CHAPTER XVIII. FROGS AND TADPOLES.

60. THE LIFE OF THE FROG.

1. =Manner of life.=—Where have you found frogs? Are they commonest in dry or in damp situations? At what time of the year have you seen them actually in water? How do frogs move about? Do they walk or hop? Chase a frog, and notice that its hops become shorter as it is pursued. Upon what does the frog feed? Have you ever seen frogs abroad in the depth of winter? Are insects common in winter? How do you suppose frogs spend the winter? What are the principal enemies of frogs?

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.

=Food.=—The frog’s method of catching its prey is very interesting, and is graphically described by Dr. Hans Gadow[26] as follows:—“The food, which consists chiefly of insects, snails, and worms, must be moving to excite interest; then the frog, whose favourite position is half squatting, half supported by the arms, erects itself and, facing the insect, turns round upon its haunches, adjusts its position anew by a shifting of the legs, and betrays its mental agitation by a few rapid movements of the throat. All this time the prey is watched intently until it moves; then there follows a jump, a flap of the tongue, and the insect is seen no more.” This flap of the tongue is well illustrated in Fig. 215. The frog’s tongue is free behind, but is attached, by its anterior end, close to the middle of the lower jaw,—an arrangement which enables it to be flicked out to its full length. Further, it is covered by a glutinous secretion, which sticks tenaciously to the prey.

The frog is fortunate also in the extremely wide gape of its mouth, which stretches, almost literally, from ear to ear. Once in the mouth, the captive is prevented from escaping by the teeth. Of these there are two sets; a row of fine teeth is present along the greater part of the margin of the upper jaw, and, in addition, two small patches of teeth—the vomerine teeth (_Vo._, Fig. 216)—occur on the roof of the mouth. Fairly large insects are promptly gulped down into the stomach; those which, owing to their minute size, escape being swallowed in the ordinary manner, are slowly but surely forced down the throat by the incessant lashing of thousands of tiny threads—too small to be seen except by high powers of the microscope—which are carried by the skin of the roof of the mouth. The action of these invisible threads may easily be seen if a frog’s mouth be opened widely and a small cork-shaving be placed near the top of the throat (Expt. =60=, 5, _b_). The shaving steadily travels backwards and is soon lost to sight.

It should be clearly understood that frogs and toads are of incalculable value in keeping down insect pests, and deserve systematic protection for this if for no other reason.

=How a frog breathes.=—When a resting frog is watched, the floor of the mouth is seen to be raised and lowered alternately. It is a common belief among children that these movements are a sign that the animal is “getting ready to spit.” Frogs do not spit, however, and the action is simply a part of the breathing process, which is performed in the following manner. The mouth being closed, the nostrils are opened, and, by alternate up-and-down movements of the floor of the mouth, the air present in the mouth-cavity is completely replaced by fresh air. The nostrils are then closed, and the slit-like glottis (_gl._, Fig. 163) which leads to the lungs (_l. lng._, _r. lng._) is opened. The foul air is forced out of the lungs and mixed with the pure air in the mouth-cavity. Then, immediately, the floor of the mouth is raised—pumping the mixed air into the lungs—and the glottis is closed. In the =lungs= an exchange takes place between the oxygen of the refreshed air, and the surplus carbon dioxide in the blood which is circulating in the capillaries (p. 242) of the walls of the lungs. In the meantime the nostrils have again been opened, and the first stages of the process are already being repeated.

A considerable part of a frog’s breathing is carried on, in these first stages, through the thin membrane forming the =roof of the mouth-cavity=. This is richly supplied with blood capillaries, and is therefore admirably adapted for the exchange of gases which constitutes respiration. Moreover, the =skin= covering the general surface of the body has also a very abundant blood-supply, and forms yet a third respiratory organ; so that it is practically impossible to drown a frog in ordinary water—in water, that is, which contains dissolved air. This power of breathing through the skin is of great importance to a frog during the winter sleep at the bottom of a pond.

=The frog’s skin.=—The frog’s skin is kept moist by a slimy fluid which is continually being discharged from small glands in its substance. The moisture not only facilitates skin-breathing as described above, but its evaporation keeps the body cool even in hot weather—a matter of vital importance to an animal to which a temperature of 40° C. or so is fatal. A third most interesting property of the frog’s skin is its power of changing somewhat in colour to match the colour of its surroundings. The change depends upon an alteration in form and size of certain small brown specks imbedded in the thickness of the skin. When the animal finds itself in dark-coloured surroundings these specks enlarge, and the skin as a whole takes on a darker hue. On a light background the reverse change takes place.

=A case of evolution.=—The advantage which accrues to a frog from being thus rendered less conspicuous to enemies and prey alike is obvious, and there is no difficulty in picturing to ourselves the probable manner in which the advantage was developed. Widely different races of animals have colour-specks in the skin, and we may assume that frogs and their ancestors have possessed such spots for countless generations. Now suppose that, ages ago, a frog happened to be born with the power of altering very slightly the size of his colour-specks. If this power rendered him less conspicuous, in however slight a degree, than his neighbour frogs he would, other things being equal, be more likely than they to escape from enemies, grow to maturity, and in due course have sons and daughters. All his offspring, and they might number thousands, would tend to inherit, to varying extents, the power of changing colour. In some—perhaps in most—the power would be practically absent; in others it might be equal to that of the parent; while in a few instances it would probably be greater. In any case, those frogs of the second generation which had the greatest power of changing their colour would be the most likely to survive the keen struggle for existence and therefore to leave offspring. The survival of the “fittest” frogs of each generation, and the transmission to the next generation, in ever-increasing intensity, of their favouring accomplishment, would naturally result at last in the production of a race of animals in which, as in the frogs of to-day, the power of changing colour is universal.

=The laying of the eggs.=—About the end of March the frogs resort in great numbers to shallow ponds and ditches, pair with much croaking, and the females lay their eggs or “spawn.” Both sexes croak, and the male of the edible frog, though not of the common grass frog, is able to make more noise in virtue of a pair of vocal sacs (Fig. 217), which he can inflate with air from the mouth (Fig. 216), and which act as resonators. After spawning, the frogs leave the water, abandoning the eggs to their fate, and resume their ordinary terrestrial life, until approaching winter prompts them to hide in the mud and go to sleep.

61. THE DEVELOPMENT OF THE FROG.

1. =A simple aquarium.=—A simple aquarium, in which the development of the frog from the egg may be watched, is easily made. Obtain a fairly large basin, and cover the bottom with sand, mud, and stones from a pond. Arrange these so that the bottom shall shelve from the surface of the water at one side to a depth of 3” or 4” at the other. Put in some stones covered with green slime, which will almost certainly be found in the pond, plant a few water-weeds, and allow the water to clear.

2. =Frog spawn.=—Having prepared the aquarium, obtain, towards the end of March (or earlier in a mild spring) a handful of frog spawn from a pond or ditch. It forms a mass of jelly in which the true eggs—small balls about ¹/₁₀” in diameter—are imbedded. Put this in the aquarium and examine it carefully every day, making the observations described below. If possible, obtain a pair of spawning frogs, and place them in a bucket with a little water, so that the earliest stages also may be studied.

3. =The eggs before hatching.=—Observe the globe of jelly which surrounds each egg. Try to pick it up between your finger and thumb. Do you think it is of any use in protecting the eggs from being eaten by fishes, birds, etc.? Remove the jelly as completely as possible from one egg, put the egg in a watch-glass with a little water, and examine it carefully with a strong lens. To which of the stages shown in Fig. 218 does it correspond? If possible, treat a newly-laid egg in the same manner and examine it every hour or so through the day. Make notes of the time elapsing between the various stages shown in Fig. 218.

4. =Hatching.=—Notice that the developing eggs change from the spherical to the ovoid form. What is the shape when the embryo begins to move? Notice the appearance of a neck and a tail. At this stage the embryo makes its way out of the jelly, or hatches. The jelly may now be thrown away, as it is of no further use. How does the embryo behave immediately after hatching? Put one in a watch-glass and with a lens try to see the _sucker_ by which it attaches itself to water-weeds, etc.

5. =Tadpolehood.=—Examine the tadpoles at frequent intervals by the help of a lens, and write careful accounts of the changes which take place in them. Notice particularly the fine threads—the external _gills_—which grow out from the sides of the neck. How many are there? After a time they shrivel up and are replaced by other gills which cannot be seen. Write a description of a tadpole at the time when it begins to feed. How large is it?

Count the tadpoles in the aquarium at intervals of a few days, carefully removing all dead ones. Supply fresh green slime from the pond from time to time. Have you any reason to suppose that tadpoles are cannibals?

How soon do the tadpoles come to the surface to breathe air? Describe the appearance of one when it begins to do this. Has it any legs? How many can you see? In what order do the legs grow out?

Notice now the dwindling of the tail. (It does not drop off, as is so often stated.) The tadpole has become a frog. What percentage of the original eggs have developed into frogs?

=The early development of the frog’s egg.=—A frog’s egg is a little spherical mass about one-tenth of an inch in diameter. When laid, it is covered by a thin gelatinous layer which soon swells up in the water to form a transparent globe of about half-an-inch diameter, in the centre of which the true egg can be seen. This jelly is extremely slippery and difficult to grasp, and is consequently an efficient protection against the attacks of birds, fishes, insect-larvae, etc., and even of parasitic fungi and other small organisms. The jelly also acts as a float. At the time of laying, each egg consists of a black and a white portion. In the lower, or white, part there is a store of food, on which the little embryo subsists until it acquires a mouth and begins to fend for itself. The development begins in the upper, or black, part of the egg, and may easily be watched with a lens. And to be appreciated properly, the changes should be watched, and not merely read about. The student should get, if possible, some freshly-laid frog’s eggs, and remove the gelatinous investment from one or two. It will require care to do this without injuring the eggs. They should now be put, with a little fresh water, into a watch-glass, and carefully examined at intervals of half an hour or so. Soon a little pit makes its appearance in the middle of the black half, and gradually extends until it becomes a groove, which little by little reaches quite round the egg (Fig. 218, _A_). In the meantime another groove begins to form at right angles to the first, and, in its turn, grows down round the egg (_B_). If we compare the whole egg to the earth, and the middle of the black half to the North Pole, these two grooves may be said to be along meridians at right angles to each other. The third groove (_C_) may be considered as along a parallel of latitude, but it is somewhat to the north of the Equator. These first three grooves deepen until the whole egg is cut up into eight separate pieces. The sight of the apparently lifeless speck dividing itself up in this regular and orderly manner, “while you wait,” is an intellectual treat which should not be missed. The cleavage of the egg goes on rapidly, but in a less regular manner now, until the whole is cut up into a hollow sphere of segments (_F_), black and small (_mi_) in the northern hemisphere, whitish and larger (_mg_) in the south.

It is worth while to pause here to consider how these early changes are assisted by the peculiar condition of the egg at the time of laying. The southern hemisphere of the egg is laden with a store of food. The food is dead, and acts as a mechanical hindrance to the activity of the living matter. In the northern half of the egg but little food is present to impede its activity, and it is plainly important that this half shall receive as much warmth as possible from the uncertain sunshine of early spring. Two circumstances ensure this. In the first place, the food-laden region is the heaviest part of the egg, so that the latter—buoyed up as it is by the jelly—tends to float with its most “alive” part upwards. Secondly, this upper part is coloured black,—a great advantage, since black objects absorb heat readily. Both these peculiarities therefore favour the more rapid development of the “northern” hemisphere. Hence the third cleft is to the north of the egg’s equator, instead of being halfway between the upper and lower poles, and hence, too, at the close of segmentation the northern segments are smaller and more numerous than the southern. In the case of the hen’s egg (Chapter XVI.), the amount of stored food contained in the egg (the yolk) is so enormous that segmentation is confined entirely to a small patch on the upper surface. Another result of the relatively small amount of stored food in the frog’s egg is that the tadpole is compelled to turn out and earn its own living at a stage when the chick’s inherited fortune is still considerable.

=The tadpole.=—The spherical mass soon becomes ovoid, and is divided into head and trunk by a neck-constriction (_J_). An occasional wriggle shows that the creature is alive. Shortly afterwards a tail grows out from the hinder end of the trunk, giving the animal something of the appearance of a fish (_L_). In this stage the tadpole makes its way out of the jelly, and thus hatches, about a fortnight after the laying of the eggs. The little creature is quite helpless; it has no mouth (the egg food is not yet exhausted, however), and its attempts at swimming are still feeble and uncertain. In this defenceless condition (Fig. 219, 1) it will be seen to attach itself to the water-weed of the aquarium by means of a sucker (Fig. 218, _L_, _sk_) on the underside of its head. It is in a very favourable position for examination, and by help of a lens, two—soon there are three—pairs of fine, thread-like outgrowths can be distinguished on the sides of the neck (Fig. 219, 2 and 2_a_). These are the =external gills=. In a few days the mouth breaks through, and the animal begins to nibble at the vegetation with little horny jaws, and soon swims about the aquarium with confidence.

The gills are the organs by which the young tadpole breathes, and each little gill-thread is seen, when viewed with a low power of the microscope, to contain tiny loops of blood-vessels. Every time the tadpole uses his jaws, wags his tail, or, in short, does anything at all, he uses up some oxygen and produces some carbon dioxide. The water of the aquarium contains dissolved oxygen—green water-weeds are kept in aquaria for the express purpose of liberating oxygen (p. 51)—and this oxygen makes its way through the excessively thin membrane which divides the blood of the gills from the water, to be swept away by the current of the blood to the various parts of the body. The carbon dioxide produced is brought to the gills by the blood stream, and passes through the membrane into the water, where it is utilised by the water-weed as food (p. 51). Animal and plant are thus mutually serviceable.

A series of slits soon opens through the sides of the neck, and along their margins are formed folds which are usually called the =internal gills=. The external gills dwindle and shrivel up as the internal gills are being formed, and at the same time a flap of skin grows backwards from each side of the head and covers over the slits so that they cannot be seen. Presently the two flaps fuse at their edges—except at one point on the left side, where a spout is left—and so enclose a chamber. The water which enters the tadpole’s mouth pours through the gill-slits, into the chamber, and out through the spout. As it swills over the folds of the internal gills there is an exchange of carbon dioxide for oxygen in the manner just described.

The tadpole in the meantime is growing strong and active, and the tail has grown out to form a powerful organ, the sinuous motion of which propels the animal with relatively great speed through the water.

From the point of view of the biologist, perhaps the most interesting feature of this stage of tadpolehood is the almost entire correspondence of the structure with that of a fish, although the adult frog is not in any sense a fish. This curious state of things is explained by supposing that frogs have descended from fish-like ancestors, and that every frog, in the course of its development, is under the necessity of repeating, in a more or less modified manner, the chief stages of its ancestral history. As Marshall happily expressed it,[27] a frog during its development climbs up its own genealogical tree.

=The metamorphosis.=—Just as the external gills are replaced by internal gills, so these, in their turn, are replaced by =lungs=, and advanced tadpoles frequently come to the surface of the water to breathe air. Limbs have now grown out from the sides of the body, and the webbed hind feet considerably assist the tail in swimming. Changes take place in nearly all the internal organs, fitting the animal for its life on land; and these changes are so extensive that there is necessarily a short period when the creature is neither tadpole nor frog, and is incapable of feeding. The tail, however, which would be useless to the terrestrial, leaping frog, is gradually =absorbed=, and forms a store of nutriment during the transformation.

The gills shrivel up, and the slits close; the outer layer of the skin (including the horny jaws) is thrown off, the hind limbs lengthen, and the animal leaves the water—a frog.

EXERCISES ON CHAPTER XVIII.

1. Make observations upon toads. In what respects do they differ in appearance from frogs? How do they move about?

2. Are the skins of toads dry or moist? In what situations have you found toads? How do they protect themselves from the heat of the summer sun?

3. At what time of the year, and in what places, do toads lay their eggs? Compare the voice with that of a frog. Do toads inflate any part of the body when they sing? Compare the vocal sacs with those of frogs.

4. Look for toad-spawn in the spring. It forms long, gelatinous ropes in which the eggs are embedded. How large are the eggs? In what respects do they differ from frogs’ eggs?

5. Keep toads’ eggs in an aquarium, and carefully compare all the stages of development with those of the frog.

6. Count how many caterpillars you can persuade a toad to eat “at one sitting.” What would be one result of the extermination of toads and frogs?

7. Describe carefully what happens when a frog leaps. Point out the special arrangements which enable a frog to leap safely. (1897)

8. Describe the ordinary process of feeding of a frog, and show how it is assisted by the peculiar structure of the frog’s mouth and tongue. (1895)

9. Explain the special use of the wide gape of the frog. Where are the teeth of the frog situated? (1898)

10. Mention some remarkable features of the mouth of a frog, and try to show that they are adapted to meet special needs. (1901)

11. Describe the process of filling the lungs with air, as observed in the frog and the rabbit. (1896)

12. What changes take place in a tadpole during the first week after hatching? Illustrate your answer by drawings. (1898)

13. Describe the appearance of one of the gills of a very young tadpole as seen by a low magnifying glass. (1901)

14. Describe the appearance, size, and mode of life of a tadpole about a fortnight after hatching. On what does it feed? Describe its mouth carefully. (1897)

15. Compare a fresh-hatched tadpole with one in which the hind limbs have recently appeared. (1898)

16. Trace the history of a tadpole from hatching to the time when the tail begins to grow less. What does it feed upon during this time? (1898)

17. Relate the life-history of the frog, from the time of hatching to the end of the first summer. (1897)

18. Give some account of the habits of a young tadpole a few days after hatching, especially with respect to locomotion and feeding. Make a drawing of such a tadpole in side view, three times the natural length. (1904)

19. Describe the hind leg of a frog, and explain the ways in which it is used. (1905)

20. Show how a full-grown frog is enabled to live either in air or in water. (1906)

FOOTNOTES:

[26] _The Cambridge Natural History_, Vol. VIII. (Macmillan.)

[27] Marshall’s _The Frog_ (Smith, Elder & Co.).