This booklet belongs to: Spring 2017 Page 1 of 10
Frog Dissection Background Amphibians are studied in science for a variety of reasons. Amphibians are unique in many ways because their anatomy allows them to live on both land and in the water. As simple as they are, they have many of the same anatomical structures as higher order animals. Objectives Identify the external and internal anatomy of a frog; Make predictions about how the anatomical structures of the frog are adaptive features; Develop dissecting and observational skills. Animal Orientations Locate the following orientations: Dorsal: The back which is usually uppermost Ventral: The lower surface Anterior (Cranial): the forward part (nose end) Posterior (Caudal): the hind part (tail end) Medial: the central longitudinal axis of the body Lateral: the longitudinal line along the sides of the body The External Anatomy 1) Collect the materials, including a frog, from your teacher. 2) Place the frog ventral side down (lying on its stomach) on the dissecting tray. 3) Carefully examine the coloration and locations of markings on the frog. The color is caused by scattered granules in the epidermis and chromatophore cells in the dermis. Chromatophore cells are cells that contain pigments. 4) Remove a 1cm x 1 cm section of the dorsal skin containing one of the frog s spots. Take a look at the skin using a stereomicroscope. 5) Locate the thin membrane that covers the eye from below. This in the nictitating membrane, which protects the eye when the frog in under water and keeps it moist when the frog is on land. Notice the large tympanic membranes behind the eyes. These membranes function as eardrums to receive sound waves. The external nares are the frog s nostrils. 6) Examine the forelegs and the hind legs of the frog, noting the number of toes. If your frog is a male, it will have roughened pads near the thumbs. These are used to hold the female during mating. It is difficult to determine the gender of the frog from its thumbs though. You will need to wait until you check its internal organs (the presence of eggs is also a give-away for a female). 7) Answer questions 1-4 in the Analysis section now. Page 2 of 10
The Oral Cavity 1) Open the mouth by cutting the jaws with the scissors. Locate the maxillary teeth around the edge of the jaw. They hold food but are not used for chewing. Feel them with your fingers; they feel prickly. 2) Locate the slit-like glottis at the back of the throat. This opening leads to the respiratory system (lungs). Above the glottis is the opening to the esophagus. 3) Find the eustachian tubes at the posterior corners of the upper jaw. Probe with your dissecting probe to find out where they lead. These tubes equalize pressure within the ear. 4) Locate in a male frog the opening that leads to the vocal sacs at the widest corner of the lower jaw. They amplify the male s mating call. 5) Notice the shape of the tongue and where it is attached. It can be flipped forward to catch prey. 6) Locate the nostril openings in the roof of the mouth. Between the nostril openings are two vomerine teeth. Feel these teeth and the maxillary teeth with your fingers. They should feel like 2 hard bumps. 7) Answer questions 5-9 in the Analysis Section. The Digestive System 1) Place your frog dorsal side down in the dissecting pan. Open the frog by cutting the skin around the abdomen in the manner shown in the diagram. 2) Insert the point of your scissors just through the muscle above the anal opening and make a cut extending to the lower jaw. Cut sideways at both the forelegs and hindlegs as shown in the diagram. Cut the muscles down toward its back until they lie flat open in the dissecting pan. NOTE: If your frog is female and contains blackish-grey eggs, they must be removed carefully before the internal organs can be observed properly. 3) Locate the esophagus at the back of the throat. Pass a probe into the stomach. Note that the lower end of the stomach is constricted. This constriction is the pyloric sphincter. It regulates the amount of food that enters the small intestine. 4) Cut open the stomach and observe its lining. If food is present in the stomach, try to identify it. Usually insect body parts will be present. Cut a window out of the stomach & look at it under the dissecting microscope. 5) Follow the digestive tract beyond the pyloric sphincter to the coiled small intestine. The first portion that usually runs parallel to the stomach is the duodenum. 6) Cut open the lower part of the small intestine. Place a piece of the small intestine on a clean glass slide with the inside surface up. Observe the surface of the small intestine under a dissecting microscope. Note the villi, the many small folds in the lining that increase the absorption of nutrients into the bloodstream. 7) Follow the digestive tract below the small intestine where it widens into the large intestine, or colon. The colon ends in the rectum, which in turns opens into the cloaca. The cloaca opens to the outside of the frog. The digestive, reproductive, and excretory systems all open into the cloaca in the frog (unlike humans). 8) Note the large brown liver. Lift the lobes of the liver to locate the greenish gallbladder. The gallbladder stores bile that is secreted by the liver. Bile aids in the digestion of fats. 9) Locate the pancreas, a soft, irregular, pinkish organ that produces digestive enzymes, found lying in a membrane between the stomach and duodenum. 10) Answer questions 10-12 in the Analysis Section. Page 3 of 10
The Respiratory & Circulatory Systems Air is drawn into the mouth by expansion of the throat. The external nares close, then the throat muscles contract and air is forced into the lunges through the glottis. Air is expelled as the nares remain closed, the throat expands, and air enters the mouth again from the lungs. The glottis closed, the nares open, and the throat contracts, forcing the air out through the nares. 1) Probe the glottis to see where it leads. Locate the trachea, the passageway between the glottis and the lungs. 2) Locate the pinkish-gray lungs. 3) Notice the three-chambered heart between the lungs and posterior to the trachea. The pointed ventricle is lighter in color than the two thinwalled atria. 4) Lift the stomach and find the spleen, a round red organ. It is generally located embedded in the mesentery that holds the internal organs in place. You will have to pull this upward to find the spleen. The spleen filters the blood, taking out improperly functioning red blood cells. 5) Answer question 13 in the analysis. The Excretory Systems 1) Examine the kidneys that lie against the dorsal body wall in the posterior region of the body cavity. 2) Each kidney has a yellow stripe, known as the adrenal body, that secretes hormones. 3) The kidneys filter the blood and produce urine, which drains into the urinary bladder, a thin walled bag that attaches to the cloaca. Page 4 of 10
The Reproductive Systems 1) A female frog has two lobed, greyish ovaries that lie close to the kidneys. In a mature female, the two ovaries might be filled with black and white eggs. 2) Locate in a male frog the white testes that can be found close to the kidneys. Look at the reproductive organs of both sexes. 3) Examine the yellow, finger-like fat bodies attached near the kidneys. Compare their size with those in the frog of the opposite sex. The fat bodies provide nourishment for the gametes. Which gametes (eggs or sperm) do you think might need more nourishment? How do the fatty bodies reflect this? 4) Answer question 14-16 in the Analysis Section. Page 5 of 10
The Frog Anatomy Page 6 of 10
Analysis Section 1) Compare the position of the frog s eye s with the position of human eyes. How is the positioning of the frog s eyes an adapative advantage for the frog? 2) What structure in the human plays a similar role to the typanic membrane in the frog? 3) Compare the colours of the dorsal and ventral surfaces of the frog. Of what adaptive value to the frog is each of these colorations? Why does a frog have counter shading? 4) Is the dispersal of the pigment in all the chromatophores uniform (even distributed) over the entire skin of the frog? 5) How (where) is the tongue attached? 6) What keeps water and food from entering the frog s lungs? 7) To what structures do the eustachian tubes lead? 8) Complete the following table: Structure Function Nictitating Membrane Tympanic Membrane Eustachian Tube Vomerine Teeth Internal Nares Tongue Glottis Page 7 of 10
9) Identify the Following Mouth Structures: Hint: Ribit ribit! 10) How does the lining of the stomach compare with the lining of the small intestine? 11) How many lobes does the frog s liver have? 12) Where does the glottis lead? 13) Sequence the passage of air into and out of a frog. Page 8 of 10
14) Are the fat bodies larger in male of female frogs? Why? 15) Label the following diagram of the internal structures of the frog: Structure Name 1 2 3 4 5 6 7 8 9 10 16) The largest parts of the frog s brain are the olfactory lobes and the optic lobes, the centers of smell and vision, respectively. How is this adaptation an advantage for the frog s lifestyle? Page 9 of 10
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