Lesson 11. STRUCTURE OF THE SKELETON OF AMBHIBIDES USING THE EXAMPLE OF THE FROG OF THE GENUS RANA
Equipment and materials
1. Skeleton of a frog.
2. Vertebrae from different parts of the body.
3. Front and hind limbs with belts (one set for two students).
5. Tables: 1) skeleton of a frog; 2) the structure of vertebrae from different parts of the body; 3) skull (top and bottom views); 4) the skeleton of the limbs and their girdles.
Introductory Notes
The skeleton of a typical representative of the amphibian class, the frog, is characterized by a combination of progressive features characteristic of terrestrial vertebrates with a number of adaptive features. As progressive signs can be named: free limbs of the five-fingered type, the formation of girdles and limbs from three homodynamic elements (built according to a single scheme), the connection of the pelvic girdle with the axial skeleton, autostyly (fusion of the palatoquadrate cartilage with the cranium), greater differentiation of the spine . At the same time, some primitive features are preserved in the frog skeleton: slight ossification of the skull, weak development of the cervical and sacral regions, and absence of ribs.
Features of specialization in the frog skeleton are manifested in significant deviations of the skeleton of the free limbs from the typical five-fingered limbs of terrestrial vertebrates, elongation of the ilium, and replacement of the caudal vertebrae with a single bone - the urostyle. However, in other groups of amphibians (caudates), not all of the listed adaptive characteristics are found.
Study the structural features of a frog.
Consider:
Scull
Brain skull
Occipital region: lateral occipital bones; foramen magnum; occipital condyles.
Sides of the skull: anterior; scaly; sphenoid-olfactory bones.
Skull roof: nasal; frontoparietal bones.
Bottom of the skull: parasphenoid; paired opener; palatine, pterygoid bones.
Visceral skull
Upper jaw: intermaxillary; maxillary; quadratozygomatic bones.
Lower jaw: dental and angular bones.
Sublingual apparatus: sublingual plate with two pairs of horns.
Spine
Spinal sections: cervical; trunk; sacral and caudal.
Trunk procoelal vertebra and its structure.
Limb belts
Shoulder girdle: shoulder blades; coracoids; collarbone; suprascapular cartilage; sternum; presternum.
Pelvic girdle: ileal; sit bones; pubic cartilage; acetabulum.
Paired limbs
Forelimb: shoulder; forearm (fused ulna and radius bones), hand (wrist, metacarpus, phalanges).
Hind limb: hip; lower leg (fused tibia and fibula), foot (two enlarged bones in the form of a ring of the tarsus, metatarsus, phalanges of the fingers).
Sketch:
1) skull (top and bottom views); 2) limb belts; 3) skeleton of paired limbs.
Skeletal structure
The skull of amphibians has undergone noticeable changes compared to that of fish. It developed features characteristic of typical land animals. The greatest transformations occurred in the visceral skull, where autostyly appeared and secondary upper jaws arose from the integumentary bones (which first appeared in bony fishes). The gill apparatus was reduced. The hyoid arch has transformed into an element of the hearing aid (stapes) and the hyoid plate.
Brain skull
The brain skull has a small number of chondral and integumentary bones, and therefore it remains predominantly cartilaginous for life (Fig. 47).
Occipital region The skull contains only two lateral occipital bones of chondral origin. Both of them carry along the articular condyle, with the help of which the skull is attached to the spine. The upper and main occipital bones do not develop, and the corresponding places remain cartilaginous.
Rice. 47. Frog skull from above ( A) and below ( B):
1
- lateral occipital bone; 2
- occipital condyle; 3
- anterior bone; 4
- sphenoid-olfactory bone; 5
- nasal bone; 6
- frontoparietal bone; 7
- scaly bone; 8
- parasphenoid; 9
- palatine bone; 10
- opener; 11
- choana; 12
- palatoquadrate cartilage; 13
- premaxillary bone; 14
- maxillary bone; 15
- quadratozygomatic bone; 16
- pterygoid bone; 17
- foramen magnum. The thick dotted line shows the cartilaginous elements of the skull
Sides of the skull in the area of the auditory capsules they consist predominantly of cartilage. Here only the anterior auricular bones develop, lying laterally and slightly in front of the occipital bones. The anterior auricular bones are adjacent to the outer side of the integumentary squamosal bones. In the area of the orbit, in its anterior part, one ring-shaped sphenoid-olfactory bone develops. On the upper and lower sides it is covered by the frontoparietal bones and parasphenoid. The olfactory capsule remains entirely cartilaginous. All bones of the sides of the braincase, with the exception of the squamosal, are of chondral origin.
Skull roof formed by the integumentary bones. The main part of the frog's braincase is covered by the frontoparietal bones, which arose as a result of the pairwise fusion of the frontal and parietal bones (a phenomenon characteristic only of tailless amphibians). In front of the frontoparietal bones, towards the end of the muzzle, there are nasal bones on each side.
Bottom of the skull lined with bones of integumentary origin - a relatively large cruciform parasphenoid and a paired vomer lying in front of it. Frogs have specific vomer teeth on their vomers. Some bones of the visceral skull take part in the formation of the bottom of the skull: in the area of the olfactory capsule, paired palatine bones adjoin the vomers, and the pterygoid bones underlie the orbits.
Visceral skull
Jaw arch amphibians consist of cartilage and bone elements. The palatoquadrate cartilage (primary upper jaw) grows to the bottom of the frog's skull with its anterior and posterior ends. On the lower surface of the palatoquadrate cartilage, the already mentioned integumentary palatine and pterygoid bones are formed. The quadrate bone does not develop, and the posterior part of the palatoquadrate cartilage remains cartilaginous.
Function upper jaws("biting" jaws) perform secondary jaws, consisting of the integumentary intermaxillary, or premaxillary, and maxillary bones. These bones bear small teeth. The maxilla on each side articulates with the thin, rod-shaped quadratojugal bone.
Lower jaw It is represented mainly by Meckel's cartilage, covered on the outside by paired integumentary dental and angular bones. The anterior end of Meckel's cartilage has become the small mental bone through chondral ossification.
Due to autostyle the hyoid arch ceases to perform the function of attaching the jaws to the skull, which was typical
for most fish. In connection with this, the hyomandibular, which served as a jaw suspension and support for the gill cover (completely reduced in amphibians), turned into a small bone - stepladder. It is located in the middle ear cavity and functions as an auditory ossicle. The hyoids, copula and remains of highly reduced branchial arches form a cartilaginous hyoid plate located between the branches of the lower jaw, and one pair of anterior horns (processes) extending from the plate. These horns are attached, going around the pharynx on both sides, to the auditory capsules. The second pair of processes, or posterior horns, of the hyoid plate are homologous to the lower elements of one of the branchial blowouts.
Spine
The frog's spine consists of nine vertebrae and, unlike the spine of fish, is divided into four sections: cervical, trunk, sacral and caudal (Fig. 48).
Cervical region It is represented by a single vertebra, differing from the trunk vertebrae in that it is devoid of transverse processes and its body is small. On the front side of this vertebra there are two articular fossae, through which the spine is connected to the skull.
Trunk section tailless amphibians usually consist of seven vertebrae (in tailed amphibians, from 14 to 63). The bodies of the trunk vertebrae have a concave surface in front and a convex surface in the back. Vertebrae of this structure are called procoelous (Fig. 49). However, the last trunk vertebra has an amphicoelous type of structure. All vertebrae of the trunk are equipped with upper arches that form the spinal canal. Each arch ends in a weakly defined superior spinous process. Long transverse processes extend from the lateral sides of the vertebral bodies. At the base of the upper arches there are two pairs of articular processes: anterior and posterior.
Rice. 48. Spinal column of a frog with pelvic girdle:
I- cervical region; II- trunk section; III- sacrum; IV- urostyle (caudal section)
Rice. 49. The structure of the trunk vertebra of a frog from the front ( A) and above ( B):
1
- spinal canal; 2
- vertebral body; 3
- superior spinous process; 4
- transverse process; 5 - articular process
The articular processes of adjacent vertebrae are connected to each other. Frogs don't have ribs.
Sacral section, like the cervical one, is represented by one vertebra. The iliac bones of the girdle of the hind limbs are attached to its long and somewhat backward transverse processes. The sacral vertebra lacks posterior articular processes. This vertebra has biconvex articular surfaces.
Tail section The spine consists of one bone called the urostyle. It is formed by the fusion of several caudal vertebrae, which are formed during the embryonic development of the frog. In tailed amphibians, the caudal region consists of 20 - 36 individual vertebrae.
Limb belts
Shoulder girdle(Fig. 50) has the appearance of an open ring, or arc, located in the front part of the trunk. Each half of this arch consists of three elements typical of terrestrial vertebrates. The dorsal part is represented by the scapula, to which the horizontally located suprascapular cartilage adjoins. Two other elements are located on the ventral side of the girdle: a relatively wide coracoid and an anterior cartilaginous procoracoid. On the latter lies the integumentary bone - the collarbone. The scapula, coracoid and procoracoid with the clavicle converge to the place of articulation of the anterior
Rice. 50. Shoulder girdle and forelimb of the frog:
1
- shoulder blade; 2
- suprascapular cartilage; 3
- coracoid; 4
- procoracoid; 5
- collarbone; 6
- sternum; 7
- presternum; 8
- supracoracoid cartilage; 9
- cartilaginous part of the sternum; 10
- cartilaginous part of the presternum; 11
- humerus; 12
- forearm (fused ulna and radius bones); 13
- carpal bones; 14
- metacarpal bones; 75 - phalanges of fingers ( II - V). Cartilage marked with dots
Rice. 51. Pelvic girdle and hind limb of the frog:
1
- ilium; 2
- ischium; 3
- pubic cartilage; 4
- femur; 5
- lower leg (fused tibia and fibula); 6
- tarsal ring (fused upper tarsal bones); 7
- other tarsal bones; 8
- metatarsal bones; 9
- rudiment of the fourth finger; 10
- phalanges of fingers ( I - V)
limbs, forming the articular fossa. The inner ends of the coracoid and procoracoid are bordered by supracoracoid cartilage. Behind the suture between the supracoracoid cartilages there is a small sternum, and in front there is a presternum. Both of these bones end in cartilage. Due to the absence of ribs, there is no chest, and the belt of the forelimbs lies freely in the thickness of the muscles.
Pelvic girdle(Fig. 51) The frog, like the shoulder, consists of three pairs of elements. The three elements of each side, by analogy with the shoulder girdle, are connected at the point of articulation of the limb and form here the articular fossa, or acetabulum. The first pair of bones is called the ilia. Their function is to attach the hind limb girdle to the axial skeleton through the transverse processes of the sacral vertebra. In the frog, due to its peculiar method of movement - jumping - the iliac bones have elongated anterior ends - the wings of the ilium. The second pair of bones - the ischium - is located below and slightly behind the acetabulum. The third paired element - the pubis - remains cartilaginous in the frog. The pubic cartilages are directed forward and downward.
Thus, the girdles of the frog's limbs are built according to a single pattern, common to both the shoulder and pelvic girdles. In general, this scheme is typical for most terrestrial vertebrates. The appearance of limb girdles of this type and the emergence of their connection with the axial skeleton (pelvic girdle) also indicates the progressiveness of the structure of the amphibian skeleton in comparison with that of fish.
Paired limbs
The fore and hind limbs of the frog (as well as their belts) are homodynamic, i.e. built according to a single scheme, also characteristic of all terrestrial vertebrates. In a typical case five-fingered limb a terrestrial vertebrate consists of three main divisions, the last (distal) of which is in turn divided into three subdivisions (Fig. 52).
Rice. 52. Scheme of the structure of the limbs of a terrestrial vertebrate:
A- forelimb: I- shoulder; II- forearm; III- brush ( 1
- wrist; 2
- metacarpus; 3
- phalanges of fingers). B- hind limb: I- hip; II- shin; III- foot ( 1
- tarsus; 2
- metatarsus; 3
- phalanges of fingers)
I department- shoulder of the forelimb; hip in the back. This section is always represented by one bone, which enters with its proximal end into the articular cavity of the corresponding girdle.
II department- forearm in the forelimb, shin in the hindlimb. The section consists of two bones: the ulna and radius in the forearm, and the tibia and fibula in the lower leg.
III department- hand in the forelimb, foot in the hindlimb. This department has three subsections.
The first subsection in the hand is the wrist. In the foot there is a tarsus. This subsection consists of 9 - 10 bones arranged in 3 rows.
The next subsection consists of five elongated bones arranged in one row: the metacarpus in the forelimb and the metatarsus in the hindlimb.
The third subsection - the phalanges of the fingers - consists of five longitudinal rows, usually with several bones in each.
The structure of the frog's limbs is somewhat different from the diagram above, which is caused by a specific method of movement.
IN forelimb(see Fig. 50) these differences are expressed in the fact that the ulna and radius bones following the shoulder fuse into one forearm bone. The number of fingers has been reduced to four. Only the rudiment of the metacarpal bone of the first inner finger is clearly visible on the specimen.
IN hind limb(see Fig. 51) the tibia is represented by only one bone, and not two, as follows from the diagram discussed above. This shin bone is formed as a result of the fusion of the tibia and fibula. The two proximal tarsal bones greatly enlarge and, fused at their ends, form a kind of ring. The remaining tarsal bones either fuse or are reduced. In front of the first inner finger, rudimentary bones of the additional sixth finger are preserved.
To the question of the sections of the frog skeleton and the name of the bones asked by the author Nail Fazullin the best answer is Skeleton
The body is divided into a head, torso and five-fingered limbs. The head is movable and connected to the body. The skeleton is divided into sections:
axial skeleton (spine);
head skeleton (skull);
skeleton of paired limbs.
The spine is divided into 4 sections: cervical, trunk, sacral and caudal. The number of vertebrae ranges from 7 in tailless amphibians to 200 in legless amphibians.
The cervical vertebra is movably attached to the occipital region of the skull (provides head mobility). Ribs are attached to the trunk vertebrae (except for anurans, which lack them). The only sacral vertebra is connected to the pelvic girdle. In tailless animals, the vertebrae of the caudal region are fused into one bone.
The flat and wide skull articulates with the spine using 2 condyles formed by the occipital bones.
The skeleton of the limbs is formed by the skeleton of the limb girdle and the skeleton of the free limbs. The shoulder girdle lies in the thickness of the muscles and includes paired shoulder blades, collarbones and crow bones connected to the sternum. The skeleton of the forelimb consists of the shoulder (humerus), forearm (radius and ulna) and hand (bones of the wrist, metacarpus and phalanges of the fingers). The pelvic girdle consists of paired iliac ischial and pubic bones fused together. It is attached to the sacral vertebra through the ilia. The skeleton of the hind limb includes the thigh, tibia (tibia and fibula) and foot. Bones of the tarsus, metatarsus and phalanges of the fingers. In anurans, the bones of the forearm and lower leg are fused. All bones of the hind limb are greatly elongated, forming powerful levers for mobile jumping.
For the first time, amphibians developed a true five-fingered limb.
Reply from Interrogation[newbie]
that's for sure, but the end is not certain
Skeletal structure of a frog
Frog
According to the sharp differences in the structure of the limbs when adapting to hock movements and the loss of the tail, the frog skeleton bears the features of extreme specialization, i.e., secondary deviations from the typical structure of the vertebrate skeleton. This especially applies to the structure of the spinal column and the front and hind limbs.
The spine contains only 9 vertebrae and, unlike the spine of fish, has 4 sections: cervical, trunk, sacral and caudal. The trunk vertebrae have a concave surface in front and a convex surface in the back, i.e. they belong to the procoel type. All of them are equipped with upper arches that limit the spinal canal and end with the spinous processes. Large transverse processes extend from the sides of the vertebrae, and in front and behind, at the base of the upper arches, there are a pair of short articular processes that serve to connect with the corresponding processes of neighboring vertebrae. There are no ribs.
The cervical region, like all amphibians, contains only one vertebra, which differs from the trunk vertebrae in that it is devoid of transverse and anterior articular processes and bears on its anterior side two articular fossae, through which it is connected to the skull.
Frog skeleton
Skeleton of a frog. I - solid skeleton; II - vertebra from above; III - vertebra in front:
1 - cervical vertebra,
2 - sacral vertebra,
3 - urostyle,
4 - sternum,
5 - cartilaginous posterior part of the sternum,
6 - presternum,
7 - coracoid,
8 - procoracoid,
9 - blade,
10 - suprascapular cartilage,
11 - ilium,
12 - ischium,
13 - pubic cartilage,
14 - humerus,
15 - forearm (radius + ulna),
16 - wrist, 17 - metacarpus,
18 - rudimentary I finger,
19 - II finger,
20 - V finger,
21 - thigh,
22 - tibia (tibia and fibula),
23 - tarsus,
24 - metatarsus,
25 - vestigial accessory finger (praehellux),
26 - I finger,
27 - vertebral body,
28 - spinal canal,
29 - articular platform,
30 - spinous process,
31 - transverse process
The number of trunk vertebrae in a frog is usually seven.
The sacral section contains only one vertebra, which in general has the typical structure of the trunk vertebra and is characterized mainly by the fact that the pelvic bones are attached to its transverse processes.
The caudal section is represented by one long bone - the urostyle (urostyl), which, as the history of its development shows, is formed by many vertebrae fused with each other.
Frogs are one of the most numerous species of amphibians. Features of the external and internal structure of 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 the moist skin of the eyelids 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 breathes atmospheric air, which enters its body through its 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 is the outer part of the hearing organ - the eardrum. The inner ear of a frog, like that of fish, is located in the bones of the skull.
The frog has well-developed paired limbs - front and hind legs. Each limb consists of three main sections. The front leg is divided into: shoulder, forearm and hand. The frog's hand ends with four fingers (its fifth finger is underdeveloped). In the hind limb, these sections are called the thigh, tibia and foot. The foot ends in five toes, which in the frog are connected by a swimming membrane. The sections of the limbs are movably articulated with each other using joints. The hind legs are much longer and stronger than the front legs; they play a major role in movement. A sitting frog rests on slightly bent forelimbs, while the hind limbs are folded and located on the sides of the body. Quickly straightening them, the frog makes a jump. The front legs protect the animal from hitting the ground. The frog swims by pulling and straightening its hind limbs, while pressing its front limbs to its body. 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 the environment enters the frog's body through the skin and through food. The frog never drinks.
Frog skeleton
Unlike fish, the frog has a cervical vertebra. It is movably articulated with the skull. It is followed by the trunk vertebrae with lateral processes (the frog's ribs are not developed). The cervical and trunk vertebrae have superior arches that protect the spinal cord. At the end of the spine of the frog and all other tailless amphibians is a long tail bone. In newts and other tailed amphibians, this section of the spine consists of a large number of flexibly articulated vertebrae. 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
The structure of the frog's muscular system is much more complex than that of 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
The digestive system of amphibians has almost the same structure as that of fish. Unlike fish, the hindgut does not open directly outward, but into a special extension called the cloaca. The ureters and excretory ducts of the reproductive organs also open into the cloaca. Respiratory system
The frog breathes atmospheric air. The lungs and skin are used for breathing. The lungs look like bags. Their walls contain a large number of blood vessels in which gas exchange occurs. The frog's throat is pulled down several times per second, creating a rarefied space in the oral cavity. Then the air penetrates through the nostrils into the oral cavity, and from there into the lungs. It is pushed back under the action of the muscles of the body walls. The frog's lungs are poorly developed, and skin respiration is as important for it as pulmonary respiration. Gas exchange is possible only when the skin is damp. If a frog is placed in a dry vessel, its skin will soon dry out and the animal may die. Immersed in water, the frog completely switches to skin respiration. Circulatory system
The frog's heart is located in the front of the body, under the sternum. It consists of three chambers: the ventricle and two atria. Both atria and then the ventricle contract alternately. In the heart of a frog, the right atrium contains only venous blood, the left - arterial, and in the ventricle the blood is mixed to a certain extent. 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 times and decreases in cold weather. 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
The forebrain is more developed than in fish, and two swellings can be distinguished in it - the cerebral hemispheres. Amphibians' bodies are close to the ground and they do not have to maintain balance. In this regard, the cerebellum, which controls the coordination of movements, is less developed in them than in fish. The structure of the sense organs corresponds to the terrestrial environment. For example, by blinking its eyelids, a frog removes dust particles adhering 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
After spending the winter in a state of torpor, amphibians wake up with the first rays of the spring sun 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
After some time, the shell of each egg swells and turns into a gelatinous transparent layer, inside of which the egg is visible. The upper half is dark and the lower half is light: the dark part of the egg uses the sun's rays better and heats up more. Clumps of eggs in many frog species float to the surface where the water is warmer. Low temperature retards 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. The hotter the weather, the faster the tadpole changes. 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.
Lifespan- 5 (18) years (bullfrog up to 16 years; toad up to 36 years).
Habitat- swamp, wet forests, meadows, in water.
Behavior- in dry weather they hide, in cloudy weather they hunt.
Eating— insects (beetles); spiders, terrestrial gastropods, fish fry.
Activity during the warm time of the day (year).
Origin of Amphibians
External structure
The frog lives in bodies of water or on their shores. Its flat, wide head smoothly transitions into a short body with a reduced tail. The mucus secreted by the skin glands not only ensures that the skin participates in gas exchange, but also protects it from microorganisms.
The skeleton consists of the spine, skull and skeleton of the limbs.
Internal structure
Adult amphibians are predators, feeding on various insects and other invertebrate animals; some aquatic amphibians catch small vertebrates.
Digestive system
The digestive system begins with a large oropharyngeal cavity, at the bottom of which the tongue is attached at the anterior end. When catching prey, the tongue is thrown out of the mouth, and the prey sticks to it. Streams of salivary glands open into the oropharyngeal cavity. Their secretion moistens the cavity and food, making it easier to swallow prey. On the upper jaw there are small conical teeth that serve only to hold prey. Food moistened with saliva enters the esophagus and then into the stomach. The glandular cells of the stomach walls secrete the enzyme pepsin, which is active in an acidic environment (hydrochloric acid is also released in the stomach).
Partially digested food moves to the duodenum, into which the bile duct of the liver flows. Pancreatic secretions also flow into the bile duct. The duodenum quietly passes into the small intestine, where nutrients are absorbed. Undigested food remains enter the wide rectum and are expelled through the cloaca.
Respiratory system
The frog breathes through its lungs and through its skin. On the walls of the paired sac-like lungs there is an extensive network of blood vessels. When the frog opens its nostrils and lowers the bottom of the oropharyngeal cavity, air enters the latter.
Then the nostrils close with valves, the bottom of the oropharyngeal cavity rises, and air passes into the lungs. Exhalation occurs due to the action of the abdominal muscles and the collapse of the pulmonary walls.
Circulatory system
The heart of adult amphibians has three chambers - two atria and one ventricle. An arterial cone with a longitudinal spiral valve inside extends from the ventricle, which distributes arterial and mixed blood into different vessels. The right atrium receives venous blood from internal organs and arterial blood from the skin, i.e. mixed blood gathers here. The left atrium receives arterial blood from the lungs. Both atria contract simultaneously and blood flows from them into the ventricle. Thanks to the longitudinal valve in the arterial cone, venous blood flows to the lungs and skin, mixed blood flows to all organs and parts of the body except the head, and arterial blood flows to the brain and other organs of the head.
Amphibians have two circuits of blood circulation, but they are not completely separated due to a single ventricle. In a large circle, blood flows from the ventricle to all organs, and from them through the veins it returns to the right atrium. In the pulmonary circle, blood flows from the ventricle to the lungs and skin, and from there, enriched with oxygen, it returns to the left atrium.
Excretory system
The excretory system is represented by two kidneys located on the sides of the sacral vertebra. The kidneys contain glomeruli, in which harmful breakdown products and some valuable substances are filtered from the blood. During the flow through the renal tubules, valuable compounds are reabsorbed, and urine flows through two ureters into the cloaca and from there into the bladder. After filling the bladder, the muscles of its walls contract, urine is discharged into the cloaca and thrown out.
Nervous system
The brain has the same sections as those of fish. The forebrain is more developed, divided into two hemispheres.
The cerebellum is small, which is explained by a sedentary lifestyle and monotony of movements.
In the eyes of adult amphibians, movable eyelids (upper and lower) and a nictitating membrane are developed; they protect the cornea from drying out and contamination.
Metabolism
Sense organs
The sense organs are more complex than those of fish; they provide orientation for amphibians in water and on land. Adult amphibians living in water have developed lateral line organs, they are scattered on the surface of the skin, especially numerous on the head. The epidermal layer of the skin contains temperature, pain and tactile receptors. Olfactory organ are represented by paired olfactory sacs, which open outward with paired external nostrils, and into the oropharyngeal cavity with internal nostrils. Some of the walls of the olfactory sacs are lined with olfactory epithelium. The olfactory organs work only in the air; in water, the external nostrils are closed. The olfactory organs of amphibians and higher chordates are part of the respiratory tract.
Cornea eyes convex, the lens has the shape of a biconvex lens. The retina contains rods and cones. Many amphibians have developed color vision.
IN organs of hearing In addition to the inner ear, the middle ear is developed. It contains a device that amplifies sound vibrations. The external opening of the middle ear cavity is covered with an elastic eardrum. The cavity contains the auditory ossicle. The middle ear cavity is connected by a narrow canal to the oral cavity.
Reproduction
The ovaries and testes of amphibians and fish are similar. External fertilization occurs in water. Gonads are paired. Paired oviducts flow into the cloaca, seminiferous tubules into the ureters. Frogs reproduce in the spring during their third year of life.
The male attaches himself to the female’s back, tightly clasping her with his front paws. The thickenings on the inner toes help to clasp the paws so as not to lose sight of the female for several days. During this time, the female releases up to 3,000 eggs, and the male immediately waters them with milk. As a result, almost all the eggs are fertilized, and none of the amphibians have to hurl millions of them, as is often the case with fish, so the eggs can be larger, which means there are more reserves in each.
The eggs are covered with a mucous membrane, which swells greatly in water. The shells, like lenses, collect the rays of the sun and heat the eggs by several degrees, accelerating their development.
Development
Fertilized eggs develop within 7-15 days. The upper, dark part of the egg gradually turns into the head and tail of the embryo, and the lower light part into the abdomen. It contains a yolk sac with nutrients, which gradually decreases. After 8-15 days, a larva emerges from the egg shell - a tadpole.
The tadpole is very different in structure from adult animals. It looks like a fish not only externally, but also in its internal structure. The caudal fin is used for movement, and the branched external gills are used for breathing. The tadpole uses horny scrapers around its mouth to obtain plant food. The lateral line helps with orientation.
Soon the external gills disappear, they are replaced by gill slits with petals, covered with a fold of skin. At this stage, the tadpole has a two-chambered heart and one circulation. Oxygen enters the blood from the gills through the three anterior arches of the gill arteries, as well as through the cutaneous vein - from the extensive surface of the tail. The main product of excretion, like in fish, is ammonia.
Comparison of the structure of larvae and adult frogs
| Sign | Larva (tadpole) | Adult animal |
| Body Shape | Fish-like, with limb buds, tail with a swimming membrane | The body is shortened, two pairs of limbs are developed, there is no tail |
| Method of transportation | Swimming with your tail | Jumping, swimming using hind limbs |
| Breath | Branchial (gills are first external, then internal) | Pulmonary and cutaneous |
| Circulatory system | Two-chambered heart, one circle of blood circulation | Three-chambered heart, two circles of blood circulation |
| Sense organs | The lateral line organs are developed, there are no eyelids in the eyes | There are no lateral line organs, eyelids are developed in the eyes |
| Jaws and feeding method | They feed mainly on plant foods (algae, etc.); they have horny plates on their jaws that scrape off soft plant tissues along with the unicellular and other small invertebrates found on them. | There are no horny plates on the jaws; with its sticky tongue it captures insects, mollusks, worms, and fish fry. |
| Lifestyle | Water | Terrestrial, semi-aquatic |
After a few weeks, metamorphosis begins - the transformation of an aquatic larva into a frog adapted to life on land. The gills become overgrown, the lateral line disappears, and the tail gradually shortens. Limbs appear, lungs are formed from intestinal protrusions, from the fourth (posterior) gill artery - the pulmonary circulation... and so on, in accordance with the main stages of amphibian evolution. Used organs do not “fall off” and are not wasted. They are disassembled into molecules and carried away by the blood to where they can be used for the “construction” of new organs. After two to three months, the tadpole turns into a frog.
The frog is a typical representative of amphibians. Using this creature as an example, you can easily study the most important characteristics of the 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.
The external structure of a frog and its habitat are two inextricably linked links in a single chain. It’s no secret that the appearance and structure are determined by the place and way of life 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 turns into a 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. Features of the external structure of the frog are associated with its lifestyle. 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.
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 large bulging eyes. 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 amphibian representatives 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 with bright colors. This indicates that the animal is very poisonous. This is what scares away enemies from such frogs.
Amphibian limbs
When citing 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 rear 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 a 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 plays a 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 require oxygen in much smaller quantities than mammals. And the reason for this is the ability to absorb part of the necessary oxygen by the surface of the skin.
The respiratory system of frogs also has its own characteristics. Amphibians breathe not only with the help of their lungs. In this process, the skin also takes active attention. As you can see, external integuments 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 life both in water and on land.
Nervous system of a frog
But the nervous system of amphibians is 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: the anterior, middle, intermediate, medulla oblongata, and cerebellum. By the way, the latter is poorly developed due to the animal’s sedentary lifestyle. But the forebrain consists of two hemispheres and, on the contrary, is quite developed.
Internal and external structure of the frog on the site.
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