Frog is a typical representative of amphibians. On the example of this animal, you can study the characteristics of the entire class. This article describes in detail the internal structure of the frog.
Body covers
The lake frog lives in reservoirs and on their banks. It has a simple external structure - a flat wide head, smoothly turning into a short body, a reduced tail, short forelimbs with four fingers and elongated hind limbs with five. A drawing showing the skeleton and main organ systems will help to understand the internal structure of the frog.
First, let's study the skin of the animal. The body of the frog is covered with smooth bare skin with a large number of multicellular glands that secrete mucus. This secret lubricates the skin, helping to retain water, promoting gas exchange. In addition, it protects against harmful microorganisms.
The frog's thin and elastic skin not only protects and perceives external stimuli, but also plays an important role in gas exchange. In addition, the frog absorbs water exclusively through the skin. That is why she needs most of the time is indampness or water.
Skeleton
The structure of the frog skeleton has features in connection with adaptation to hocking movements. It consists of the skull, spine, girdles and limb skeleton. The skull is flattened, wide. In mature individuals, it retains a large amount of cartilaginous tissue, which makes frogs related to lobe-finned fish.
The short spine is represented by four sections: trunk, sacral, cervical and tail. The cervical region consists of only one ring-shaped vertebra, but thanks to its mobility, the frog can tilt its head.
The trunk section includes seven vertebrae. The animal has no ribs. The sacral region is also represented by a single vertebra, to which the pelvic bones are attached. The last, caudal, section is represented by a long bone, the urostyle, which is formed from 12 fused vertebrae.
The structure of the frog skeleton is interesting due to the peculiarities of the formation of the limbs, the belts of which connect the skeletons of the limbs with the spine. The forelimb belt includes the sternum, two shoulder blades, two crow bones and two collarbones, the forelimb itself consists of the shoulder, forearm and hand and four fingers (the fifth finger is in its infancy).
Girdle of the hind limbs due to the large load is more massive than the shoulder. It is represented by fused pelvic bones. The skeleton of the hind limbs includes the thigh, lower leg and foot with five fingers. The length of the hind legs is two to three times longer than the front.
Muscles
The muscles of the frog can be divided into segmented muscles of the trunk and limbs, part of the muscles of the trunk have a metameric structure (akin to the muscles of fish). The muscles of the hind limbs and jaws are especially well developed.
Digestive system
The structural features of a frog are clearly seen in the structure of its digestive system. All internal organs of an amphibian are located in the coelomic cavity. This is a kind of sac, the walls of which consist of epithelial cells. The cavity contains a small amount of fluid. Most of the bag is occupied by the digestive organs.
The digestive system begins with the oropharyngeal cavity. A tongue is attached to its bottom, which the frog uses to catch insects. Due to its unusual structure, it is able to throw itself out of its mouth with great speed and stick the victim to itself.
On the palatine bones, as well as on the lower and upper jaws of an amphibian, there are small conical teeth. They serve not for chewing, but primarily for holding prey in the mouth. This is another similarity between amphibian and fish. The secret secreted by the salivary glands moisturizes the oropharyngeal cavity and food. This makes it easier to swallow. Frog saliva does not contain digestive enzymes.
The frog's digestive tract begins with the pharynx. Next comes the esophagus, and then the stomach. Behind the stomach is the duodenum, the rest of the intestine is laid in the form of loops. The intestine ends with a cloaca. Frogs also have digestive glands - the liver and pancreas.
Caught with the help of the tongue, the prey is in the oropharynx, and then through the pharynx it enters the stomach through the esophagus. Cells located on the walls of the stomach secrete hydrochloric acid and pepsin, which contribute to the digestion of food. Next, the semi-digested mass goes to the duodenum, into which the secrets of the pancreas also pour out and the bile duct of the liver flows.
Gradually the duodenum passes into the small intestine, where all useful substances are absorbed. The remains of food that is not digested enter the last section of the intestine - a short and wide rectum, ending in a cloaca.
The internal structure of a frog and its larva is different. Adults are predators and feed mainly on insects, but tadpoles are real herbivores. Horny plates are located on their jaws, with the help of which the larvae scrape off small algae along with single-celled organisms living in them.
Respiratory system
Interesting features of the internal structure of the frog also concern breathing. The fact is that, along with the lungs, an amphibian skin filled with capillaries plays a huge role in the process of gas exchange. The lungs are thin-walled paired sacs with a cellular inner surface and an extensive network of blood vessels.
How does a frog breathe? Amphibian uses valves capable of opening and closing nostrils and bottom movementsoropharynx. In order to take a breath, the nostrils open, and the bottom of the oropharyngeal cavity descends, and the air enters the frog's mouth. In order for it to pass into the lungs, the nostrils close and the bottom of the oropharynx rises. Exhalation is produced by the collapse of the lung walls and the movements of the abdominal muscles.
In males, the laryngeal fissure is surrounded by special arytenoid cartilages, on which the vocal cords are stretched. The high volume of sound is provided by the vocal sacs, which are formed by the mucous membrane of the oropharynx.
Excretory system
The internal structure of the frog, or rather, its excretory system, is also very curious, since the waste products of an amphibian can be excreted through the lungs and skin. But still, most of them are excreted by the kidneys, which are located at the sacral vertebra. The kidneys themselves are elongated bodies adjacent to the back. These organs have special glomeruli that can filter decay products from the blood.
Urine passes through the ureters to the bladder, where it is stored. After filling the bladder, the muscles at the abdominal surface of the cloaca contract and the fluid is thrown out through the cloaca.
Circulatory system
The internal structure of a frog is more complex than that of fish. The heart of an adult frog is three-chambered, consisting of a ventricle and two atria. Due to the single ventricle, arterial and venous blood is partially mixed, the two circles of blood circulation are not completely separated. The arterial cone, which has a longitudinal spiral valve, departs from the ventricle and distributesmixed and arterial blood into different vessels.
Mixed blood is collected in the right atrium: venous blood comes from the internal organs, and arterial blood comes from the skin. Arterial blood enters the left atrium from the lungs.
The atria contract simultaneously, and blood from both enters the single ventricle. Due to the structure of the longitudinal valve, arterial blood enters the organs of the head and brain, mixed blood - to the organs and parts of the body, and venous - to the skin and lungs. It can be difficult for students to understand the internal structure of a frog. A diagram of the circulatory system of an amphibian will help visualize how blood circulation works.
The circulatory system of tadpoles has only one circulation, one atrium and one ventricle, like in fish.
The structure of the blood of a frog and a person is different. The erythrocytes of a frog have a nucleus, an oval shape, while in humans they have a biconcave shape, the nucleus is absent.
Endocrine system
The frog's endocrine system includes the thyroid, sex and pancreas, adrenal glands and pituitary gland. The thyroid gland produces the hormones necessary to complete metamorphosis and maintain metabolism, the gonads are responsible for reproduction. The pancreas is involved in the digestion of food, the adrenal glands help regulate metabolism. The pituitary gland produces a number of hormones that affect the development, growth and color of the animal.
Nervous system
Frog nervous systemcharacterized by a low degree of development, it is similar in characteristics to the nervous system of fish, but has more progressive features. The brain is divided into 5 sections: middle, intermediate, forebrain, medulla oblongata and cerebellum. The forebrain is well developed and is divided into two hemispheres, each of which has a lateral ventricle - a special cavity.
Due to monotonous movements and a generally sedentary lifestyle, the cerebellum is small in size. The medulla oblongata is larger. In total, ten pairs of nerves emerge from the frog's brain.
Sense Organs
Significant changes in the sense organs of amphibians are associated with the exit from the aquatic environment to land. They are already more complicated than those of fish, as they should help to navigate both in water and on land. Tadpoles have developed lateral line organs.
Pain, touch and temperature receptors are hidden in the epidermis layer. The papillae on the tongue, palate, and jaws function as organs of taste. The olfactory organs consist of paired olfactory sacs that open with both external and internal nostrils to the environment and the oropharyngeal cavity, respectively. In the water, the nostrils are closed, the organs of smell do not work.
As organs of hearing, the middle ear is developed, in which there is an apparatus that amplifies sound vibrations due to the eardrum.
The structure of the frog's eye is complex, because it needs to see both under water and on land. Movable eyelids and a nictitating membrane protect the eyes of adults. Tadpoles have no eyelids. The cornea of the frog eye is convex, the lens is biconvex. Amphibians see fairly far and have color vision.
