1. Seize the posterior or hind end of the tongue and pull it forward.

a. Tell what you have done and state which end of the tongue is attached to the floor of the mouth. b. Describe the shape of the tip end of the tongue. 2. The living frog can extend its tongue much farther than you have been able to do in the case of the preserved frog, and in the living frog the tongue is covered with a very sticky substance. The tongue is used to catch insects at some distance from the animal (Fig. 79). Tell how you think the frog could use its tongue to catch insects and get them into its mouth.1

3. Look for teeth on the jaws of a skeleton of a frog, or if you cannot obtain a skeleton, rub the finger over the jaws of a preserved specimen.

a. Which of the jaws has teeth?

b. Describe the location of the teeth on the jaw.
c. State the shape and size of the jaw teeth.

d. What is the probable use of the jaw teeth?

4. (Optional.) Look on the roof of the mouth for two relatively large palate teeth. Rub the fingers over the surface of the palate teeth.

a. Tell what you have done.

b. What have you found out about the palate teeth?
c. What is the probable use of the palate teeth?

H. How a frog swallows.

1. Gently touch the eyes of a living frog until it draws them into the head. Tell what you have done and observed.

2. Look at the roof of the mouth of a preserved specimen while you push the eyes into the head. Tell what you have done and describe the effect produced in the roof of the mouth.

3. How will the act of pushing the eyes into the head be useful to a frog in swallowing?1

1 If possible live frogs should be fed on meal worms, or other insects, and the feeding movements observed.

I. (Optional.) Sketch of the mouth cavity.

1. Open wide the mouth of a large frog and make a sketch to show the shape of the mouth cavity twice the natu

ral size, and the shape and thickness of the upper and lower jaws.

2. Draw the following parts to show their location, size, and shape jaw teeth, palate teeth, tongue, glottis, nostril openings, swellings caused by the eyes.

3. Farthest back in the throat find an opening that extends crosswise. It is the opening into the gullet and is just behind the glottis. Push the handle of the forceps into this opening and draw it (in your sketch) partly opened.

4. Label upper jaw, lower jaw, jaw teeth, palate teeth, tongue, glottis, opening of gullet, nostril openings, swellings caused by eyes.

J. Structure of arms and leg

Place a frog in a glass jar at least half full of water to cause the animal to extend the hind legs.

1. Make a sketch (natural size) of an arm to show the shape and size of the following parts: upper arm, elbow, forearm, hand, number of fingers. Label each part.

2. Draw one of the legs (natural size) to show the following parts thigh (next the body), knee, shank, ankle (elongated region above foot), foot, toes, web between toes. Label each part.

K. How a frog swims.

Place an active frog in a sink or other receptacle large enough to afford it room to swim. The water should be deep enough so that the frog will not strike the bottom with the legs. Get the frog to swim the full length of

the receptacle as many times as may be necessary to answer the following:

1. Tell what you have done.

2. Describe the movements of the hind legs in swimming. 3. In which of these movements are the toes spread out? 4. In which of these movements, therefore, can the frog get the best hold upon the water?

5. In which direction must the frog push the harder in order to move in the direction that it does?

6. In what respects are the posterior appendages well fitted for swimming?

7. In what respects are the anterior appendages not as well fitted as the legs for swimming?

L. How a frog jumps.

Place a frog where there is plenty of room, and get it to jump as many times as necessary to answer the following:

1. Tell what you have done.

2. Describe the position of the parts of the legs just before the frog jumps.

3. Describe the two movements made by the parts of the leg in the act of jumping and when about

to land.

4. In which of these two movements must the frog use the greater force?

5. Which movement, therefore, throws the frog into the air?

6. In what respects are the legs better adapted for jumping than the arms?

To the Teacher. - Put into a covered jar enough frogs to supply each two students with a specimen. Put into the jar some ether, or better, saturate a small sponge with the ether and place it in the jar. When the animals are dead, dissect them as follows: lift up the skin of the ventral wall of the abdomen with

the forceps; carefully insert the point of the scissors near the posterior end of the trunk, and carefully cut forward on one side of the body as far as the tip of the head, and back on the other side of the trunk, until the skin is completely removed from the ventral surface. In a similar manner remove the muscular wall that covers the trunk, being careful not to injure the internal organs. If time allows, remove also the skin from one leg; call attention to the thinness of the skin and to the underlying blood vessels; show the characteristics and action of the leg muscles.

If the specimen is a female, remove nearly all the eggs and throw them away. Insert a blowpipe in the glottis and partly inflate the lungs. Wash the specimens thoroughly to remove all traces of blood and cover them with water in a dissecting pan.

If the specimens are needed on successive days, they should be wrapped in a wet cloth immediately after the class work of each day and kept in a cold place. Use only specimens that are fresh.

1. Make an outline drawing, natural size (or twice natural size if the frogs are small), of the ventral view of the head and trunk regions of a dissected specimen, together with the base of each of the four appendages, and draw nothing else until directed to do so.

2. The heart is a cone-shaped body midway between the arms. Draw the heart to show its position, shape, and relative size.

3. On either side of the heart are the lungs. Stretch one of them a little by pulling on it; then letting

it go.

a. State whether or not the lungs are elastic. Are they hollow or solid? Of what advantage are these two characteristics of structure?

b. What is the color of the lungs? State whether or not you find tiny blood vessels on the surface.

c. Draw the lungs in position to show their situation, size, and shape.

4. On the frog's right side, and behind the heart and lungs is the reddish, several-lobed liver. Lay the liver over to one side and find between the lobes on the underside a thin-walled, green sac, the bile sac (or gall bladder). Sketch in your drawing the liver to show it in this position together with the gall bladder.

5. On the frog's left side and under the liver in its natural position is a whitish, oblong body, which narrows at its hinder end. This body is the stomach. Push the handle of the dissecting needle down the gullet into the stomach.

a. Tell what you have just done.

b. What organ does the handle enter?

c. Push the stomach to the frog's left and draw it in this position to show its shape and relative

size.

6. Extending from the stomach is a tubular structure of considerable length, the small intestine. At the lower end of the small intestine the tube becomes larger and then disappears between the two thighs. This last part of the tube is called the cloaca or large intestine.

Draw the small and large intestines.

7. Between the stomach and the first loop of the small intestine is a thin pink body, the pancreas,

which is a very important digestive gland.

Draw the pancreas.

8. Label heart, lungs, liver, stomach, small intestine, large intestine, bile sac, pancreas.

9. Push the small intestine to one side and find two red bodies on either side of the spinal column. These bodies are the kidneys. The kidneys remove the nitrogenous waste (urea) from the blood.

Make a sketch of the kidneys twice the natural size.