Frogs are the most famous among amphibians. These animals live practically in the whole world: from the tropics to the desert. The external structure of the frog is very similar to the structure of other animals of this class. The temperature of her body varies depending on the temperature of the environment. The size of an adult can range from 1 centimeter to 32.
There are about 4000 species of frogs. It is believed that they first appeared in Africa, and then on other continents.
For the winter, frogs hibernate. They hide at the bottom of water bodies or in burrows.
Origin of Amphibians
The first amphibians appeared about 300 million years ago. The external structure of the frog, their way of life and close connection with water indicate that the amphibians originated from fish. Scientists could find the remains of extinct species. Unlike modern amphibians, their bodies were covered with scales. And the structure of the skull is similar to the structure of brush blooded fishes.
Also prehistoric frogs had fins and lungs that emerged from the swim bladder. And they had a tail that the modern frog does not have.
Frogs lived only in fresh water and with the help of fins they could crawl overland, moving from one pond to another. But the development of the frog went further, and in the process of evolution, she developed limbs.
A significant part of life frogs are conducted in fresh water bodies or on the coast. Food frogs catch on the surface, but in case of danger quickly go to the bottom. Some species almost never leave the water, while others live in water only during the mating season.
In the process of evolution, the internal and external structure of the frog has changed. She has adapted to live not only near water bodies. Also frogs live in places with high humidity: in marshes, in tropical forests. There are species that live on trees and almost never peel off.
The frog skeleton is very similar to the perch skeleton, but due to the peculiarities of the way of life it has a number of features. The most important difference is the presence of limbs. The forelimbs are connected to the spine with the help of the limb bones. Hind legs are attached to the spine by the hip bone.
The frog skull has fewer bones than the skull of the fishes. But the gill bones and gill cover are absent. Breathing occurs with the help of the lungs.
The spine of the frog consists of 9 vertebrae and has 4 divisions: cervical, trunk, sacral and caudal. The vertebrae of the trunk refer to the trunk, are provided with upper arches and limit the spinal canal. The number of vertebrae is almost equal to seven for all frogs. There are no ribs for this amphibian.
The sacral department has one vertebra, and it connects the spine and pelvic bones. The tail is absent from the amphibian, but the tail section of the spine is one long bone, which was formed due to several fused vertebrae.
The cervical region consists of only one vertebra and connects the head and spine. This frog skeleton differs from the structure of fish. They do not have such a department of the spine.
The muscles of the frog are very different from the muscles of the fishes. She's not only moving in the water, but also living on land. The most developed muscles of frogs and toads are the muscles of the hind limbs. Thanks to them they can make jumps. Unlike fish, frogs can move their head slightly.
External description of the frog
What is the external structure of the frog? It consists of the trunk, head, front and hind limbs. The border between the body and the body is not very clear, the neck is practically absent. The frog's body is not much bigger than the head. Features of the external structure of the frog is that it does not have a tail and it practically lacks a neck. Head is large. Eyes are large and a bit bulky. They are closed with transparent eyelids, which prevent drying out, clogging and damage. Below the eyes are the nostrils. Eyes and nostrils are located at the top of the head and during swimming are above the water. This allows the amphibian to breathe air and monitor what is happening over the water. On the upper jaw there is a row of small teeth.
Ears as such have no frogs, but behind each eye there is a small circle, protected by the skin. It's the eardrum. The amphibian skin is soft and covered with mucus. Its peculiarity is to shift relative to the body. This is because under the skin there is a large amount of space - the so-called lymphatic sacs. Frog skin is naked and thin. This facilitates the penetration of fluid and gases into her body.
The peculiarity of the frog is that it can live without skin. This fact is indicated by periodic molting, during which the animal releases it, and then eats it.
In most cases, amphibians mimic the environment. Therefore, the color repeats the drawing of the place where the frog dwells. Some species have special cells that can change the color of the skin depending on the environment.
In tropical areas, you can meet amphibians, which are painted in very bright colors. Such coloring means that the animal is poisonous. This frightens off enemies.
There are many beautiful colors of this animal. In India there lives an iridescent frog, which is the object of worship. Her skin is colored with all the colors of the rainbow.
Another unusual sight is the glass frog. Her skin is completely transparent, and you can see her insides.
Many species have toxic glands in the skin that cause paralysis of respiration in predators if they attempt to attack. Other frogs produce mucus, after contact with which blisters and burns appear on the skin.
On the territory of Russia live mainly only non-poisonous species of frogs. But in Africa, on the contrary, a large number of dangerous amphibians.
Previously, frogs could be used to kill insects. For example, in 1935 a very toxic reed frog was brought to Australia. But it did more harm than good. Because of its virulence, it harms the ecosystem, but does not want to fight insect pests.
The frog has well developed hind legs. The forelimbs are used mainly for support in a sitting position and for landing. The hind legs are longer and stronger than the forelegs. To move on water and land, the hind legs are used. The frog pushes with force and lands on the front legs. This protects it from impact.
To move around in the water, the frog also uses its hind legs. On the paws there are webs that are stretched between the fingers. In addition, much easier movement in the water is that the frog is smooth and slippery with mucus.
But movement is not limited only to water and land. The external structure of the frog can provide them with movement elsewhere. Individual species are able to plan in the air and climb trees. Features of some species of frogs are that they are equipped with special discs-suckers, which help to stick to different surfaces. Or they have special growths.
Other amphibians are able to dig in the ground, for example, the shuttle-maker does it in the afternoon. At night she goes on a hunt. Burial is due to corneous calluses on the legs. Some species can wait under the ground for cold or drought. And frogs that live in the desert, can be under the thickness of sand up to three years.
Adult toads and frogs eat small invertebrates, insects and in some cases vertebrates. Frogs are predators in their nature. They can not disdain and their kindred.
The frog lurks its victim motionless, sitting in a secluded corner. When she notices movement, she shoots a long tongue and eats her prey.
The digestive system begins with the oropharyngeal cavity, to which a long tongue is attached. When the frog finds its prey, it "shoots" this tongue, and the prey to it sticks. Although the toad has teeth, it does not chew on them, but only keeps the prey. After the amphibian has caught the victim, the food gets straight to the esophagus, and then to the stomach.
Toads and frogs breathe through the lungs and through the skin. Their lungs have a saccular shape and a network of vessels. Air enters the lungs through the nostrils. Also, the lungs are used not only for breathing, but also for "singing". By the way, females do not make any sounds, only males "sing" to attract a couple.
The organs of the senses of the frog help orientate it on land and in water. In adults, amphibians, as well as fish, have very developed lateral line organs. These organs help to navigate in space. The largest number of them is located on the head. The lateral line organs look like two longitudinal strips along the entire body, beginning with the head of the frog.
Many amphibians have developed color vision.
Frogs begin to multiply only in the third year of life. In the spring, when the mating season begins, the male chooses his female and holds it for several days. During this period, it can allocate up to 3 thousand eggs. They are covered with mucous membrane and swell in water. The shell attracts sunlight, which makes the development of eggs faster.
Development of the frog
The frog embryo (tadpole) is in the eggs for about one to two weeks. At the end of this time, a tadpole appears. The internal and external structure of the frog is very different from the structure of the tadpole. Most of all, it looks like a fish. The tadpole has no limbs, and the tail is used for movement in water. The tadpole breathes with the help of external gills.
Like fish and amphibians, the tadpole has a lateral line for orienting. At this stage, the frog embryo does not go to land. Unlike an adult, the tadpole is herbivorous.
Gradually, metamorphoses take place with him: the tail disappears, paws appear, changes occur in the structure of the skeleton. And after about 4 months there is a little frog who can get out on land.
Frogs that live in Europe, usually do not grow more than 10 centimeters. But in North America and Africa, real giants can live. The largest frog - a Goliath frog - reaches a size of 90 centimeters and can weigh 6 kilograms.
The jumping champion is a woody African frog. It is capable of jumping for distances up to 5 meters.
The longest life in an African burrowing frog. She lives up to 25 years. This frog digs a hole and lives there until the drought runs out.
Most recently in New Guinea, the smallest frog was found. Its length is 7.7 millimeters.
The record holder for poison does not look dangerous at all. This is a tiny frog about 3 centimeters long. This is the most poisonous vertebrate on earth, including snakes. She lives in the tropical forests of Colombia. With its poison the Indians greased the arrows. The poison of one such frog was enough for 50 arrows.