Frogs are the most famous among amphibians. These animals live almost all over the 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. Her body temperature changes depending on the temperature of the environment. The size of an adult can range from 1 centimeter to 32.
There are about 4000 types of frogs. It is believed that they first appeared in Africa, and then on other continents.
Frogs hibernate during the winter. They hide at the bottom of ponds or in burrows.
The origin of amphibians
The first amphibians appeared about 300 million years ago. The external structure of the frog, their lifestyle and close relationship with water indicate that amphibians are descended from fish. Scientists were able to find the remains of extinct species. Unlike modern amphibians, their body was covered with scales. And the structure of the skull is similar to the structure of lobe-finned fish.
Prehistoric frogs also had fins and lungs that emerged from the swim bladder. And they had a tail that the modern frog doesn't have.
Frogs lived only in fresh water and with the help of fins couldcrawl on land, moving from one reservoir to another. But the development of the frog went further, and in the process of evolution, it appeared limbs.
Habitats
Frogs spend a significant part of their lives in fresh water or on the coast. Frogs catch food on the surface, but in case of danger they quickly go to the bottom. Some species almost never leave the water, while others live in the water only during the mating season.
In the process of evolution, the internal and external structure of the frog has changed. She adapted to live not only near water bodies. Frogs also live in places with high humidity: in swamps, in tropical forests. There are species that live in trees and almost never leave them.
Skeleton
The skeleton of a frog is very similar to the skeleton of a perch, but due to lifestyle features, 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 bones of the girdle of the limbs. The hind limbs are attached to the spine by the hip bone.
The skull of a frog has fewer bones than a fish skull. But gill bones and gill covers are absent. Breathing occurs with the help of the lungs.
The frog's spine consists of 9 vertebrae and has 4 sections: cervical, trunk, sacral and caudal. The vertebrae of the trunk are procoelous, equipped with upper arches and limiting the spinal canal. The number of vertebrae in almost all frogs is seven. This amphibian has no ribs.
The sacral region has one vertebra, and itconnects the spine and pelvic bones. The amphibian has no tail, but the caudal spine is one long bone, which was formed by several fused vertebrae.
The cervical region consists of only one vertebra and connects the head and spine. This skeleton of a frog differs from the structure of fish. They do not have such a section of the spine.
Muscular structure
The muscles of a frog are very different from the muscles of fish. She not only moves in the water, but also lives on land. The most developed muscles of the frog and toad are the muscles of the hind limbs. Thanks to them, they can make jumps. Unlike fish, frogs can move their heads slightly.
External description of the frog
What is the external structure of a frog? It consists of the body, head, fore and hind limbs. The boundary between the body and the trunk is not very clear, the neck is practically absent. The frog's body is slightly larger than the head. Features of the external structure of the frog is that it does not have a tail and it has practically no neck. The head is large. The eyes are large and slightly protruding. They are covered with transparent eyelids that prevent drying, clogging and damage. Below the eyes are the nostrils. The eyes and nostrils are at the top of the head and are above the water when swimming. This allows the amphibian to breathe air and control what is happening above the water. The upper jaw has a row of small teeth.
Frogs do not have ears as such, but behind each eye there is a small circle protected by skin. This is a tympanic membrane. Leatheramphibian soft and covered with mucus. Its feature is to shift relative to the body. This is because there is a large amount of space under the skin - the so-called lymphatic sacs. The skin of the frog is naked and thin. This makes it easier for liquids and gases to enter her body.
The peculiarity of a frog is that it can live without skin. This fact is evidenced by periodic molting, during which the animal sheds it, and then eats it.
Coloring
In most cases, amphibians mimic the environment. Therefore, the color repeats the pattern of the place where the frog lives. Some species have special cells that can change skin color depending on the environment.
In tropical areas, you can find amphibians, which are painted in very bright colors. This coloring means that the animal is poisonous. This scares off enemies.
There are many beautiful colors of this animal. In India, the rainbow frog lives, which is an object of worship. Her skin is dyed with all the colors of the rainbow.
Another unusual look is the glass frog. Her skin is completely transparent and her insides can be seen.
Poisonousness
Many species have venom glands in their skin that cause respiratory paralysis in predators if they try to attack. Other frogs produce mucus that causes blisters and burns on the skin upon contact.
On the territory of Russia live mostly only non-poisonous speciesfrogs. But in Africa, on the contrary, a large number of dangerous amphibians.
Earlier, frogs could be used to kill insects. For example, in 1935, a very poisonous cane frog was brought to Australia. But it did more harm than good. Because of its toxicity, it harms the ecosystem, but does not want to fight insect pests.
Movement
The frog has well-developed hind legs. The forelimbs are used primarily for support while sitting and for landing. The hind legs are longer and stronger than the front. Hind limbs are used for movement on water and land. The frog pushes off with force and lands on its front legs. This prevents her from being hit.
To move in the water, the frog also uses its hind legs. On the paws there are membranes that are stretched between the fingers. In addition, the fact that the frog is smooth and slippery from mucus makes it much easier to move in the water.
But movement is not limited to water and land. The external structure of the frog can provide them with movement in other places. Some species are able to glide in the air and climb trees. Features of some frog species are that they are equipped with special suction cups that help stick to different surfaces. Or have special growths.
Other amphibians know how to burrow into the ground, for example, the shuttle lady does it during the day. She goes hunting at night. Burial occurs due to horny calluses on the paws. Some species can wait out cold or drought underground. And the frogs that live in the desert can stay under the sand for up to three years.
Food
Adult toads and frogs feed on small invertebrates, insects, and in some cases vertebrates. Frogs are predators by nature. They may not disdain their relatives either.
The frog lies in wait for its prey motionless, sitting in a secluded corner. When she notices movement, she shoots out her long tongue and eats her prey.
Digestive system
The digestive system begins with the oropharyngeal cavity, to which a long tongue is attached. When the frog finds its prey, it "shoots" with this tongue, and the prey sticks to it. Although the toad has teeth, it does not chew food with them, but only holds the prey. After the amphibian has caught the victim, the food goes straight into the esophagus, and then into the stomach.
Respiratory system
Toads and frogs breathe with their lungs and through their skin. Their lungs are bag-shaped and have a network of blood 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.
Sense Organs
The frog's sense organs help it navigate on land and in water. In adult amphibians, as well as in fish, the organs of the lateral line are very developed. 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 alongwhole body, starting with the head of the frog.
Also, there are pain and temperature receptors on the skin. The tactile organ (nose) only works if the frog's head is above the surface of the water. In the water, the nasal cavities are closed.
Many amphibians have developed color vision.
Reproduction
Frogs begin to breed only in the third year of life. In the spring, when the mating season begins, the male chooses a female for himself and keeps her for several days. During this period, she can allocate up to 3 thousand eggs. They are covered with a mucous membrane and swell in water. The shell attracts sunlight to itself, which makes the development of eggs faster.
Frog Development
The frog embryo (tadpole) is in the egg for about one to two weeks. After this time, a tadpole appears. The internal and external structure of a frog is very different from that of a tadpole. Most of all, it looks like a fish. The tadpole has no limbs and uses its tail to move through the water. The tadpole breathes with the help of external gills.
Like fish and amphibians, the tadpole has a lateral line for orientation. At this stage, the frog embryo does not come ashore. Unlike the adult, the tadpole is herbivorous.
Gradually, metamorphoses occur with him: the tail disappears, paws appear, changes in the structure of the skeleton occur. And after about 4 months, a small frog appears, which is able to get out on land.
Record Frogs
Frogs that live in Europe usually do not grow more than 10 centimeters. But real giants can live in North America and Africa. The largest frog, the goliath frog, measures 90 centimeters in size and can weigh up to 6 kilograms.
Champion in jumping - African tree frog. She is capable of jumping up to 5 meters.
The African burrowing frog has the longest lifespan. She lives up to 25 years. This frog digs its own hole and lives there until the drought ends.
Recently, the smallest frog was discovered in New Guinea. Its length is 7.7 mm.
The record holder for toxicity doesn't look dangerous at all. This is a tiny frog about 3 centimeters long. It is the most venomous vertebrate on earth, including snakes. She lives in the rainforests of Colombia. The Indians smeared their arrows with her poison. The poison of one such frog was enough for 50 arrows.
