External structure and lifestyle of the lake frog. Features of the external structure and movement of a frog in connection with its habitat - Knowledge Hypermarket
Like Frogs are one of the most numerous species of amphibians. Features of external and internal structure frogs are characteristic of most individuals from this class.
External structure of a frog
The frog's body is short, the large flat head without sharp boundaries merges into the body. Unlike fish, the head of amphibians is movably articulated with the body. Although the frog does not have a neck, it can tilt its head slightly.Two large bulging eyes are noticeable on the head, protected by eyelids: leathery - upper and transparent movable - lower. The frog blinks frequently, while wet skin the eyelid moistens the surface of the eyes, protecting them from drying out. This feature developed in the frog in connection with its terrestrial lifestyle. (Fish, whose eyes are constantly in the water, do not have eyelids.) A pair of nostrils is visible on the head in front of the eyes. These are not only the openings of the olfactory organs. The frog is breathing atmospheric air, which enters her body through the nostrils. The eyes and nostrils are located on the upper side of the head. When the frog hides in the water, it puts them out. At the same time, she can breathe atmospheric air and see what is happening outside the water. Behind each eye on the frog's head there is a small circle covered with skin. This outer part organ of hearing - eardrum. The inner ear of a frog, like that of fish, is located in the bones of the skull.
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All modern amphibians have naked skin. In a frog, it is always moist thanks to the liquid mucous secretions of the skin glands. Water from environment enters the frog's body through the skin and with food. The frog never drinks.
Frog skeleton
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A frog's skull has fewer bones than a fish's skull. Due to pulmonary respiration, the frog does not have gills.
The skeleton of the limbs corresponds to their division into three sections and is connected to the spine through the bones of the limb girdles. The girdle of the forelimbs - the sternum, two crow bones, two clavicles and two shoulder blades - has the appearance of an arc and is located in the thickness of the muscles. The hind limb girdle is formed by fused pelvic bones and is attached tightly to the spine. It serves as a support for the hind limbs.
Internal structure of a frog
Muscles
Structure muscular system frogs are much more complex than fish. After all, the frog not only swims, but also moves on land. Through contractions of muscles or muscle groups, the frog can perform complex movements. Her limb muscles are especially well developed.Digestive system
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Respiratory system
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Circulatory system
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The special arrangement of the vessels originating from the ventricle leads to the fact that only the frog’s brain is supplied with pure arterial blood, while the whole body receives mixed blood.
In a frog, blood from the ventricle of the heart flows through the arteries to all organs and tissues, and from them through the veins it flows into the right atrium - this is a large circle of blood circulation. In addition, blood flows from the ventricle to the lungs and skin, and from the lungs back to the left atrium of the heart - this is the pulmonary circulation. All vertebrates, except fish, have two circles of blood circulation: small - from the heart to the respiratory organs and back to the heart; large - from the heart through the arteries to all organs and from them back to the heart.
Metabolism
Metabolism in amphibians is slow. The frog's body temperature depends on the ambient temperature: it increases in warm time and goes down in the cold. When the air becomes hot, the frog's body temperature decreases due to the evaporation of moisture from the skin. Like fish, frogs and other amphibians are cold-blooded animals. Therefore, when it gets colder, frogs become inactive, and during the winter they go into hibernation.Central nervous system and sensory organs
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The structure of the sense organs corresponds terrestrial environment. For example, by blinking its eyelids, a frog removes dust particles stuck to the eye and moistens the surface of the eye. Like fish, the frog has an inner ear. However, sound waves travel much worse in air than in water. Therefore, for better hearing, the frog also has a middle ear. It begins with the sound-receiving eardrum - a thin round film behind the eye. From it, sound vibrations are transmitted through the auditory bone to the inner ear.
Reproduction and development of amphibians
Reproductive organs
The reproductive organs of amphibians are very similar in structure to the reproductive organs of fish. All amphibians are dioecious.Spawning
Having spent the winter in a state of torpor, amphibians with the first rays spring sun wake up and soon begin to reproduce. Males of some species of frogs croak loudly. The amplification of sounds is facilitated by special bags - resonators, which, when croaking, swell on the sides of the male’s head. When breeding, animals split into pairs. The germ cells enter the cloaca through tubular ducts and are thrown out from there. Female amphibians lay eggs in the water, similar to fish eggs. Males release fluid containing sperm onto her.Development
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Low temperature delays development. If the weather is warm, the egg divides repeatedly and develops into a multicellular embryo. After one or two weeks, the frog larva, a tadpole, hatches from the egg. Outwardly, it resembles a small fish with a large tail. The tadpole first breathes through external gills (in the form of small tufts on the sides of the head). Soon they are replaced by internal gills. The tadpole has one circulation and a two-chambered heart; a lateral line is visible on the skin. Thus, amphibian larvae have some structural features of fish.
During the first days, the tadpole lives off the nutritional reserves of the eggs. Then a mouth appears, equipped with horny jaws. The tadpole begins to feed on algae, protozoa and other aquatic organisms. Further changes in the tadpole occur the sooner the more hotter weather. First his hind legs appear, then his front legs. The lungs are developing. The tadpole begins to rise to the surface of the water and swallow air. The tail gradually shortens, the tadpole becomes a young frog and comes ashore. From the moment the eggs are laid until the end of the transformation of the tadpole into a frog, about 2-3 months pass. Baby frogs, like adult frogs, eat animal food. They can reproduce from the third year of life.
Features of the external and internal structure of amphibians using the example of a frog.
The frog is a typical representative of amphibians. Its body is divided into a head, a torso and two pairs of limbs. On the head there are large eyes (only notices moving prey) with movable eyelids, nostrils, eardrums and mouth. The hind limbs are longer and stronger than the forelimbs. The skin is bare, with a large number of glands. The mucus they secrete moisturizes the skin, which promotes gas exchange. The skeleton consists of a skull, a trunk skeleton and a limb skeleton. The skull is mainly cartilaginous. It is connected by a joint to the cervical vertebra, which provides some mobility of the head. The spine consists of the cervical, trunk, sacral sections and the caudal bone - urostyle. The cervical and sacral sections consist of one vertebra each, the trunk - of 7. In the skeleton of the forelimb, the humerus, bones of the forearm and hand are distinguished, and in the skeleton of the hind limb - the femur, bones of the lower leg and foot. The shoulder girdle is represented by paired shoulder blades, clavicles, crow bones and the unpaired sternum bone. The hind limb girdle consists of the pelvic bones. The digestive system begins with the oropharyngeal cavity with a long tongue and ducts salivary glands. Next comes the esophagus, stomach, intestines, consisting of the duodenum and small intestine, rectum, ending in the cloaca. There is a liver with a gallbladder and a pancreas. The frog is a predator, feeds on insects and earthworms. Adult animals breathe through their lungs, while larvae breathe through their gills. Since there is no chest, breathing is carried out by contracting the muscles of the floor of the mouth. Skin respiration plays an important role. The heart has three chambers - two atria and a ventricle. In connection with the appearance of pulmonary respiration, a second circle of blood circulation appears - pulmonary or pulmonary. The blood in the ventricle is partially mixed. Excretory organs are paired trunk kidneys. Through the ureters, urine first enters the cloaca, and then into the bladder. The accumulated urine is discharged through the cloaca. The brain has the same sections as those of fish. But amphibians have a better developed forebrain and a weaker cerebellum. Sense organs: eyes with eyelids, middle ear with one auditory ossicle and eustachian tube, externally limited by the eardrum. There are olfactory organs. Frogs are dioecious. Fertilization is external; development with metamorphosis. Larvae (tadpoles) develop in water, breathe with gills, have one circulation, a two-chambered heart and lateral line organs. Metamorphosis is accompanied by significant changes in most organs.
Amphibians, or amphibians, are cold-blooded predatory animals that thrive both in water and on land. Initially, they breathe using gills, and then adults switch to pulmonary breathing. The article will examine in detail the internal structure of amphibians using the example of a frog.
Habitat
Amphibians live in two environments: on land and in water, they jump well and swim well, and even climb trees. Thanks to their characteristics, they feel great both in damp places (swamps, wet forests and meadows), and on the banks of freshwater bodies. The entire development process takes place in water. There they reproduce, the development of larvae occurs, as well as the growth of fry, and only mature individuals are found on land.
The behavior of frogs also depends on the humidity of the environment. They can't stand it sunny weather, and in the evening and rainy days go hunting. Those that live in or near water also look for food during the daytime. With the onset of cold weather, animals burrow into the silt at the bottom of reservoirs and spend the entire cold season there. They can breathe through their skin, so there is no need to rise to the surface. Some animals spend the winter season on the surface of the earth, burrowing under heaps of fallen leaves and large stones. All processes in the body slow down and only with the arrival of warmth do they, even from a frozen state, return to normal life.
Characteristics of the external structure of the frog
Schoolchildren usually study the internal structure of a frog in 7th grade. However, first let’s get acquainted with the external structure. The frog's body consists of a head and a body with a length of 8 mm to 32 cm. The color can be solid (green, brown, yellow) or variegated. The cervical region is not pronounced, the head immediately passes into the body. The animal has developed fore and hind limbs. The skin is bare and mucous, the horny ones are poorly developed. The epidermis contains a large number of multicellular glands that produce a mucous substance that protects the skin from drying out. Typically ground limbs the five-fingered type have a complex muscular structure. Hind limbs due to in a special way movements have become more developed than the front ones, which consist of the shoulder, forearm and hand. There are four toes; males have a swelling at the base of the inner one, which is a genital wart. The long hind limb consists of a thigh, lower leg and foot, which has five toes connected by a swimming membrane.
frog head
On the flat head there are:
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External and internal structure of a frog
The frog, like all amphibians, can be without water for a long time, but it needs it to reproduce. Having changed, the larvae lose their resemblance to fish and turn into amphibians. The body is elongated, there are two pairs of limbs. The head, which goes into the body, unlike fish, is capable of turning. The skeleton consists of bones, although there is a lot of cartilage; The spine has many vertebrae. There are no ribs, which means there is no chest. Thanks to its strong skeleton and developed muscles, the animal is adapted for life on land. The hind and forelimbs have three joints. The skin is smooth and contains many glands to moisturize it. The frog breathes through the lungs and skin.
The structure of the internal organs of the frog suggests the presence of a three-chambered heart, consisting of one ventricle and two atria, as well as two circulation circles. Food passes from the pharynx through the esophagus, stomach and short intestine. To digest it, secretions are produced by the liver, stomach walls and pancreas. At the end of the rectum there is a cloaca into which the female's oviduct opens. Animals have two kidneys and a bladder. The small braincase contains the developed forebrain and cerebellum. Frogs have organs of vision, hearing, touch, taste, and smell.
Internal structure of a frog
The muscles have a rather complex structure and are quite well developed compared to fish. Thanks to the coordinated work of a group of muscles, the frog can move, and in addition, they also take part in breathing.
The skeleton includes the following sections: spine, girdles and skeleton of the limbs, skull. The latter is connected to the spine using the cervical vertebra. This makes it possible to tilt your head. IN trunk section seven vertebrae, no ribs. The sacral region, like the cervical region, is represented by one vertebra. The long bone forms tail section. The thighs, legs, and feet form the hind limbs, and the shoulders, forearms and hands form the forelimbs. They are connected to the spine using a girdle of limbs: anterior and posterior. The first includes two shoulder blades and the sternum, and the second - pelvic bones, which have grown together.
Nervous system
The frog's nervous system is more complex than that of fish. Its internal structure is as follows: nerves, spinal cord and brain. The latter has three sections: a more developed, compared to fish, forebrain and a small cerebellum, since frogs lead a sedentary lifestyle and make only monotonous movements, as well as the cerebral hemispheres. Adults have developed upper and lower eyelids, as well as a nictitating membrane, thanks to which the cornea does not dry out and is protected from contamination.
Circulatory system
The circulatory system is represented by a three-chambered heart. From the lungs, arterial blood enters the left atrium. The right atrium receives venous blood from the internal organs, and arterial blood from the dermis.
With the simultaneous contraction of the atria, blood enters the ventricle. With the help of a special valve, venous blood flows to the lungs and skin, and arterial blood flows to the brain and head organs. It reaches all other organs, as well as parts of the body. mixed blood. The frog has two circles of blood circulation, and they are united by a common ventricle.
Respiratory system
The skin takes part in breathing, and the internal structure of the frog allows breathing with the help of the lungs, which have a network of blood vessels.
The frog opens its nostrils, the bottom of the oropharyngeal cavity drops and air enters it. Next, the nostrils close, and the bottom rises, and air enters the lungs. When the pulmonary walls collapse and the abdominal muscles contract, exhalation occurs.
Digestive system
It begins with a rather large oropharyngeal cavity. When it sees prey, the frog throws out its tongue and the prey sticks to it. Small teeth are located on the upper jaw and serve to hold prey. The structure and activity of the frog's internal organs contribute to the processing of food. It is moistened by the secretion of the salivary glands in the oropharyngeal cavity and enters the esophagus, and then into the stomach. Incompletely digested food enters the duodenum and then into the small intestine, where nutrients are absorbed. Undigested residues exit through the cloaca, having first passed through the rectum (hind) intestine.
Excretory system
On the sides of the sacral vertebra there are two kidneys containing glomeruli and filtering waste products and some nutrients from the blood.
The latter are absorbed in the renal tubules. Urine enters the bladder, having first passed through the ureters and cloaca. The internal structure of the frog allows the muscles of the bladder to contract when it is full. Urine enters the cloaca and then comes out.
Metabolism
It proceeds quite slowly. The frog's body temperature also depends on the ambient temperature. In cold weather it decreases and increases in warm period. IN extreme heat Due to the evaporation of moisture from the skin, the animal’s body temperature decreases. Due to the fact that these are cold-blooded animals, when cold weather sets in, they become inactive, choosing warmer places. And in winter they completely hibernate.
Sense organs
The structure and functions of the frog’s internal organs help it adapt to its living conditions:
- The frog is able to blink, having a movable upper eyelid and the so-called nictitating membrane. It moistens the surface of the eye and removes dirt particles stuck to it. The animal reacts more to a moving object, but the stationary object does not see well enough.
- The hearing aid consists of the inner and middle ear. The latter is a cavity that opens on one side into the oropharynx, and the other goes to the surface of the head, separated from the external environment by the eardrum, which is connected to the inner ear with a stirrup. Through it, sound vibrations are transmitted to the inner ear from the eardrum.
- The animal is quite good at navigating by smell. The olfactory organs communicate with the external environment through the nostrils.
Conclusion
Thus, the peculiarities of the internal structure of the frog, like other amphibians, consist in a more complex structure of the nervous system, as well as sensory organs. In addition, they have lungs and two circulation circles.
January 11, 2018The frog is typical representative amphibians. Using this creature as an example, you can easily study the most important characteristics class. To do this, we will look at the internal and external structure of the lake frog, which will further help us understand the structural features of all amphibians.
External structure
The external structure of a frog and its habitat are two inextricably linked links in a single chain. It's no secret that appearance and the structure is determined by the place and lifestyle of the animal.
The lake frog lives on the banks and in reservoirs, which is why it got its name. However, a river, swamp, pond and other places can also serve as a place of residence.
The external structure of the frog is very simple. The wide flat head goes into small body with a reduced tail. The frog has very short forelimbs equipped with four fingers. The hind limbs, on the contrary, are elongated and have five fingers.
Covers of an animal
Speaking about the external structure of a frog, it is necessary to pay attention to its skin. The body of an amphibian creature is covered with smooth skin, on the surface of which there is an impressive number of glands that constantly secrete mucus. The secretion lubricates the surface, helping to retain moisture and promoting gas exchange. Mucus also protects the animal from penetration of harmful microorganisms into the body.
The amphibian's thin skin protects the body and also participates in gas exchange. Peculiarities external structure frogs are associated with her way of life. For example, water enters the animal’s body only through the skin. It is for this reason that the frog must spend most of its time in dampness or water.
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Characteristics of the external structure of the frog: body parts
Speaking about the external structure of the frog, we can distinguish the following parts of the body - the hind and forelimbs, the head and the torso. One of the features of the amphibian is the almost complete absence of a neck. In general, the separation line between the body and the head is not clear, or rather, it is practically non-existent. The frog's body is slightly larger than its head. The animal completely lacks a tail.
The large head has big eyes stick out. They are covered by clear eyelids designed to prevent damage, drying out and clogging.
Below the eyes are the nostrils. In general, it is worth noting that the eyes and nostrils are located close to each other at the top of the head for a reason. The fact is that while swimming they are constantly above the surface of the water. Thanks to this, the animal can breathe and see everything that happens above the lake.
The upper jaw of frogs is armed with a row of small teeth. But they don't have ears. Their role is played by the eardrums located behind the animal’s eyes.
Animal color
Describing the external structure of the frog, let us pay attention to its color, which is largely determined by its habitat. Most amphibians mimic external nature. And some varieties even have special cells that can change the color of the skin depending on environmental conditions. Therefore, the color of the animal often repeats the pattern of the places where the creature lives.
In the tropics there are amphibians that have bright colors. This indicates that the animal is very poisonous. This is what scares away enemies from such frogs.
Amphibian limbs
When presenting the external structure of the lake frog, we mentioned the unusual limbs of the animal. The peculiarity of their structure is due to the fact that the front legs are used for landing and for support in a sitting position. The hind limbs are much stronger and longer than the forelimbs. It is the hind legs that are used to move through water or on land. There are membranes on the limbs of the animal, which greatly facilitate movement in the water. The external structure of the frog is such that it allows the animal to move not only in water, but also on land.
Some representatives can even climb trees or glide in the air. Frogs have a lot of adaptations depending on their habitat. Nature has endowed some with special suction cups, thanks to which they stick to any surface, while others can remain under layers of sand or soil for a long time. In all these cases, their strong limbs help amphibians move.
The skeleton of a frog, according to biologists, is very similar to the skeleton of a perch. However, due to the nature of the lifestyle, there are significant differences. As we know, frogs have limbs. The front ones are attached to the spine with the help of peculiar bones. But the hind ones are connected to the spine with the help of the hip bones. The structure of the skull of an amphibian is much simpler than that of fish, since it has fewer bones.
The frog's spine consists of a total of nine vertebrae. Moreover, it is divided into four sections: cervical, sacral, caudal and trunk. But the animal has no ribs.
Digestive system
Frogs and toads feed on insects and small invertebrates. Such animals are classified as predators. Sometimes they even disdain their relatives. Frogs guard their prey in a secluded place, motionless. As soon as the prey approaches, they shoot out their tongue and consume the prey.
The digestive system of frogs has its own characteristics. The fact is that in the oropharyngeal cavity of the animal there is long tongue. It is with this that the frog catches its prey. After the prey has stuck to the surface of the tongue, it enters the oral cavity. The animal does not use its teeth to chew food; it only needs them to hold the victim. Next, the food immediately enters the esophagus and stomach. And then the mass moves into the duodenum, which passes into the small intestine. The remains of undigested food enter the large intestine.
The internal structure of the green frog is more complex than that of fish. The fact is that the animal has a three-chambered heart. It is located in the front of the body. Consists of two atria and a ventricle. There is a gradual alternating contraction of both atria and the ventricle.
The right atrium contains only venous blood, and the left atrium contains arterial blood. But it is in the ventricle that their mixing occurs.
The peculiar arrangement of the vessels led to the fact that the head of the animal is supplied with arterial blood, but the rest of the body receives mixed blood. Frogs have two circles of blood circulation.
Amphibian Blood
Blood is a connective tissue that performs very important role in any organism. It is she who transports metabolic products and nutrients. As you know, blood consists of red blood cells and white blood cells.
The external structure and shape of frog red blood cells differs from human red blood cells. In the animal they are oval and have nuclei. But in humans, red blood cells are biconcave and lack nuclei. This increases the area occupied by oxygen molecules. This means that the human circulatory system is more perfect. In addition, frog red blood cells are larger and there are much fewer of them than in human blood. All this suggests that amphibians need oxygen in much smaller quantities than mammals. And the reason for this is the ability to assimilate part required oxygen surface of the skin.
It also has its own characteristics respiratory system frogs. Amphibians breathe not only with the help of their lungs. The skin also plays an active role in this process. As you can see, outer coverings play a vital role in the life of amphibians. The lungs of an animal are thin-walled paired sacs that have a cellular internal structure and a highly branched network of blood vessels.
How does breathing occur? Amphibians use valves that close and open their nostrils. During inhalation, the nostrils open and the oropharyngeal cavity descends. This is how air gets inside. In order for it to get further into the lungs, the nostrils close, and the bottom of the oropharynx, on the contrary, rises. Exhalation is carried out due to the work of the walls of the lungs and the movement of the abdominal muscles.
Sense organs
The sense organs of amphibians have their own characteristics, due to the fact that animals also live on land. The lateral line organs help the frog navigate in space. The largest number of them is located on the head. Outwardly, they resemble two stripes that run along the body.
Temperature and pain receptors are located on the skin of amphibians. The nose as a tactile organ only works if it is located above the water. In water, the nostrils are always closed. Even the respiratory system is fully adapted to the amphibian’s habitat both in water and on land.
Nervous system of a frog
And here nervous system amphibians are not very developed. It is in many ways similar to the nervous system of fish. In this regard, amphibians are not far along the evolutionary ladder. The amphibian brain consists of five sections: anterior, middle, intermediate, medulla, cerebellum. By the way, the latter is poorly developed due to sedentary lifestyle animal life. But the forebrain consists of two hemispheres and, on the contrary, is quite developed.
Lesson 10. INTERNAL STRUCTURE OF AMPHIBIDES USING THE EXAMPLE OF THE FROG OF THE GENUS RANA
Equipment and materials
1. Freshly killed frogs (one for two students).
2. Ready-made preparations: 1) dissected frog; 2) digestive system; 3) injected circulatory system; 4) excretory organs; 5) reproductive organs; 6) brain.
3. Tables: 1) appearance of the frog; 2) general location internal organs; 3) digestive system; 4) respiratory organs; 5) circulatory system; 6) excretory organs; 7) reproductive organs of the male and female; 8) brain.
4. Dissecting instruments: scalpel; scissors; tweezers; dissecting needle; stationery pins (one set for two students).
5. Baths (one for two students).
6. Glass tubes with a drawn-out spout, connected to a rubber bulb (2 - 4 per group).
Introductory Notes
Amphibians, or amphibians, are the first relatively small group of primitive terrestrial vertebrates. However, they still maintain a close connection with the aquatic environment. This is most fully manifested during the period of embryonic and initial postembryonic development. The laying of caviar (eggs) and its development in the vast majority of amphibians occurs in water. The larvae emerging from the eggs - tadpoles - also live in aquatic environment. They have the characteristics of typical aquatic animals: gill breathing, a two-chambered heart, one circle of blood circulation, lateral line organs, etc. After metamorphosis, amphibians acquire the characteristics of terrestrial vertebrates.
Pulmonary breathing is characteristic of adult amphibians. The circulatory system changes accordingly: the heart becomes three-chambered; pulmonary circulation occurs; the branchial arteries are replaced by homologous carotid arteries, systemic aortic arches and pulmonary arteries. The posterior vena cava, characteristic of terrestrial vertebrates, appears. The sense organs are noticeably improved: the shape of the cornea of the eye becomes convex, the lens becomes lenticular, movable eyelids and the middle ear cavity with the eardrum and auditory bone - the stirrup - appear. Digestive tract much more differentiated than in fish. Ground limbs of the five-fingered type appear. The limb girdles become more complex. The belt of the hind limbs is firmly connected to the axial skeleton etc.
However, despite these transformations, amphibians are still poorly adapted to living on land. This is reflected in poor development of the lungs, and therefore bare skin plays an important role in the breathing process. The skin, which is easily permeable to gases and water, does not protect the body from drying out, which necessitates the need to constantly replenish water losses. At the row aquatic species External gills are preserved for life, so many experts consider amphibians to be a transitional group between fish and true terrestrial vertebrates. The three-chambered heart does not provide complete separation of blood, and more or less mixed blood is distributed throughout the body. The limbs are still poorly developed and cannot hold the body in an elevated position above the ground. Moche reproductive system in almost all amphibians it is not fundamentally different from that of fish. Amphibians, like fish, are characterized by poikilothermy (inconstancy of body temperature).
Consider the features of the internal structure of the frog.
Digestive system: oropharyngeal cavity; teeth; esophagus; stomach; duodenum; small and rectal intestine; liver; gallbladder; pancreas.
Respiratory system: laryngeal fissure; larynx; bronchi; lungs.
Circulatory system: three-chambered heart (two atria and a ventricle); abdominal aorta; two systemic aortic arches; anterior vena cava, posterior vena cava, two circles of blood circulation. Using the preparation and drawing, trace the blood circulation pattern.
Excretory organs: kidneys; ureters; bladder.
Reproductive organs: testes; vas deferens; seminal vesicles; ovaries; oviducts; fat bodies.
Central nervous system: brain (large hemispheres of the forebrain with the olfactory lobe, diencephalon, optic lobes of the midbrain, cerebellum, medulla oblongata); spinal cord.
Sketch:
1) general location of internal organs; 2) brain (top view); 3) diagram circulatory system(homework).
Internal structure
Opening
Whenever possible, freshly killed frogs are most convenient for dissection. large sizes. Animals are killed
Rice. 39. Dissected frog:
1 - heart; 2 - lung; 3 - liver; 4 - gallbladder; 5 - stomach, 6 - pancreas; 7 - duodenum; 8 - small intestine; 9 - rectum; 10 - spleen; 11 - cloaca; 12 - bladder; 13 - kidney; 14 - ureter; 15 - right ovary (left ovary removed); 16 - fat body; 17 - right oviduct; 18 - uterine section of the oviduct; 19 - dorsal aorta; 20 - posterior vena cava; 21 - carotid artery; 22 - left aortic arch; 23 - pulmonary artery
20 - 30 minutes before the start of class. For this purpose, frogs are placed in a tightly closed vessel containing cotton wool, abundantly moistened with chloroform or ether.
Place the frog in the bath with its belly up and, stretching its limbs, attach them with pins. After pulling back the skin at the back of the abdomen with tweezers, use scissors to make a small transverse incision in front of the base of the limbs. Then insert the scissors into the hole formed and from there make a longitudinal incision in the skin along the midline of the body up to the chin. In order not to damage the underlying organs, when cutting, it is necessary to pull the scissors upward. At the level of the forelimbs, cut the skin perpendicular to the longitudinal section to the base of the forelimbs. Turn the resulting flaps of skin to the sides and secure them with pins. After this, look at the opened muscles and some blood vessels.
In the middle part of the body, above the abdominal cavity, lies the rectus abdominis muscle, divided by transverse tendon septa into separate segments. In the area of the forelimbs there is a paired pectoral muscle, which extends from the middle of the body (from the sternum) in three bundles to the forelimbs. Ahead pectoral muscle between the branches of the lower jaw there is a submandibular muscle, which plays an important role in the breathing mechanism. Noteworthy is the dark blood vessel - the abdominal vein, which stretches along the midline of the rectus abdominis muscle. In addition, a large number of vessels are found located on the inner surface of the skin. These are branches of the skin arteries and veins.
Continuing with the dissection, cut through the wall of the body cavity. The longitudinal incision should not be made along the midline, but to the side of the abdominal vein in order to avoid bleeding. When cutting the bones of the forelimb girdle, care must be taken to avoid damaging the underlying heart. After this, turn to the sides and secure the muscle flaps with pins, reattach the forelimbs (their tension weakened after cutting shoulder girdle) and rinse the product carefully with water. It is not recommended to remove any of the internal organs. You can only carefully straighten the intestines and lay them out next to the animal (Fig. 39).
General location of internal organs
At the top of the body cavity lies the three-chambered heart. In a recently killed frog it continues to pulsate. The dark-colored atria and the lighter
ventricle (note the asynchronous contraction of these chambers).
On the sides of the heart lie dark gray thin-walled lungs. As a rule, they collapse when opened and are therefore difficult to see. In order to see them better, insert the thin end of a glass tube into the laryngeal slit and, using a rubber bulb, carefully fill the lungs with air. Note the thin-walledness of the lung sacs, the weak cellularity of their surface and the network of blood vessels in their walls.
Below the heart is a large three-lobed liver. A round greenish-brown gall bladder is visible between the lobes of the liver. Under the liver on the left side of the body is the stomach, which passes into the duodenum. In the loop between the duodenum and stomach, a small orange-yellow pancreas is attached to the mesentery. The duodenum passes into the small intestine, which is curled into a ball. The large intestine is poorly visible, but the rectum, on the contrary, is very clearly defined. On the mesentery approximately at the level leading edge In the rectum lies a burgundy rounded body - the spleen. Above the rectum, at the point where it exits into the cloaca, there is a transparent, two-lobed bladder (often upon opening it is damaged, collapses and is difficult to see).
The kidneys are located on dorsal side abdominal cavity and covered by the intestines, and in female frogs, by the genitals. By lifting the intestines (and ovaries in females) with tweezers, you can see the kidneys and the fat bodies lying in front of them, which are represented by multi-lobed flat formations. If the male is opened, a pair of oval testes is found under the intestines. In a sexually mature female, the entire posterior part of the body cavity is occupied by ovaries filled with eggs (spawn) and long oviducts rolled into a complex ball. It should be emphasized that the reproductive system of females is usually so developed that it even covers the intestines. Therefore, to examine the latter, it is necessary to move the ovaries and oviducts to the sides.
Organ systems
Digestive system
Compared to the digestive system bony fish The digestive system of amphibians is more complex and differentiated. The digestive tube begins with an oral slit leading into the oropharyngeal cavity (the latter was studied during an external examination of the frog). The tongue is placed in this cavity. The ducts of the salivary glands, which first appeared in
amphibians. However, in frogs these glands serve only to wet the food bolus and are not yet involved in the chemical processing of food. The oropharyngeal cavity passes into a short but wide esophagus (Fig. 40), and the latter into a relatively voluminous stomach, which has a slightly curved shape.
The pyloric part of the stomach, bending strongly, passes into the duodenum, which is the beginning of the small intestine. As already indicated, the pancreas lies in the loop between the stomach and duodenum. The small intestine forms many bends and loops and smoothly passes into the large intestine, which ends in a clearly visible rectum. The rectum opens into the cloaca. The entire intestine is suspended from the walls of the cavity on special folds of the peritoneum - the mesentery. The digestive glands - the liver with the gallbladder and the pancreas - are well developed. The liver ducts, together with the gallbladder duct, open into the duodenum. The pancreatic ducts flow into the gallbladder duct, so this gland does not have independent communication with the intestines.
Rice. 40. Digestive tract of a frog:
1 - esophagus; 2 - stomach; 3 - duodenum; 4 - small intestine; 5 - rectum; 6 - cloaca; 7 - the place where the rectum flows into the cloaca; 8 - bladder
Respiratory system
The respiratory organs of amphibians are of a completely different type than those of fish. They are lightweight - two thin-walled bags oval shape with narrow lower ends. The inner surface of the lungs is slightly honeycombed. When the lungs are filled with air (see page 87), a network of blood vessels is clearly visible on their walls. However, due to the imperfection of the lungs (small oxidation surface), the skin plays an important role in breathing. For example, in green frogs, over 50% of the oxygen necessary for blood oxidation passes through the skin. In connection with pulmonary breathing, internal nostrils, or choanae, appear, connecting the nasal cavity with the oropharyngeal cavity. Airways due to
lack of cervical spine very short. They are represented by the nasal and oropharyngeal cavities, as well as the larynx. The larynx opens directly into the lungs with two openings.
The breathing mechanism of a frog is of the pressure type. The role of the pump is performed by the oropharyngeal cavity. When its bottom is lowered, the volume of the cavity increases and air through the external nostrils (the valves of which are open at this time) and then through the choanae is sucked into the cavity. In this case, the laryngeal fissure is closed. Then the laryngeal slit opens, the valves of the nostrils close and air from the lungs, as a result of contraction of the abdominal muscles, is also pushed into the oral cavity. After this, the mixed air from the oropharyngeal cavity, when its bottom is raised, is pushed into the lungs (the nostril valves continue to be closed). Exhalation occurs when the valves of the nostrils open due to the contraction of the elastic walls of the lungs.
Circulatory system
The circulatory system of amphibians, in connection with pulmonary respiration, has undergone significant transformations and is significantly different from that of fish. In connection with the appearance of the lungs, a second circle of blood circulation and a three-chambered heart arose. The branchial arteries were replaced by the carotid arteries, systemic aortic arches and pulmonary arteries. In higher (tailless) amphibians, the posterior cardinal veins disappeared and the posterior vena cava, characteristic of terrestrial vertebrates, appeared, and the abdominal vein appeared. Due to cutaneous respiration great development reached the skin blood vessels, which is specific feature amphibians.
The frog's heart is three-chambered (Fig. 41), it consists of the right and left atria and ventricle. Both thin-walled atria communicate with the ventricle through one common opening. The right atrium is more
Rice. 41. Scheme of an opened frog heart from the ventral side:
1 - right atrium; 2 - left atrium; 3 - ventricle; 4 - valves; covering a common hole; leading from both atria into the ventricle; 5 - arterial cone; 6 - common arterial trunk; 7 - pulmonary cutaneous artery; 8 - aortic arch; 9 - common carotid artery; 10 - carotid gland; 11 - spiral valve of the arterial cone
voluminous - blood from the whole body collects through the veins into it, while the left one receives blood only from the lungs.
The ventricle is thick-walled, its inner surface is covered with numerous projections, between which there are pocket-like depressions. In addition to the indicated main parts of the heart, there is a venous sinus (sinus), which communicates with the right atrium, and an arterial cone extending from the right side of the ventricle.
Three pairs of arterial vessels (arterial arches), homologous to the gill arteries of fish, extend from the conus arteriosus. Each vessel arising from the conus arteriosus begins with an independent opening. All three vessels (arches) of the left and, respectively, the right side first go through a common arterial trunk, surrounded by a common membrane, and then branch (see Fig. 41).
The vessels of the first pair (counting from the head), homologous to the first pair of gill arteries of fish, are called carotid arteries. The carotid arteries carry blood to the head. These vessels depart from the common arterial trunk in the form of the common carotid arteries, each of which almost immediately splits into the external and internal carotid arteries (Fig. 42). At the site of their separation lies the carotid gland, which apparently regulates blood pressure in the carotid arteries.
Rice. 42. Diagram of the arterial system of a frog:
1 - ventricle; 2 - right atrium; 3 - left atrium; 4 - arterial cone; 5 - common carotid artery; 6 - systemic aortic arches; 7 - subclavian artery; 8 - dorsal aorta; 9 - iliac artery; 10 - femoral artery; 11 - sciatic artery; 12 - enteromesenteric artery; 13 - pulmonary artery; 14 - cutaneous arteries; 15 - carotid gland; 16 - external carotid artery; 17 - internal carotid artery. Arteries with venous blood are painted black, arteries with arterial and mixed blood are shaded
Through the vessels of the second pair (homologous to the second pair of gill arteries of fish) - the systemic arches of the aorta - blood is directed to the back of the body. The systemic arches go around the heart on the right and left sides, respectively, and merge under the spine into a common trunk - the dorsal aorta. The subclavian arteries depart from the systemic arches, carrying blood to the forelimbs.
Through the vessels of the third pair, homologous to the fourth pair of gill arteries of fish (vessels homologous to the third pair of gill arteries are absent in the frog), the pulmonary arteries, blood is sent to the lungs. From each pulmonary artery there arises a large cutaneous artery, through which blood is directed to the skin for oxidation (see Fig. 42). From the dorsal aorta, blood is carried through a number of arteries to internal organs and hind limbs.
Venous blood from the anterior end of the body is collected through two pairs of jugular veins (Fig. 43). The latter, merging with the cutaneous veins, which have already absorbed the subclavian veins, form two anterior vena cava. These veins carry mixed blood into the venous sinus, since oxygenated arterial blood moves from the skin through the cutaneous veins. Blood from the hind limbs and back of the body moves through the iliac veins to the kidneys, where it passes through the portal system. The vessels leaving the kidneys merge to form
Rice. 43. Scheme of the venous system of a frog:
1 - venous sinus (shown as if visible through the contours of the heart); 2 - external jugular vein; 3 - internal jugular vein; 4 - great cutaneous vein; 5 - subclavian vein; 6 - anterior vena cava; 7 - posterior vena cava; 8 - femoral vein; 9 - sciatic vein; 10 - iliac vein; 11 - renal portal system; 12 - subintestinal vein; 13 - portal system of the liver; 14 - hepatic veins; 15 - abdominal vein; 16 - pulmonary vein Veins with arterial blood are shaded
powerful posterior vena cava. The lower (posterior) section of this vein is homologous to the posterior cardinal veins of fish, while its upper (anterior) section is a neoplasm. Through the posterior vena cava, blood is directed to the venous sinus, from which it then enters the right atrium.
From the intestines, blood is collected by the subintestinal vein, which flows into the liver, where the portal system functions. Blood also passes through the portal system of the liver from the abdominal vein, which carries it from the hind limbs. From the liver, blood flows through the hepatic veins into the posterior vena cava.
From the lungs, blood moves through the pulmonary veins to the left atrium.
The blood circulation in the heart of a frog can be schematically represented as follows. Mixed blood enters the right atrium (venous blood comes from all parts of the body, arterial blood comes from the skin), and arterial blood (from the lungs) enters the left atrium. When the atria contract, blood flows through the common opening into the ventricle. This is where further mixing of the blood occurs. However, venous blood predominates in the right part of the ventricle, and arterial blood predominates in the left. The opening leading from the ventricle to the conus arteriosus is located on the right side of the ventricle. Therefore, when the ventricle contracts, the first portion of blood, containing more venous blood, enters the opening of the nearest pulmonary arch, the next portion - with a predominance of arterial blood - enters the systemic arches of the aorta, and a portion with lowest content venous blood.
Excretory organs
The excretory organs (Fig. 44 and 45) are represented in amphibians, as well as in fish, by the trunk kidneys (mesonephros). They are elongated, compact, reddish-brown bodies that lie on the sides of the spine. From each kidney a thin Wolffian canal stretches to the cloaca. In female Wolffrogs, the canal serves only as an excretory duct, or ureter, while in males
Rice. 44. Genitourinary organs male frog:
1 - testis; 2 - fat body; 3 - kidney; 4 - ureter; 5 - seminal vesicle; 6 - cloaca; 7 - bladder; 8 - posterior vena cava; 9 - seminiferous tubules; 10 - adrenal gland
Rice. 45. Urogenital organs of a female frog:
1 - oviduct funnel; 2 - oviduct; 3 - uterine section of the oviduct; 4 - cloaca; 5 - bladder; b - right ovary; 7 - kidney; 8 - fat body
it simultaneously performs the function of the reproductive duct, or vas deferens (about this, see page 93). In the cloaca, the Wolffian canals open with independent openings. It also opens separately into the cloaca and bladder. Urine enters first into the cloaca, and from it into the bladder. After filling the latter, through the same hole, urine is discharged again into the cloaca, and then out.
Reproductive organs
The reproductive organs of amphibians are represented by paired gonads. In males, these are oval-shaped testes, attached by the mesentery to the anterior part of the kidneys (see Fig. 44). Thin seminiferous tubules stretch from the testes to the kidneys. Sexual products from the testis are sent through these tubules to the bodies of the kidneys, then into the already known Wolffian canals and through them to the cloaca. Before flowing into the cloaca, the Wolffian canals form small expansions - seminal vesicles, which serve to temporarily reserve sperm.
The ovaries of females (see Fig. 45) are thin-walled sacs, in adults filled with pigmented eggs. In the lateral parts of the body cavity there are highly convoluted light oviducts, or Müllerian canals. These genital canals are not directly connected to the ovaries; they open through small funnels near the lungs into the body cavity. Before flowing into the cloaca, each oviduct expands into the so-called “uterus”. Mature eggs fall out through breaks in the walls of the ovary into the body cavity, then are captured by the funnels of the oviducts and move along them to the cloaca. Passing through the oviducts, the eggs are covered with a gelatinous membrane. In the “uterus” the formation of lumps ready for laying eggs occurs. Thus, in females the excretory and reproductive ducts are completely separated.
In front of the kidneys in both sexes lie yellow multilobed fatty bodies (in males they are more developed), the function of which is to supply nutrients gonads during the breeding season.
central nervous system
Compared to the brain of fish, the brain of amphibians has a number of progressive features. This mainly concerns the forebrain, which in amphibians is relatively larger than in fish, its hemispheres are completely separated, and the nervous substance also lines the sides and roof in addition to the bottom of the lateral ventricles, i.e., amphibians have a real brain vault - the archipallium. Among bony fishes, a true brain vault is characteristic only of lungfishes.
To study the structure of the brain, remove the skin from the animal's head. Next, make a small transverse incision into the skin and muscle just behind the head. Having bent the frog's body along the cut, insert the tip of the scissors into the opened occipital region and carefully cut the skull from the side to the eye. Do the same on the other side. Carefully lift the cut roof of the skull up with tweezers, bend it forward and cut it off. If after this part of the brain remains covered by bones, they should be broken off with tweezers.
The frog's brain consists of five sections (Fig. 46). In front is the forebrain, consisting of two elongated hemispheres separated by a deep fissure. In front of the hemispheres
Rice. 46. Frog brain from above (A) and below (B):
1 - cerebral hemispheres of the forebrain; 2 - olfactory lobe; 3 - olfactory nerve; 4 - diencephalon; 5 - visual chiasm; 6 - funnel; 7 - pituitary gland; 8 - optic lobes of the midbrain; 9 - cerebellum; 10 - medulla oblongata; 11 - spinal cord
the common olfactory lobe arises from which two olfactory nerves originate. Behind the forebrain is the diencephalon. On its roof is the epiphysis (endocrine gland). The midbrain is presented in the form of two rounded optic lobes. Behind the optic lobes lies the underdeveloped cerebellum. Immediately behind it is the medulla oblongata with the rhomboid fossa (fourth ventricle). The medulla oblongata gradually passes into the spinal cord.
To view the brain from the bottom, cut off the nerves extending from the brain and carefully lift it up by the medulla oblongata. On the lower side of the brain there is an optic chiasm, or chiasm, an infundibulum extending from the bottom of the diencephalon, and the pituitary gland (lower cerebral gland). 10 pairs of head nerves arise from the amphibian brain, the eleventh pair is not developed, and the twelfth departs outside the skull.