Adaptations for Life.--The most common frog in the eastern part of the
United States is the leopard frog. It is recognized by its greenish brown
body with dark spots, each spot being outlined in a lighter-colored
background. In spite of the apparent lack of harmony with their
surroundings, their color appears to give almost perfect protection. In
some species of frogs the color of the skin changes with the surroundings
of the frog, another means of protection.
Adaptations for life in the water are numerous. The ovoid body, the head
merging into the trunk, the slimy covering (for the frog is provided, like
the fish, with mucus cells in the skin), and the powerful legs with webbed
feet, are all evidences of the life which the frog leads.
Locomotion.--You will notice that the appendages have the same general
position on the body and same number of parts as do your own (upper arm,
forearm, and hand; thigh, shank, and foot, the latter much longer
relatively than your own). Note that while the hand has four fingers, the
foot has five toes, the latter connected by a web. In swimming the frog
uses the stroke we all aim to make when we are learning to swim. Most of
the energy is liberated from the powerful backward push of the hind legs,
which in a resting position are held doubled up close to the body. On land,
locomotion may be by hopping or crawling.
[Illustration: This diagram shows how the frog uses its tongue to catch
insects.]
Sense Organs.--The frog is well provided with sense organs. The eyes are
large, globular, and placed at the side of the head. When they are closed,
a delicate fold, or third eyelid, called the _nictitating membrane_, is
drawn over each eye. Frogs probably see best moving objects at a few feet
from them. Their vision is much keener than that of the fish. The external
ear (_tympanum_) is located just behind the eye on the side of the body.
Frogs hear sounds and distinguish various calls of their own kind, as is
proved by the fact that frogs recognize the warning notes of their mates
when any one is approaching. The inner ear also has to do with balancing
the body as it has in fishes and other vertebrates. Taste and smell are
probably not strong sensations in a frog or toad. They bite at moving
objects of almost any kind when hungry. The long flexible tongue, which is
fastened at the front, is used to catch insects. Experience has taught
these animals that moving things, insects, worms, and the like, make good
food. These they swallow whole, the tiny teeth being used to hold the food.
Touch is a well-developed sense. They also respond to changes in
temperature under water, remaining there in a dormant state for the winter
when the temperature of the air becomes colder than that of the water.
Breathing.--The frog breathes by raising and lowering the floor of the
mouth, pulling in air through the two nostril holes. Then the little flaps
over the holes are closed, and the frog swallows this air, forcing it down
into the baglike lungs. The skin is provided with many tiny blood vessels,
and in winter, while the frogs are dormant at the bottom of the ponds, it
serves as the only organ of respiration.
[Illustration: Internal organs of a frog: M, mouth; T, tongue; Lu, lungs;
H, heart; St, stomach; I, small intestine; L, liver; G, gall bladder; P,
pancreas; C, cloaca; B, urinary bladder; S, spleen; K, kidney; Od, oviduct;
O, ovary; Br, brain; Sc, spinal cord; Ba, back bone.]
The Food Tube and its Glands.--The mouth leads like a funnel into a short
tube, the _gullet_. On the lower floor of the mouth can be seen the
slitlike _glottis_ leading to the lungs. The gullet widens almost at once
into a long _stomach_, which in turn leads into a much coiled intestine.
This widens abruptly at the lower end to form the _large intestine_. The
latter leads into the _cloaca_ (Latin, _sewer_), into which open the
_kidneys_, _urinary bladder_, and reproductive organs (_ovaries_ or
_spermaries_). Several _glands_, the function of which is to produce
digestive fluids, open into the food tube. These digestive fluids, by means
of the ferments or enzymes contained in them, change insoluble food
materials into a soluble form. This allows of the absorption of food
material through the walls of the food tube into the blood. The glands
(having the same names and uses as those in man) are the _salivary glands_,
which pour their juices into the mouth, the _gastric glands_ in the walls
of the stomach, and the _liver_ and _pancreas_, which open into the
intestine.
Circulation.--The frog has a well-developed heart, composed of a
thick-walled muscular ventricle and two thin-walled auricles. The heart
pumps the blood through a system of closed tubes to all parts of the body.
Blood enters the right auricle from all parts of the body; it then contains
considerable carbon dioxide; the blood entering the left auricle comes from
the lungs, hence it contains a considerable amount of oxygen. Blood leaves
the heart through the ventricle, which thus pumps some blood containing
much and some containing little oxygen. Before the blood from the tissues
and lungs has time to mix, however, it leaves the ventricle and by a
delicate adjustment in the vessels leaving the heart most of the blood
containing much oxygen is passed to all the various organs of the body,
while the blood deficient in oxygen, but containing a large amount of
carbon dioxide, is pumped to the lungs, where an exchange of oxygen and
carbon dioxide takes place by osmosis.
In the tissues of the body wherever work is done the process of burning or
oxidation must take place, for by such means only is the energy necessary
to do the work released. Food in the blood is taken to the muscle cells or
other cells of the body and there oxidized. The products of the
burning--carbon dioxide--and any other organic wastes given off from the
tissues must be eliminated from the body. As we know, the carbon dioxide
passes off through the lungs and to some extent through the skin of the
frog, while the nitrogenous wastes, poisons which must be taken from the
blood, are eliminated from it in the kidneys.
Change of Form in Development of the Frog.--Not all vertebrates develop
directly into an adult. The frog, for example, changes its form completely
before it becomes an adult. This change in form is known as a
_metamorphosis_. Let us examine the development of the common leopard frog.
[Illustration: Development of a frog. 1, two cell stage; 2, four cell
stage; 3, 8 cells are formed, notice the upper cells are smaller; in (4)
the lower cells are seen to be much larger because of the yolk; 5, the egg
has continued to divide and has formed a gastrula; 6, 7, the body is
lengthening, head is seen at the right hand end; 8, the young tadpole with
external gills; 9, 10, the gills are internal, hind legs beginning to form;
11, the hind legs show plainly; 12, 13, 14, later stages in development;
15, the adult frog. Figures 1, 2, 3, 4, 5, 6, and 7 are very much enlarged.
(Drawn after Leukart and Kny by Frank M. Wheat.)]
The eggs of this frog are laid in shallow water in the early spring. Masses
of several hundred, which may be found attached to twigs or other supports
under water, are deposited at a single laying. Immediately before leaving
the body of the female they receive a coating of jellylike material, which
swells up after the eggs are laid. Thus they are protected from the attack
of fish or other animals which might use them as food. The upper side of
the egg is dark, the light-colored side being weighted down with a supply
of yolk (food). The fertilized egg soon segments (divides into many cells),
and in a few days, if the weather is warm, these eggs have each grown into
an oblong body which shows the form of a tadpole. Shortly after the tadpole
wriggles out of the jellylike case and begins life outside the egg. At
first it remains attached to some water weed by means of a pair of
suckerlike projections; later a mouth is formed, and the tadpole begins to
feed upon algae or other tiny water plants. At this time, about two weeks
after the eggs were laid, gills are present on the outside of the body.
Soon after, the external gills are replaced by gills which grow out under a
fold of the skin which forms an operculum somewhat as in the fish. Water
reaches the gills through the mouth and passes out through a hole on the
left side of the body. As the tadpole grows larger, legs appear, the hind
legs first, although for a time locomotion is performed by means of the
tail. In the leopard frog the change from the egg to adult is completed in
one summer. In late July or early August, the tadpole begins to eat less,
the tail becomes smaller (being absorbed into other parts of the body), and
before long the transformation from the tadpole to the young frog is
complete. In the green frog and bullfrog the metamorphosis is not completed
until the beginning of the second summer. The large tadpoles of such forms
bury themselves in the soft mud of the pond bottom during the winter.
Shortly after the legs appear, the gills begin to be absorbed, and lungs
take their place. At this time the young animal may be seen coming to the
surface of the water for air. Changes in the diet of the animal also occur;
the long, coiled intestine is transformed into a much shorter one. The
animal, now insectivorous in its diet, becomes provided with tiny teeth and
a mobile tongue, instead of keeping the horny jaws used in scraping off
algae. After the tail has been completely absorbed and the legs have become
full grown, there is no further structural change, and the metamorphosis is
complete.
[Illustration: At the left is a hen's egg, opened to show the embryo at the
center (the spot surrounded by a lighter area). At the right is an English
sparrow one day after hatching.]
Development of Birds.--The white of the hen's egg is put on during the
passage of the real egg (which is in the yolk or yellow portion) to the
outside of the body. Before the egg is laid a shell is secreted over its
surface. If the fertilized egg of a hen be broken and carefully examined,
on the surface of the yolk will be found a little circular disk. This is
the beginning of the growth of an _embryo_ chick. If a series of eggs taken
from an incubator at periods of twenty-four hours or less apart were
examined, this spot would be found at first to increase in size; later the
little embryo would be found lying on the surface. Still later small blood
vessels could be made out reaching into the yolk for food, the tiny heart
beating as early as the second day of incubation. After about three weeks
of incubation the little chick hatches; that is, breaks the shell, and
emerges in almost the same form as the adult.
[Illustration: The embryo (_e_) of a mammal, showing the absorbing organ in
black, the branch-like processes which absorb blood from the mother being
shown at (_v_); _ct_, the tube connecting the embryo with the absorbing
organ or placenta.]
Development of a Mammal.--In mammals after fertilization the egg undergoes
development within the body of the mother. Instead of blood vessels
connecting the embryo with the yolk as in the chick, here the blood vessels
are attached to an absorbing organ, known as the _placenta_. This structure
sends branch-like processes into the wall of the _uterus_ (the organ which
holds the embryo) and absorbs nourishment and oxygen by osmosis from the
blood of the mother. After a length of time which varies in different
species of mammals (from about three weeks in a guinea pig to twenty-two
months in an elephant), the young animal is expelled by muscular
contraction of the uterus, or is born. The young, usually, are born in a
helpless condition, then nourished by milk furnished by the mother until
they are able to take other food. Thus we see as we go higher in the scale
of life fewer eggs formed, but those few eggs are more carefully protected
and cared for by the parents. The chances of their growth into adults are
much greater than in the cases when many eggs are produced.