The frog belongs to the class of vertebrates known as the Amphibia. The name Amphibia refers to the fact that these animals pass through an immature stage during which they live in water and breathe by gills, before they reach the adult stage, when they breathe by lungs and are able to live on land. The toads, newts and salamanders are also amphibians, and this class is easily distinguished from fish by the possession of limbs, which typically have five digits. It is this feature, rather than the possession of lungs, which marks off the adult amphibian from a fish, since certain fish, known as lung fish, are able to use the swim bladder as a kind of air breathing lung. On the other hand, the amphibian can be distinguished from a reptile by its moist, slimy skin, which differs greatly from the dry scaly covering of the reptile.
The body of the frog shows features which adapt it to both its land and water life. The body has no neck, is broadest towards the middle and flat behind, and tapers towards each end. This barge-like shape allows the frog to move easily through the water in a horizontal plane. On land the frog progresses in a series of leaps made by its powerful hind legs. It has no tail, for this would interfere with the jumping.
The head is broad and flattened forthe water horizontally, and the mouth is very wide. The large eyes protrude from the head, and the nostrils are raised on slight prominences at the end of the snout so that when the body is floating with the snout just below the surface the eyes and nostrils are above the surface. The eyes have upper and lower eyelids, which are practically immovable, and also a thin movable film called the nictitating membrane. This is a transparent fold arising from the lower edge of the eye and which can be drawn across the eye. Behind the eye there is a circular patch of tightly stretched skin, which is the ear-drum or tympanic membrane, for the frog has no outer ear-trumpet.
The smooth skin fits the body loosely, and is kept moist by the secretion of small glands distributed all over the body. The skin has a yellowish-brown or green ground colour marked with spots and streaks of black. On the hind legs there are cross bars of brown or green, and the tympanic membranes are coloured brown. The ground colour of the skin changes in intensity according to the amount of light reflected from the surroundings. In dark situations the pigment in certain cells in the skin expands, and the frog assumes a deeper colour. Well-lit backgrounds cause the pigment in the cells to contract, and the frog becomes lighter in colour. In this way the frog is able to harmonize with its background and so escape notice by its enemies.
The Legs and Movement of the Frog
The front legs of the frog are short and consist of upper arm, forearm and hand. The hand has only four fingers, for the thumb is not present. During the breeding season the male can be distinguished by the possession of black pads under the first finger of the fore limb. The hind legs are much longer than the front ones, and consist of thigh, shin and foot. Owing to the elongation of two bones in the ankle region, the foot is very long and has two joints, one between the ankle bones and the shin, and the other between the bones of the ankle and the foot. The foot has five long toes which are joined by a thin web of skin.
On land the frog sometimes crawls along, but it generally moves by jumping. In the restingthe hind legs are folded up in the form of a letter Z, but when the frog leaps the legs are straightened out quickly, thus lifting the animal off the ground. The fore legs do not help in the jumping, but are used as shock absorbers on landing and for propping up the fore end of the body when the animal is at rest.
The frog feeds on insects, slugs and worms and does much good by destroying garden pests. It catches its food by means of its forked tongue, which is a fleshy tab fixed at the front of the mouth with its free end lying towards the throat. When an insect is seen moving, the frog suddenly shoots out the back of its tongue and catches its prey. The tongue is hinged at the front, and the tip of the tongue has a sticky secretion obtained from the roof of the mouth, and which helps in the capture of the insect. The victim is firmly held by two patches of teeth on the roof of the mouth, and also by the eyes, which are withdrawn into their sockets so as to bulge into the roof of the mouth. The movement of the eyes also helps to push the food towards the gullet.
If frogs are kept in captivity, once they have moulted they will generallyreadily on earthworms—if not supplied too often ; once, or at most twice, a week is sufficient. An aquarium tank with several bricks as resting-places forms a suitable environment, but the inhabitants should be transferred into a clean tank at least once a fortnight. One drop of ferric chloride added to the water seems to keep them more healthy.
THE GENERAL INTERNAL STRUCTURE OF THE FROG
The frog has a row of small teeth along the edges of the upper jaw and, as well, two small patches of teeth on the roof of the mouth. At the front of the mouth, behind the upper jaws, is a pair of openings, the internal nostrils. These lead into short passages which open on the outer side of the head by the external nostrils. On the roof of the mouth behind the vomer teeth are two large rounded projections caused by the bulging of the eyes into the mouth. The tongue lies on the floor of the mouth and is attached by the front end, so that the free end lies backward. Behind the tongue is a slight elevation on which there is a slit, the glottis, leading to the lungs. The mouth narrows towards the back to form a region called the pharynx. On each side of the pharynx just above the angle of the jaws is a small hole, the opening of the Eustachian tube which passes to the ear.
The Skin and Lymph Cavities
The skin of the frog fits the body very loosely, and if a slit is made in the skin of the abdomen it will be seen that there is a space between it and the muscles of the body wall. This space is filled with a liquid, termed lymph, and is termed a subcutaneous lymph sinus. Other lymph sinuses occur beneath the skin of the back, sides, throat and legs. The sinuses are separated from each other by partitions where the skin is attached to the muscles beneath.
The Abdominal Cavity
The abdomen contains a large coelom, which in its anterior region is protected by bones forming the ventral part of the pectoral girdle. Lying in the coelom above the ventral part of the pectoral girdle is the heart, a pink conical body which is surrounded by a thin-walled bag, the pericardium. Other structures lying in the abdomen are the digestive organs, the lungs and the reproductive organs. The coelom is lined by a thin membrane, the peritoneum, and contains a lymphatic fluid. Above the peritoneum lie the kidneys in a large lymphatic space called the cisterna magna.
Frogs are cold-blooded creatures, and the colder the weather the less active they become. They avoid the cold and frost of winter by hibernating, and pass this season hidden in holes in banks, in nooks and crevices in stone walls and similar places wherever it is damp ; sometimes they bury themselves in the mud at the side of a. The frogs emerge from hibernation just before the breeding season, which usually occurs in March or April but may occur in February if the winter is mild. The female frog is easily distinguished from the male at this time of the year, for her body is large and swollen owing to the contained eggs, and it is brownish in colour with yellow spots ; the body of the male is thinner and has a darker olive-brown colour. Another difference lies in the presence, during the breeding season, of a black breeding pad under the first finger of the fore limb of the male, which is absent in the female.
When pairing takes place the male mounts on the back of the female, and clasping her with his arms just below her armpits holds himself there by grasping the skin with the breeding pads. The female carries the male about in thisfor several days until the eggs are laid, when the male fertilizes them by shedding spermatozoa over them.
The eggs are surrounded by a jelly which acts as a buffer, protecting the eggs from injury when the water is disturbed, and prevents small water fleas from nibbling them. Bacteria, also, are unable to reach the eggs, and because of its slippery nature the jelly makes it very difficult for water birds to eat them. Another useful function of the jelly is the separation of the eggs from one another so that they are not overcrowded, and each obtains a sufficient supply of oxygen. In the spaces between the spheres of jelly there are usually minute green plants which give off oxygen and so benefit the developing eggs.
The Hatching of the Egg
Each egg has a black upper surface and is cream on the lower surface. The protoplasm of the egg is loaded with yolk granules, which are most abundant in the lower part of the egg and consequently cause it to float with the black surface always uppermost. The egg commences to develop immediately after fertilization, but the only change which can be seen with the naked eye during the first week is the gradual darkening of the lower surface until the egg is black all over. Then the egg lengthens within the jelly and gradually a small tadpole is formed, showing head, body and tail regions, and which is curved round the jelly because of the lack of space. About fourteen days after the fertilization the tadpoles wriggle out of the jelly, which has become slimy and less firm. They fix themselves to water weeds or to the jelly by means of a sticky substance secreted by a U-shaped cement gland on the under side of the head.
Development of the Mouth and Gills
At this stage the tadpole has no mouth, and slight pits mark the position where the eyes, ears and nostrils will develop. On either side of the head two small projections grow out and are closely followed by a third. These are the external gills, which are the breathing organs, and which branch and become tufted in appearance. Cilia covering the whole animal cause currents of water to How over the gills. At this time the mouth develops, and is provided with a pair of horny jaws with which later the tadpole nibbles water weeds. It is entirely vegetarian at this stage, and its long intestine can be seen coiled up within its body. The external gills function for a few days only and then shrivel and disappear. Four gill slits become apparent on each side of the tadpole, and internal gills like those of a fish develop upon the walls of the slits. The gill slits do not remain visible long, for on either side a flap of skin, the opercular fold, forms in front of the slits and grows backwards so as to cover them. At this stage the tadpole is breathing in a similar manner to a fish, for water enters the pharynx through the mouth and passes out through the gill slits and under the edges of the operculum. Gradually the edges of the opercula fuse with the body and with each other beneath the pharyngeal region so that the gill slits become entirely enclosed, though on the left side a spout-like opening is left through which waterafter passing out from the gill slits.
Development of Lungs and Limbs
The tadpole is now about four weeks old, and not only resembles a fish in its manner of breathing but has a fish-like tail fringed with a continuous tail fin. The tail shows V-shaped markings due to the presence of muscle segments similar to those of a fish. In the next few weeks there is little change in the appearance of the tadpole, which feeds actively and grows larger. During the sixth or seventh week the hind limbs appear as small knobs at the base of the tail, and at the end of a fortnight have become perfectly formed. The fore limbs develop at the same time, but are hidden beneath the skin. When it is about two months old the lungs of the tadpole commence to develop, and it begins to pay short visits to the surface for air. As time goes on these visits become more frequent, and the internal gills gradually degenerate. At about three months the tadpole stopsand casts its skin ; and at the same time the mouth widens and loses its horny jaws. The front legs appear, the right pushing through the skin while the left pushes its way out down the opercular spout. During this time the tail slowly shrinks and is absorbed by white corpuscles in the blood stream. The small, tailed frog can now leave the water and feed on land. It is no longer a vegetarian, but feeds on worms, slugs and insects, and as a result of this habit the intestine, which has lost its coiled arrangement, becomes shorter. Eventually all traces of the tail disappear, and a perfect little frog is the final result of this long development. The frog docs not become sexually mature until its third year.
Judging by size, a frog can live for about five years under natural conditions, but specimens have been kept in captivity for eleven years.
The other British amphibians are the toad and the newts , of which there are three species. Although the toad is similar 90 crawling slowly or by short jumps on all four legs. It is a nocturnal animal and spends the day hidden in some crevice or hedge bottom. The eggs of the toad are laid in two rows in a long string of jelly, which is 10 or 12 ft. long and is wrapped round water plants by the toad. The eggs hatch into tadpoles, which pass through stages similar to those of the frog tadpole.
A newt is similar to a frog in general structure, but has a longer, narrower body and is provided with a tail. It is often mistaken for a lizard, but it is easily distinguished from a reptile by its soft, moist skin and its clawless toes ; the lizard has a dry, scaly skin and its toes have long pointed nails. Newts are generally looked upon as land animals, but they take to water in spring to lay their eggs. These are laid singly at the rate of two or three a day for several weeks. The newt bends theof a water plant and lays the egg in the fold, where the jelly causes it to stick and also prevents the from unbending. The eggs hatch into tadpoles agreeing in general structure and habits with those of a frog, but differing in that the front legs appear before the hind ones, and the external gills persist until the young newt is ready to leave the water.