalpels, and probes - dissecting tray III. Safety 1. YOU MUST WEAR YOUR GOGGLES AT ALL TIMES DURING THIS LAB. 2. IN ADDITION, YOU MUST BE EXTREMELY CAREFUL WITH THE SCISSORS, SCALPELS, AND PROBES. They are all very sharp and can hurt students. 3. Aprons and latex gloves are available for use. IV. Procedures 1. Obtain a frog, disposable gloves, dissecting tray, and instruments, and bring them to your table. 2. Place the frog in your dissection tray. ALL FROG PARTS MUST REMAIN IN THE TRAY. Orientation Dorsal toward the back Ventral toward the belly Lateral toward the sides Median toward the middle Anterior toward the head Posterior toward the tail External Structures 3. Identify the eyes, which have a non-moveable upper and lower lid, but can be covered with a nictitating membrane which serves to moisten the eye. 4. Identify the tympanic membrane, or eardrum. It is located behind each eye. (See Figure 1). 5. Examine the external nares (nostrils). Insert a probe into the external nares and note that it protrudes from one of the paired

small openings, the internal nares inside the mouth cavity. (See Figure 1). 6. Identify the paired appendages. Find the digits, which are like fingers on both the fore and hind limbs. (See Figure 2).

2 small openings, the internal nares inside the mouth cavity. (See Figure 1). 6. Identify the paired appendages. Find the digits, which are like fingers on both the fore and hind limbs. (See Figure 2). 7. Determine if your frog is male or female by looking at the innermost finger of the forelimb. The male pad is an enlarged darker patched pad that the make uses to grasp a female during mating. (See Figure 2). 8. and complete question 1 through 3. Mouth Anatomy 9. Pry open the mouth. Use scissors to cut open the corner of the mouth where the maxilla (upper jaw) and mandible (lower jaw) join together. 10. Find the tongue and pull it out. 11. Find the Eustachian tube opening into the angle of the jaws. These tubes lead to the ears. Eustachian tubes equalize air pressure in the ears. This ensures proper tightness of the membrane for good hearing. With your probe, follow a Eustachian tube up to the tympanic membrane. 12. Feel the maxillary teeth that are along the rim of the upper jaw. (See Figure 3). Notice that only the upper jaw has teeth. Another set of teeth, the vomerine teeth, is present just behind the mid-portion of the upper jaw. Vomerine teeth are absent in toads. 13. If your frog is a male, locate the openings to vocal sacs in the floor of the lower jaw near the hinge joints. Consider why males would have vocal sacs. 14. Locate the glottis. It is a slit opening for the voice box on the floor of the mouth through which air passes in and out of the trachea, the short tube from the glottis to the lungs. 15. Find the esophagus at the rear of the mouth. It lies dorsal and posterior to the glottis and leads to the stomach. 16. and complete questions 4 through 7.

Internal Structures 17. Place the frog ventral side up (on its back) and pin down its four limbs. READ TWICE, CUT ONCE. 18. Lift the frog s skin with forceps between the rear legs. 19.

3 Internal Structures 17. Place the frog ventral side up (on its back) and pin down its four limbs. READ TWICE, CUT ONCE. 18. Lift the frog s skin with forceps between the rear legs. 19. Make a small cut through the lifted skin with the scalpel. Take care to cut only the skin. You are making a starting place for the scissors. 20. Use the scissors to continue the incision up to the midline all the way through the frog s skin. 21. Stop cutting when your scissors reach the frog s neck. Be careful not to cut too deeply. 22. Now, you will cut horizontally. Still using the scissors, make horizontal incisions just above the rear legs and between the front legs of the frog. 23. Be careful to cut only skin, not muscle. 24. Once you have finished the incisions between the front and rear legs of the frog you need to separate the skin flaps from the muscle below. To do this: Pick up the flap of skin with the forceps, and use a scalpel to help separate the skin from the muscle below. 25. Once the skin flaps have been cut, pin them to the dissection tray using several pins. First Muscle Incision 26. Now that the skin has been removed, begin the abdominal muscle incision by using the forceps to lift the muscle midway between the rear legs of the frog. Next use the scalpel to start the incision in the direction of the chin. 27. Use the scissors to continue the incision up the middle of the frog to just below the front legs. 28. DO NOT CUT TOO DEEPLY, the muscle is thin. It is easy to damage the organs underneath. Cutting the Bone 29. When you reach a point just below the front legs, turn the scissor blades sideways to cut through the bones in the chest. This should prevent damage to the heart or other internal organs. Be careful that you don t cut too deeply. When your scissors reach a point just below the frog s neck, you have cut far enough. Second Muscle Incision 30. Make a horizontal incision, just as you did with the skin, but using the scalpel. The first incision is between the front legs, the second is between the rear legs. 31. Use the forceps to hold the muscle flaps while separating the muscle from the organ tissues below with a scalpel. 32. Once the muscle flaps have been separated from the underlying tissue, pin the muscle flaps back far enough to allow easy access to the internal organs. 33. and complete question Locate any yellow fat bodies and carefully remove them.

Respiratory System and Liver 35. Insert a probe into the glottis, and observe its passage into the trachea. Enlarge the glottis by making short cuts above and below it.

Identify the lungs, two small sacs on either side of the midline and partially hidden under the liver. Trace the path of air from the external nares to the lungs.

4 Respiratory System and Liver 35. Insert a probe into the glottis, and observe its passage into the trachea. Enlarge the glottis by making short cuts above and below it. When the glottis is spread open, you will see a fold on either side. These are the vocal cords used in croaking. 36. Identify the lungs, two small sacs on either side of the midline and partially hidden under the liver. Trace the path of air from the external nares to the lungs. The lungs are shallow and do not supply enough oxygen to support the frog. As such, the frog has two other respiratory surfaces on its body that it uses to exchange gas with the surroundings: the skin and on the lining of the mouth. While completely submerged all of the frog's respiration takes place through the skin. The skin is composed of thin membranous tissue that is quite permeable to water and contains a large network of blood vessels. The thin membranous skin allows the respiratory gases to readily diffuse directly down their gradients between the blood vessels and the surroundings. When the frog is out of the water, mucus glands in the skin keep the frog moist, which helps absorb dissolved oxygen from the air. 37. Locate the liver, the large, prominent, dark-brown organ in the mid-ventral portion of the trunk. The liver serves to produce bile and clean waste products from the body. 38. Under the liver, locate the gall bladder, the storage center for bile produced by the liver. 39. and complete question 9.

5 Circulatory System 40. Lift the liver gently. Identify the heart, a small triangular-shaped organ between the front legs, just above the liver. It is covered by a membranous covering (the pericardium). 41. With forceps, lift the covering and gently slit it open. 42. Amphibian hearts have three chambers. The heart consists of a single, thick-walled ventricle and two (right and left) anterior, thin-walled atria. 43. and complete questions 10 and 11.

Digestive System 44. Identify the esophagus, a very short connection between the mouth and stomach. Lift the left liver lobe, and identify the stomach, which is whitish in color and J-shaped.

Slit open the side of the stomach and notice its ribbed internal surface. Try to identify its contents. 46. and complete questions 12 and 13. 47.

6 Digestive System 44. Identify the esophagus, a very short connection between the mouth and stomach. Lift the left liver lobe, and identify the stomach, which is whitish in color and J-shaped. The stomach connects with the esophagus anteriorly and with the small intestine posteriorly. 45. Cut the stomach out of the frog. Slit open the side of the stomach and notice its ribbed internal surface. Try to identify its contents. 46. and complete questions 12 and Find the small intestine and the large intestine, which enters the cloaca. The cloaca lies beneath the pubic bone and is a general receptacle for the intestine, the reproductive system, and the urinary system. It opens to the outside by way of the anus. Trace the path of food in the digestive tract from mouth to the cloaca. 48. Remove the small intestine from the body cavity. Stretch the small intestine out and measure the length. Now measure the length of your frog. Record your measurements in centimeters on question As you lift the small intestine, you will see the pancreas. This organ appears as a thin, yellowish ribbon, between the small intestine and the stomach. 50. Locate the spleen, a small pea-shaped body near the stomach. It produces new blood cells and disposes of old ones. It is more difficult to find the spleen in a female frog. 51. and complete question 15.

Urogenital System 52. The urogenital system consists of both the urinary system and the reproductive system. Identify the kidneys, which are long narrow organs lying against the dorsal wall.

It may be difficult to locate. Identify the urinary bladder, attached to the ventral wall of the cloaca. In frogs, urine backs up into the bladder from the cloaca. 54.

7 Urogenital System 52. The urogenital system consists of both the urinary system and the reproductive system. Identify the kidneys, which are long narrow organs lying against the dorsal wall. Female kidneys are located in the same place as male kidneys but may be difficult to see until after the removal of the ovaries and oviducts. 53. The bladder is a thin sac attached to the cloaca. It may be difficult to locate. Identify the urinary bladder, attached to the ventral wall of the cloaca. In frogs, urine backs up into the bladder from the cloaca. 54. The light stripe lying on the ventral side of each kidney is the adrenal gland. Male Anatomy 55. Locate the testes in the male frog. They are yellow or tan-colored, bean-shaped organs near the anterior end of each kidney. Several small ducts, the vasa efferential, carry sperm into the kidney ducts that also carry urine from the kidneys. Fat bodies, which store fat, are attached to the testes. Female Anatomy 56. Locate the ovaries in the female frog. They are attached to the dorsal wall. Fat bodies are attached to the ovaries. Highly colored oviducts lead to the cloaca. The ostium (opening) of the oviducts is dorsal to the liver. 57. A mature female may have its body cavity full of black and white eggs. The black portion of the egg contains the living embryo. The white portion is yolk, containing stored food. 58. An immature female will have 2 grayish or pinkish ovaries on either side of the abdominal cavity. (The testes of the male would be in corresponding positions). 59. and complete question 16.

Central Nervous System 60. Carefully cut away the skull to expose the brain. Identify the following structures of the brain: a. Cerebrum: The cerebrum is the largest part of the brain.

8 Central Nervous System 60. Carefully cut away the skull to expose the brain. Identify the following structures of the brain: a. Cerebrum: The cerebrum is the largest part of the brain. It is responsible for voluntary activities, movement, speech, reasoning, emotions, and personality. b. Olfactory lobe: This lobe of the brain is responsible for smell. c. Optic lobe: This lobe of the brain is responsible for vision. d. Cerebellum: This structure is located at the back of the head. It coordinated balance and the action of muscles. e. Medulla oblongata: The medulla oblongata controls involuntary actions necessary for life including breathing, blood pressure, and heart rate. It also has a function in sneezing and sleep. It is located before the start of the spinal cord. 61. and complete question 17 through 20. Cleanup 62. Dispose of all frog parts by wrapping the frog in the paper towel and placing the frog back inside the plastic bag. Throw all dissectibles away in the metal trash bin. 63. Rinse and dry all equipment used. Reline the dissection trays with 3 paper towels.

Frog Dissection. Name: Block:

Frog Dissection. Name: Block: Name: Block: Frog Dissection Background: As members of the class Amphibia, frogs may live some of their adult lives on land, but they must return to water to reproduce. Eggs are laid and fertilized in