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 ambient temperature. 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 ponds or in holes.
Origin of Amphibians
The first amphibians appeared about 300 million years ago. The external structure of the frog, their lifestyle and close connection with water indicate that amphibians 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 the carp-fish.
Also, prehistoric frogs had fins and lungs that emerged from the swimming bladder. And they had a tail that a modern frog does not have.
Frogs lived only in fresh water and with the help of fins could crawl overland, moving from one reservoir to another. But the development of the frog went further, and in the process of evolution, limbs appeared in it.
Habitat
Frogs spend a significant part of their life 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 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 ponds. Frogs also live in places with high humidity: in swamps, in tropical forests. There are species that live on trees and almost never get off them.
Skeleton
The skeleton of a frog is very similar to the skeleton of a perch, but due to the nature of its lifestyle it has a number of features. The most important difference is the presence of limbs. The forelimbs are connected to the spine using the bones of the limb belt. The hind limbs are attached to the spine of the hip bone.
A frog skull has fewer bones than a fish skull. But the gill bones and gill covers are missing. Breathing occurs with the help of the lungs.
The frog spine consists of 9 vertebrae and has 4 departments: cervical, trunk, sacral and caudal. The vertebrae of the body are percussion, are equipped with upper arches and limit the spinal canal. The number of vertebrae in almost all frogs is seven. This amphibian has no ribs.
The sacral section has one vertebra, and it connects the spine and pelvic bones. The amphibian has no tail, but the caudal spine is one long bone, which was formed due to several fused vertebrae.
The cervical spine consists of only one vertebra and connects the head and spine. This differs the skeleton of a frog from the structure of fish. They have no such spine.
Muscle structure
The muscles of the frog are very different from the muscles of the fish. She not only moves in 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 jump. Unlike fish, frogs are able to slightly move their heads.
The external description of the frog
What is the external structure of the frog? It consists of the trunk, head, fore 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 larger than the head. The external structure of the frog is peculiar in that it has no tail and the neck is practically absent from it. The head is large. The eyes are large and a little bulging. They are covered with transparent eyelids that prevent drying out, clogging and damage. Below the eyes are the nostrils. The eyes and nostrils are located in the upper part of the head and are above water during swimming. This allows the amphibian to breathe air and control what is happening above the water. There are a number of small teeth on the upper jaw.
The frogs do not have ears per se, but behind each eye there is a small circle protected by skin. This is the eardrum. The amphibian skin is 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 lymph bags. The skin of the frog 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 evidenced by periodic molting, during which the animal discards 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 that are painted in very bright colors. Such a coloring means that the animal is poisonous. This scares off enemies.
There are many beautiful colors of this animal. In India, a rainbow frog lives, which is the subject of worship. Her skin is dyed with all the colors of the rainbow.
Another unusual look is a glass frog. Her skin is completely transparent and her insides can be seen.
Virulence
Many species have poisonous glands in their skin that cause respiratory paralysis in predators if they try 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-toxic 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 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 mainly used for support in a sitting position and for landing. The hind legs are longer and stronger than the front. The hind limbs are used to travel on land and water. The frog pushes with force and lands on its front legs. This protects her from being hit.
The frog also uses hind legs to move in the water. On the legs there are membranes that are stretched between the fingers. In addition, it is much easier to move in the water because the frog is smooth and slippery from mucus.
But movement is not limited only to water and land. The external structure of the frog can provide them with movement in other places. Individual species are able to plan in the air and climb trees. The peculiarities of frogs of some species are that they are equipped with special suction discs that help stick to different surfaces. Or have special growths.
Other amphibians can bury themselves in the ground, for example, the shuttle does this during the day. She goes hunting at night. Instillation occurs due to corneous calluses on the legs. Some species may bide cold or drought underground. And the frogs that live in the desert are able to be under the thickness of sand for up to three years.
Food
Adult toads and frogs feed on small invertebrate animals, insects and, in some cases, vertebrates. Frogs are predators by nature. They may not disdain their relatives.
A frog lies in wait for its victim motionless, sitting in a secluded corner. When she sees movement, she shoots with 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 a frog finds its prey, it “shoots” with this tongue, and 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, food enters the esophagus and then into the stomach.
Respiratory system
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.
Sensory organs
The senses of the frog help to navigate it on land and in water. In adult amphibians, as well as in fish, lateral line organs are very developed. These organs help to navigate in space. The largest number of them is located on the head. The organs of the lateral line look like two longitudinal strips along the entire body, starting with the frog’s head.
Also on the skin are pain and temperature receptors. The tactile organ (nose) works only if the frog’s head is above the surface of the water. In water, the nasal cavities are closed.
Many amphibians have developed color vision.
Breeding
Frogs begin to breed only in the third year of life. In the spring, when the mating season begins, the male chooses a female and holds it for several days. During this period, she 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.
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 the frog is very different from the structure of the tadpole. Most of all, he looks like a fish. The tadpole has no limbs; a tail is used to move in water. The tadpole breathes with the help of external gills.
Like fish and amphibians, the tadpole has a side line for orientation. At this stage, the frog embryo does not go to land. Unlike an adult, the tadpole is herbivorous.
Gradually metamorphoses occur with it: 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 to 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 - reaches 90 centimeters in size and can weigh 6 kilograms.
Champion in jumping - African tree frog. She is able to jump over distances of up to 5 meters.
The longest life span of an African digging frog. She lives up to 25 years. This frog digs its hole and lives there until the drought ends.
More recently, the smallest frog was discovered in New Guinea. Its length is 7.7 mm.
The record holder for toxicity 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 rainforests of Colombia. Its venom Indians smeared arrows. The poison of one such frog was enough for 50 arrows.