The frog is a typical representative of amphibians. Using this animal as an example, we can study the characteristics of the entire class. This article describes in detail the internal structure of the frog.
Integument
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 legs with five. It will help to understand the internal structure of the frog, a figure showing the skeleton and the main organ systems.
First, we study the skin of the animal. The frog's body is covered with smooth bare skin with a large number of multicellular glands secreting mucus. This secret lubricates the skin, helping to retain water, promoting gas exchange. In addition, it protects against harmful microorganisms.
The thin and elastic skin of the frog not only protects and perceives external stimuli, but also plays a large role in gas exchange. In addition, the frog absorbs water exclusively through the skin. That is why she needs most of the time is in damp or water.
Skeleton
The structure of the skeleton of a frog has features in connection with adaptation to hocking movements. It consists of a skull, spine, belts and skeleton of limbs. The skull is flattened, wide. In mature individuals, it retains a large amount of cartilaginous tissue, which makes relatives of frogs with cysterae fishes.
The short spine is represented by four sections: trunk, sacral, cervical and caudal. The cervical spine consists of only one annular vertebra, but thanks to its mobility, the frog can tilt its head.
The trunk includes seven vertebrae. There are no ribs in the animal. The sacral section is also represented by a single vertebra, to which the bones of the pelvis join. The last, caudal, department is represented by a long bone, urostyle, which is formed from 12 fused vertebrae.
The structure of the skeleton of a frog is interesting due to the features of the formation of the limbs, the belts of which connect the skeletons of the limbs with the spine. The belt of the forelimbs includes the sternum, two shoulder blades, two crow bones and two collarbones, the front limb itself consists of a shoulder, forearm and hand and four fingers (the fifth finger is in its infancy).
The belt of the hind limbs is more massive than the shoulder because of the heavy load. It is represented by fused bones of the pelvis. The skeleton of the hind limbs includes a hip, lower leg and foot with five fingers. The length of the hind legs is two to three times greater than the front.
Musculature
The muscles of the frog can be divided into segmented muscles of the trunk and limbs, some 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 the frog are clearly visible on the example of the structure of its digestive system. All the internal organs of the amphibian are in the coelomic cavity. This is a kind of sac, the walls of which are composed of epithelial cells. Inside 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. At its bottom is attached a tongue that the frog uses to catch insects. Due to the unusual structure, he is able to eject from his mouth with a high speed and stick the victim to himself.
On the palatine bones, as well as on the lower and upper jaw of an amphibian, there are small teeth of a conical shape. They do not serve for chewing, but primarily for keeping prey in the mouth. This is another similarity of amphibian fish. The secret secreted by the salivary glands moisturizes the oropharyngeal cavity and food. This makes swallowing easier. The digestive enzymes of the saliva of a frog does not contain.
The frog's digestive tract begins with a pharynx. This is followed by 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 bowel ends with cesspool. 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 help digest food. Next, the semi-digested mass follows into 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 remnants of food that has not been digested fall into the last section of the intestine - the short and wide rectum, ending in a cesspool.
The internal structure of the frog and its larva is different. Adults are predators and feed mainly on insects, but tadpoles are real herbivores. Horn plates are located on their jaws, with the help of which the larvae scrape off small algae together with the unicellular organisms living in them.
Respiratory system
Interesting features of the internal structure of the frog also relate to breathing. The fact is that along with the lungs, amphibian skin filled with capillaries plays a huge role in the gas exchange process. The lungs are thin-walled paired bags with a cellular inner surface and an extensive network of blood vessels.
How does a frog breathe? Amphibian uses valves that can open and close the nostrils, and the movements of the bottom of the oropharynx. In order to take a breath, the nostrils open, and the bottom of the oropharyngeal cavity lowers, and the air enters the mouth of the frog. So that it passes into the lungs, the nostrils close, and the bottom of the oropharynx rises. The exhalation is due to the falling of the pulmonary walls and the movements of the abdominal muscles.
In males, laryngeal fissure is surrounded by special arytenoid cartilage, over which the vocal cords are stretched. High sound volume is provided thanks to the voice bags, 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 amphibian vital products can be excreted through the lungs and skin. But still, most of them are secreted 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 capable of filtering decay products from the blood.
Urine in the ureters is excreted into the bladder, where it accumulates. After filling the bladder, the muscles at the abdominal surface of the cloaca contract and the fluid is expelled through the cloaca to the outside.
Circulatory system
The internal structure of the 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 a single ventricle, arterial and venous blood partially mixes, two circles of blood circulation are not completely disconnected. An arterial cone having a longitudinal spiral valve departs from the ventricle and distributes the mixed and arterial blood to different vessels.
Mixed blood is collected in the right atrium: venous comes from the internal organs, and arterial from the skin. Arterial blood enters the left atrium from the lungs.
The atria are reduced simultaneously, and blood from both ends up in the only ventricle. Due to the structure of the longitudinal valve, arterial blood enters the organs of the head and brain, mixed - to the organs and parts of the body, and venous - to the skin and lungs. It may be difficult for students to understand the internal structure of a frog. The circulatory system of amphibians will help to visualize how blood circulation works.
The tadpole's circulatory system 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 frog erythrocytes have a nucleus, an oval shape, and in humans - a biconcave shape, the nucleus is absent.
Endocrine system
The frog's endocrine system includes the thyroid, reproductive, and pancreas, adrenal glands, and the pituitary gland. The thyroid gland produces the hormones necessary to complete metamorphosis and maintain metabolism; the sex glands 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
The nervous system of the frog is characterized 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, anterior, 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 has a small size. The medulla oblongata is larger. In total, ten pairs of nerves exit the frog’s brain.
Sensory organs
Significant changes in the sensory organs of amphibians are associated with the exit from the aquatic environment to land. They are already more complicated than fish, as they should help to navigate both in water and on land. Tadpoles have developed lateral line organs.
Pain, tactile and temperature receptors are hidden in the epidermis layer. The papillae on the tongue, palate, and jaw function as organs of taste. The olfactory organs consist of paired olfactory sacs, which open both the external and internal nostrils, into the environment and the oropharyngeal cavity, respectively. In the water, the nostrils are closed, the sense of smell does not work.
As hearing organs, 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 be seen both under water and on land. Moving eyelids and a blinking membrane protect the eyes of adults. Tadpoles have no eyelids. The cornea of the frog’s eye is convex, the lens is biconvex. Amphibians see far enough and have color vision.