An Introduction to Nature-study

by E. Stenhouse

2. External characters.—Catch a frog; is its body dry or moist? Does it feel cold or warm? How does it behave when caught? Does it soon become tame if well treated? Put a frog under an inverted pickle-bottle and observe it carefully. What is its size? What was the size of the smallest frog you have seen? At what time of the year can the smallest frogs be seen? What is the colour of the frog? Put one frog with dark-coloured leaves and soil, and another with light-coloured leaves, and try to find out if the skin of the first becomes lighter and that of the second darker in colour. Is it an advantage to the frog to be able to change slightly in colour? Why? What is the position of the frog when at rest? Notice the shortness of the body, the hump on the back, the relative lengths of the limbs, the 4 unwebbed fingers, the 5 webbed toes, and the absence of a tail. What is the advantage of the long hind-limbs? Mention other leaping animals whose hind-limbs are longer than the fore-limbs. Is a short-bodied animal (i.e. one with its four limbs pretty close together) less likely to be injured by the fall, after a leap, than a long-bodied animal? Why? Put the frog into water, and watch it swim. What is the use of the webs between the toes? Examine the head. Notice the very wide stretch of the mouth, the small openings of the nostrils, the large prominent eyes, and the ear-drum—a dark-coloured disc on each side, a little below and behind the eye.

3. Breathing.—Watch the up-and-down movements of the floor of the mouth, by which the animal pumps air, through its nostrils, in and out of its mouth-cavity.

4. Method of feeding.—Put live insects, small worms, etc., under the upturned bottle enclosing the frog, and watch the animal’s method of feeding. Try to see how it uses its tongue. What is the advantage of the wide gape of the mouth? Can you get the frog to accept dead insects?

5. The inside of the mouth.—To kill a frog painlessly, soak about a teaspoonful of chloroform on cotton wool, and put it with the frog under a bottle or tumbler. After 15 or 20 minutes the frog will be quite dead.

(a).—Open the jaws widely, and examine the inside of the mouth. Notice how the rounded eyeballs project into the mouth-cavity when they are pressed from the outside. Pass your finger-end round the margins of the jaws, and feel the row of fine teeth borne by the upper jaw; the lower jaw is destitute of teeth. Feel also, in the roof of the mouth, two small patches of teeth (Vo., [Fig. 216]); these are carried by small bones called the vomers, and are therefore called the vomerine teeth. Just to the outside of the vomerine teeth and in front of the eyeball, notice on each side the internal opening of the nostril (Ch., [Fig. 216]); pass into one nostril a stiff bristle, and observe that it emerges at the external nostril. Pull forward the tongue (T, [Fig. 216]) and observe that it is attached at its front end; feel how sticky the tongue is. Behind the eyes, and at the angles of the jaws, notice the openings of the Eustachian tubes (E, [Fig. 216]); push a stiff bristle into one and observe, from the outside, that the end of the bristle presses the inner surface of the ear-drum.

(b).—If the roof of the mouth has dried, moisten it with water, and place a very small cork-shaving or a tiny snip of paper on it, far back between the eyeballs. Notice that the shaving travels slowly down the throat. This experiment ought to be made soon after the frog has been killed.

6. The bones.—Feel, through the body-wall, the various parts of the skeleton, making out the skull, backbone, limb-bones and their manner of attachment, and breast-bone (protecting the heart). Observe the absence of ribs.

7. The skin.—Examine the skin of the dead frog, and notice that it is damp or even somewhat slimy, and that it differs from the skin of a mammal or bird in being naked, bearing neither hairs, feathers, nor scales. Pinch up the skin, and notice how very loosely it is attached to the underlying body-wall. Snip through the thin skin, turn it back from the body-wall, and see the network of blood-vessels upon its inner surface.

The habits of the frog.—The common grass frog ([Fig. 214]) is to be found in abundance, from early spring to October, in ditches, marshy land, and other damp places. As winter approaches, frogs generally bury themselves in the mud at the bottom of ponds, etc., and remain there in a state of torpor until the spring, when they emerge and the females lay their eggs. Frogs are said to be “cold-blooded” because their temperature never varies much from that of their surroundings. Birds and mammals, on the other hand, maintain an almost constant temperature,—a healthy man’s blood, for example, being just as warm in the depth of winter as it is on a hot summer day. So long as they are not actually frozen hard, therefore, frogs can endure the winter cold without much inconvenience. Although the frog is essentially a land animal, it is quite at home in the water, and swims gracefully and easily by the help of the webs which connect the long toes of its hind feet. On land, it progresses by long leaps, its limbs and body being well adapted to this habit. As is usual in leaping animals, the hind legs and feet are markedly longer than the fore limbs; and the shortness of the body enables the two pairs of limbs to be brought together to break the shock of the fall.