State in detail the differences in structure and in position between the fore and hind limbs. How do you account for these differences?

_Feeding._

4. Induce a frog or toad to eat by dangling food, such as a piece of raw meat or meal worms, small earthworms, etc., before it. How does it seize the food? What will it eat? How is the mouth adapted to this manner of feeding?

_Respiration._

5. The frog has no diaphragm, and therefore no chest cavity; watch very carefully the movements of the mouth, the nostrils, the throat, and the sides of the body to determine how the problem of breathing (how the air is gotten into and out of the lungs) without a diaphragm is solved. Write a detailed account of the frog's method of breathing which shall explain just how the air is forced into the lungs.

6. What would be the effect of propping open the mouth of the frog?

Why? Does the frog breathe in the usual manner while under the water?

If not, how do you explain its ability to remain under water for a long period of time?

_Sense Organs._

7. Investigate the efficiency of the five special senses in the frog by devising experiments to test each sense; as, for example, giving a frog its liberty on the floor and trying to catch it again, to test the sense of sight. Write an account of your experiments and their results. Which of the senses is best developed? Give reasons for your answer.

8. Compare the eye of the frog with respect to its shape, movements, parts, and protective structures with that of the fish. In what respects are they similar? in what respects different? Why should they differ?

9. Where are the frogs' ears located? What do you think of the efficiency of an eardrum situated on the surface of the body? Why?

10. The frog has certain other responses. Try turning the jar or cage containing a frog around to face the frog in another direction. What happens? How do you explain this response on the basis of use? What other responses have you noticed?

_Protection._

11. Has the frog an exoskeleton? Describe the color scheme of the frog and explain how it may be protective. Why are frogs brighter in spring than in fall?

12. Why do frogs usually live near water? Do they ever leave the vicinity of streams? If so, when?

_Summary._

In what ways is the frog adapted to water life? In what ways to life on land? In what respects do toads differ from frogs?

The Frog's Mouth

_Materials._

Preserved or freshly killed frogs in dishes or shallow pans of water; forceps and a bristle.

_Observations._

Open the frog's mouth as widely as possible and, if necessary, insert a splinter to hold the jaws apart. Identify the following structures:--

_Tongue._ Draw it forward until the free end extends from the mouth and is outspread; observe its form, extent, and attachment.

_Teeth._ Find those on the jaws and on the roof of the mouth.

_Nostrils._ Push the bristle inward through a nostril to determine its direction and extent.

_Vocal cords._ These form a hard white ma.s.s in the floor of the mouth, well back behind the tongue.

_Glottis_, the slit inclosed between the vocal cords, opening into the trachea.

_Esophagus_, the pa.s.sage to the stomach, at the posterior end of the mouth.

_Eustachian tubes_, small pa.s.sages outward to the ears at the junction of the upper and lower jaws.

_Questions._

1. Describe the probable action of the frog's tongue in catching a bug.

2. What advantage can you ascribe to the peculiar mode of attachment of the tongue?

3. Of what use is the notch in the inner end of the tongue? (Note its position when the tongue lies at rest in the mouth.)

4. If the frog chewed its food, how would the existing structure of the nostrils be very inconvenient?

5. Recall either the frog's habit of feeding or the structure of the nostrils. Do you think the nostrils are of much service in smelling?

State the reason for your answer.

6. Of what use are the vocal cords and why are they so muscular?

Consider their use in sound making and also their condition during swallowing.

7. Of what use are the teeth? Recall the form and use of the fish's teeth.

_Suggested drawings._

a. The mouth, wide open and with tongue extended.

b. A diagram showing the path of air and of food through the frog's mouth.

The Organs of Digestion, Absorption, and Excretion

_Materials._

Freshly killed or preserved frogs in dishes or shallow pans of water, forceps, and, if the pupil is to do any dissecting, scissors.

_Directions and Observations._

The specimens may have been opened by the teacher, or may be dissected by the pupil as follows:--