Trunk department of amphibians. Amphibians

Helpful Hints 15.07.2019

Helpful Hints

TOPIC 10. Amphibian skeleton

SYSTEMATIC POSITION OF THE OBJECT

Subtype Vertebrates, Vertebrata
Class Amphibians, Amphibia
Order Anura, Anura (Ecaudata)
Representative - Lake frog, Rana ridibunda Pall.

MATERIAL AND EQUIPMENT

For one or two students you need:
1. A disassembled frog skeleton mounted on cardboard tablets.
2. Dissecting needles - 2.

EXERCISE

Understand the structural features of the skeleton of amphibians. Make the following drawings:
1. Frog skull from above.
2. Skull from below.
3. The spinal column and the pelvic girdle attached to it from above.
4. Belt of the forelimbs (straightened) from below.
5. Skeleton of the forelimb.
6. Pelvic girdle on the side.
7. Skeleton of the hind limb.

Additional task

Compare, without sketching, the skeletons of an anuran (frog) and a tailed amphibian (wet preparation).

DESCRIPTION OF THE SKELETON

The skeleton of amphibians, like other vertebrates, is divided into the axial skeleton (vertebral column), the skull (cerebral and visceral), paired limbs and their belts.

In almost all parts of the skeleton, cartilage plays a rather important role.

Axial skeleton. The axial skeleton in amphibians is represented by the vertebral column (columna veriebralis; Fig. 54), consisting of ossified vertebrae; the notochord in the adult state is usually reduced. Compared to fish, the axial skeleton of amphibians consists of a larger number of sections.

Rice. 54. Axial skeleton and pelvic girdle of a frog (top view):
1 - cervical vertebra, 2 - trunk vertebrae, 3 - sacral vertebra. 4 - urostyle (fused tail vertebrae), 5 - pelvic girdle, 6 - acetabulum

1. The cervical region (pars cervicalis; Fig. 54, 1) in all amphibians is represented by one cervical vertebra, which, with the help of two articular areas, articulates movably with the skull.

2. The trunk section (pars thoracalis; Fig. 54, 2) of the frog spine consists of 7 vertebrae (in tailed amphibians - from 14 to 63).

3. The sacral section (pars sacralis; Fig. 54, 3) in all amphibians is represented by one sacral vertebra, to the massive transverse processes of which the ilium bones of the pelvic girdle are attached (Fig. 54, 5).

4. The tail section (pars caudalis) in the larvae of tailless amphibians consists of a fairly large number of individual vertebrae, which during metamorphosis merge into one tail bone - the urostyle (urostyl; Fig. 54, 4). In tailed amphibians, 26-36 individual vertebrae are preserved in the tail.

Rice. 55. Trunk vertebra of a frog
A - general view; B - longitudinal section:
1 - vertebral body, 2 - superior arch, 3 - canal for the spinal cord, 4 - spinous process, 5 - transverse process, 6 - articular process

Compared with fish, amphibians are characterized by a greater differentiation of the spinal column into sections, a change in the shape of the vertebral bodies, and a stronger development of articular processes. These transformations are associated with a terrestrial lifestyle and provide greater strength of the axial skeleton while maintaining its mobility, a strong connection of the pelvic girdle with it, and allow some mobility of the skull in a vertical plane relative to the body (the ability to raise and lower the head).

Scull. The axial, or brain, skull of amphibians, like the skull of cartilaginous fish, is of the platybasal type: with a wide base and widely spaced eye sockets, between which the anterior end of the brain is located. In the skull, compared to bony fish, a lot of cartilage is preserved, and the number of ossifications is relatively small.

Rice. 56. Frog Skull
A - from above; B - from below; B - behind; G - the lower jaw from above, the dotted line shows the cartilaginous areas of the skull
1 - lateral occipital bone, 2 - occipital condyle, 3 - anterior ear bone, 4 - sphenoid-olfactory bone, 5 - nasal bone, 6 - fronto-parietal bone, 7 - squamous bone, 8 - parasphenoid, 9 - palatine bone, 10 - vomer, 11 - choana, 12 - palatine-square cartilage, 13 - premaxillary bone, 14 - maxillary bone, 15 - square-zygomatic bone, 16 - pterygoid bone, 17 - Meckel's cartilage, 18 - chin-jaw bone, 19 - dentary, 20 - angular bone, 21 - foramen magnum

In the cartilage of the occipital part of the brain skull, only paired lateral occipital bones (occipitale laterale; Fig. 56, 1) are formed, bordering the large occipital foramen (foramen occipitale magnum; Fig. 56, 21). Each of them forms a condyle (condylus occipitalis; Fig. 56, 2) for articulation with the cervical vertebra. In the region of the auditory capsule, instead of five pairs of ear bones characteristic of teleost fish, only one pair appears in amphibians - the anterior bones (prooticum; Fig. 56, 3). In the anterior part of the brain skull, during ossification of the cartilage, an unpaired sphenoid-olfactory bone (sphenethmoideum, Fig. 56, 4) is formed, which looks like a bone ring girdle.

Rice. 57. Cartilaginous skull of a tadpole:
1 - brain skull, 2 - palatine-square cartilage, 3 - Meckel's cartilage, 4 - gill arches, 5 - jaw joint

The rest of the brain skull remains cartilaginous. It is strengthened by integumentary (skin) bones. Above in the anterior part of the skull lie paired nasal bones (nasale; Fig. 56, 5), having an elongated triangular shape, then paired fronto-parietal bones (frontoparietale, Fig. 56, 6) merged from the frontal and parietal bones, and outward from the ear bones - having a complex shape scaly bones (squamosum; Fig. 56, 7). The bottom of the brain skull is covered by a powerful cruciform integumentary bone - the parasphenoid (parasphenoideum; Fig. 56, 8). In front of it, there are also integumentary paired palatine bones (palatinum; Fig. 56, 9) and paired vomers (vomer; Fig. 56, 10); small teeth sit on the coulters. In front of the openers are paired internal nostrils - choanas (Fig. 56, 11).

The visceral region of the amphibian skull also retains much cartilage. Throughout life, the palatine-square cartilage (cartilago palatoquadratum; Fig. 56, 12) is preserved, growing with its anterior end to the olfactory region of the brain skull, and with its posterior end to the base of the skull in front of the auditory capsule (Fig. 57, 2). Therefore, the skull of amphibians, as well as other terrestrial vertebrates, is autostylic according to the type of attachment of the jaw arch.

The bones of the secondary upper jaw that arise in the skin are adjacent to the palatine-square cartilage: paired premaxillary bones (intermaxillare or praemaxillare; Fig. 56, 13), bearing teeth and maxillary bones (maxillare; Fig. 56, 14). Behind them, strengthening the back of the palatine-square cartilage, an integumentary square-zygomatic bone (quadratojugale; Fig. 56, 15) is formed from above, and also an integumentary - pterygoid bone (pterygoideum; Fig. 56, 16) from below.

The primary lower jaw - Meckel's cartilage (cartilago Meckeli; Fig. 56, 17) also remains cartilaginous, only its very front end ossifies into small paired chin-jaw bones (mento-mandibulare, Fig. 56, 18). They are joined by integumentary dentitions (dentale; Fig. 56, 19), in amphibians devoid of teeth. The back of the Meckel cartilage is overgrown with a long integumentary angular bone (angulare; Fig. 56, 20) and several other small integumentary bones. Through the articular process of Meckel's cartilage, the lower jaw articulates movably with the posterior part of the palatine-square cartilage (Fig. 57, 5).

Rice. 58. Schematic section through the auditory region of the head of a frog:
1 - brain, 2 - auditory capsule with semicircular canals, 3 - middle ear cavity, 4 - stirrup, 5 - tympanic membrane, 6 - Eustachian tube, 7 - oral cavity

The hyoid arch began to decrease even in the ancestors of modern amphibians, and the cavity of the spiracle (the remainder gill slit between the maxillary and hyoid arches) in connection with the transition to life in the air, it was transformed into the cavity of the middle ear (Fig. 58, 3). Located next to the splash, the upper element of the hyoid arch - pendants (hyomandibulare) turned into an auditory ossicle-column, or stirrup (columel1a, or stapes; Fig. 58, 4). In modern tailless amphibians, the stirrup has the appearance of a thin rod-shaped bone lying perpendicular to the brain skull under the squamosal and square-zygomatic bones. With one end, the stirrup rests against the center of the tympanic membrane (Fig. 58, 5), and with the other end against the oval window of the auditory capsule. This mechanism, which amplifies sound vibrations and provides the possibility of hearing in the air, is secondarily reduced in some modern amphibians to varying degrees. Additional mechanisms that ensure the perception of sound waves propagating along a solid substrate are their lower jaw, as well as the transmission of sound vibrations through the blood trunks.

Rice. 59. Hyoid apparatus of the frog:
1 - body, 2 - horns

The lower element of the hyoid arch is the hyoid (hyoideum) and the gill arches functioning in the larvae of amphibians during metamorphosis turn into the hyoid apparatus (Fig. 59). In tailless amphibians, it is a cartilaginous plate with two main bunks of processes - horns. The anterior, longer horns (modified hyoids) go back and up and attach to the walls of the auditory capsules of the brain skull. The sublingual apparatus strengthens the bottom of the oral strip: the muscles located between the branches of the lower jaw are attached to it.

It is assumed that the laryngeal cartilages also represent the transformed remains of the gill arches.

Paired limbs and their girdles. The limbs of amphibians, kyaks and limbs of other classes of terrestrial vertebrates represent in the diagram a system of levers movably connected to each other. The schemes of the structure of the fore and hind limbs are of the same type (Fig. 60):

Rice. 60. Scheme of the structure of paired limbs (left) of terrestrial vertebrates
A - forelimb; B - hind limb:
a - shoulder - thigh, b - forearm - lower leg, a - hand-foot;
1 - humerus, 2 - ulna, 3 - radius, 4 - wrist, 5 - metacarpus, 6 - phalanges of fingers, 7 - femur, 8 - tibia, 9 - fibula, 10 - tarsus, 11 - metatarsus, 12 - patella, I - V - fingers

Fore limb: Hind limb:
I. Shoulder (humerus; Fig. 60, 1).
II. Forearm (antebrachium):
radius (radius; fig. 60, 3),
ulna (ulna; Fig. 60, 2)

III. Brush (manus):
wrist (carpus; Fig. 60, 4),
metacarpus (metacarpus; fig. 60, 5),
phalanges of fingers (phalanges digitorum; Fig. 60, 6). I. Thigh (femur; fig. 60, 7).
II. Lower leg (crus): tibia (tibia; Fig. 60, 5),
fibula (fibula; Fig. 60, 9).
III. Foot (pes): tarsus (tarsus; Fig. 60, 10),
metatarsus (metatarsus; fig. 60, 11),
phalanges of fingers (phalanges digitorum; Fig. 60, 6)

The proximal end has a rounded head of the shoulder (caput humeri; Fig. 61, 2), which enters the articular fossa of the girdle of the forelimbs; at the distal end - a hemispherical surface for articulation with the bones of the forearm. The surface of the shoulder has ridges to which muscles attach.
In tailless amphibians, the ulna (ulna; Fig. 61, 4) lying on the outside and the radius (radius; Fig. 61, 5) bones on the inside merge into a single forearm bone (antebrachium, Fig. 61, 3); longitudinal groove shows the border of their confluence. In tailed amphibians, these bones are independent.

The proximal ends of both bones form an articular fossa for connection with the shoulder; behind this fossa is the olecranon (Fig. 61, 6) of the ulna, which limits the extension of the limb.

The wrist (carpus, Fig. 61, 7) consists of two rows of small bones. Five elongated bones of the metacarpus (metacarpus; Fig. 61, 8) adjoin the distal row of bones of the wrist. The phalanges of the fingers (phalanges digitorum; Fig. 61, 9) are articulated with the distal ends of the metacarpal bones. In amphibians, the first (thumb) finger is greatly reduced and the hand ends with only four well-developed fingers.

Rice. 61. Forelimb and shoulder girdle of a frog:
1 - humerus, 2 - head of the shoulder, 3 - forearm, 4 - ulna,
5 - radius, 6 - olecranon, 7 - wrist, 8 - metacarpus, 9 - phalanges of the fingers, 10 - scapula, 11 - suprascapular cartilage, 12 - coracoid, 13 - articular cavity for the head of the shoulder, 14 - procoracoid cartilage, 15 - clavicle, 16 - sternum, 17 - presternum, I - reduced first finger, II - V - well developed fingers

The girdle of the forelimbs, or shoulder girdle, in amphibians, as well as in shark fish, lies in the thickness of the muscles of the body, connecting it with the axial skeleton. From the upper (dorsal) scapular part of the primary belt, a scapula (scapula; Fig. 61, 10) is formed; its uppermost part remains cartilaginous in the form of a wide suprascapular cartilage (cartilago suprascapularis, Fig. 61, 11). On the anterior-outer surface of the suprascapular cartilage, some anurans have a small ossification - the remnant of the kleytrum of fish-like ancestors. The ossified coracoid part of the belt turned into a powerful coracoid bone (coracoideum; Fig. 61,12), together with the scapula forming the articular cavity for the head of the shoulder (Fig. 61, 13). Anterior to the coracoid, behind a small opening, lies a cartilaginous procoracoid (cartilago procoracoidea; Fig. 61, 14), on which a thin integumentary bone, the clavicle (clavicula; Fig. 61, 15), rests. The non-ossified cartilaginous inner ends of the coracoids and procoracoids of the right and left sides fuse together in the midline. Behind the coracoids is a bony sternum (sternum, Fig. 62, 16) with a cartilaginous posterior end. Ahead of the procoracoids, the presternum (praesternum; Fig. 61, 17) also has a cartilaginous end. In the girdle of the forelimbs of caudate amphibians, there is noticeably more cartilage, and the ossifications are smaller; clavicles often do not develop.

The shoulder girdle serves as a support for the forelimbs and an attachment point for the muscles that control them.

The chest in amphibians does not develop: the sternum does not articulate with the ribs.

Rice. 62. Hind limb (A) and pelvic girdle (B) of a frog from the side:
1 - femur, 2 - femoral head, 3 - lower leg, 4 - tibia, 5 - fibula, 6 - tarsus, 7 - tibiale, 8 - fibulare, 9 - metatarsus, 10 - phalanges of fingers, 11 - rudiment VI finger, 12 - ilium, 13 - ischium, 14 - pubic cartilage, 15 - acetabulum, I - V - fingers

The proximal row of tarsal bones (tarsus, Fig. 62, 6) of anurans consists of two elongated bones that form an additional limb lever. The inner one is called tibiale (astragalus; Fig. 62, 7; adjoins the tibial edge of the lower leg), the outer one is called fibulare (calcaneus, Fig. 62, 8). An ankle joint is formed between the lower leg and these bones. From the distal row of tarsal bones in amphibians, only 2-3 small bones are preserved. The metatarsus (metatarsus; Fig. 62, 9) is formed by five long bones, to which the phalanges of the fingers (phalanges digitorum; Fig. 62, 10) are attached. The longest toe in frogs is IV. To the side of the I (inner) finger is a small rudiment of the VI ("pre-first") finger (praehallus; Fig. 62, 11).

The belt of the hind limbs, or pelvic, in amphibians, as in all terrestrial vertebrates, consists of three paired elements; moreover, all of them together form an articular acetabulum (acetabulum; Fig. 62, 15) for connection with the femoral head. The long, forward-directed ilium bones (ilium; Fig. 62, 12) are attached with their ends to the transverse processes of the sacral vertebra (see Fig. 54). The lower part of the pelvic girdle in amphibians does not ossify and is represented by pubic cartilage (cartilago pubis, Fig. 62, 14). Behind him lie the paired ischial bones (ischium; Fig. 62, 13).

In tailed amphibians, compared to anurans, there is much more cartilage in the pelvic girdle, and the formed bones are small.

CONCLUSION

Amphibians (Amphibia) are the first class of terrestrial vertebrates. However, representatives of the class still retain a constant connection with water. This duality is clearly manifested in the features of embryonic and postembryonic development. Oocytes (caviar) can only develop in water (or in rare cases in an extremely humid environment). A larva emerges from the egg - a tadpole, which has distinct signs of a typical aquatic animal: it has gills and gill arches supporting them, a two-chambered heart, one circle of blood circulation, paired limbs of a terrestrial type are absent, the main organ of movement is a powerful caudal fin, lateral line organs are developed etc. During metamorphosis (transformation), the larva loses some of the characteristics characteristic of aquatic animals, and acquires features that ensure the transition to a terrestrial (or rather, terrestrial-aquatic) lifestyle: paired limbs of a terrestrial type appear, lungs develop, gills are reduced and the skeletal apparatus supporting them, the circulatory system is rebuilt - two incompletely separated circles of blood circulation are formed, etc.

The duality of organization as an adaptation to a terrestrial-aquatic lifestyle is also well expressed in adults.

The terrestrial way of life is provided by a number of structural features: a greater differentiation of the spinal column into sections and a stronger connection of the vertebral bodies with each other (replacement of amphicoelous vertebrae by procoelous or opisthocoelous ones); the appearance of paired limbs of the ground type; complication of the structure and greater strength of the limb belts (at the same time, a sufficiently strong connection of the pelvic girdle with the axial skeleton is already established); a strong reduction of the metameric muscles and its replacement by a sufficiently powerful complex complex of muscles; the appearance of the eyelids (protecting the eyes from mechanical damage, preventing the cornea from drying out, etc.); the formation of a middle ear cavity with a tympanic membrane and an auditory ossicle - a stirrup (ensuring the possibility of hearing in the air). An important role was played by the disappearance of the gills and the development of the lungs, larynx and choanae, creating the possibility of air breathing; the occurrence of two circles of blood circulation; great differentiation of the digestive system (large energy costs while maintaining the body in the air), etc.

The general lengthening of the hind limbs, the isolation of an additional lever in them (due to a sharp lengthening of the two proximal tarsal bones) and the possibility of a strong bend in the middle of the body at the point of articulation of the branches of the iliac bones with the transverse processes of the sacral vertebrae are adaptations to movement by jumping in tailless amphibians. Crawling tailed amphibians do not have these features. The fusion into a single whole of two bones of the forearm and two bones of the lower leg is associated with a sharp decrease in the need for rotational movements of the foot and hand when moving by jumping. In caudate amphibians, both the forearm and the lower leg consist of two independent elements, providing the rotational movements of the hand and foot necessary for crawling.

"Water" features of the structure are manifested in a number of features: a relatively weak development of ossification of the skeleton, an abundance of mucous glands in the skin (mucus covering the skin reduces friction when moving in water, prevents bacteria and fungi from penetrating the skin, etc.), preservation of the tail, often flattened laterally and bordered by a leathery fold (newts and other tailed amphibians), a great similarity of the genitourinary system with most groups of fish, external fertilization, which is characteristic of the vast majority of amphibian species, etc.

With a relatively small surface of the lungs of amphibians, sufficiently powerful additional respiratory organs are needed. Always moist (due to the abundance of mucous glands), easily permeable to moisture and gases, the skin and partly the mucous membrane of the oral cavity becomes such an organ. In an active pond frog, the lungs absorb up to 50% of the oxygen consumed by the body and release only about 14% of carbon dioxide; through skin respiration, up to 50% of oxygen is absorbed and up to 86% of carbon dioxide is released. Have a leading more earthly life common frog during pulmonary respiration, up to 67% of oxygen enters and up to 26% of carbon dioxide is released, and 33% of oxygen is absorbed by skin respiration and 74% of carbon dioxide is released. With an increase in the level of metabolism (increase in overall activity and all metabolic processes with an increase in the temperature of the environment), the specific role of the lungs in providing the body with oxygen increases markedly. A decrease in the temperature of the environment causes a decrease in the level of metabolism. At the same time, skin respiration almost completely provides both the saturation of the body with oxygen and the release of carbon dioxide, and the relative importance of the lungs in respiration decreases sharply.

This dual nature of breathing is explained not only by the insufficient development of the surface of the lungs and the imperfection of pulmonary ventilation (“swallowing” of air in the absence of chest); it is necessary for the amphibious way of life of representatives of this class. It is this duality of the respiratory organs that provides amphibians with the possibility of a long stay in the water (up to wintering at the bottom of the reservoir of many species of anurans, when, with a sharp decrease in the level of metabolism, skin respiration completely provides all the body's needs for oxygen and carbon dioxide release).

Extensive subcutaneous lymphatic cavities serve as reservoirs for reserve water. Moisture loss is also reduced due to the reabsorption of water in the bladder, posterior intestine and cloaca. Moisture loss is very sharply reduced due to the adaptive features of behavior: amphibians show increased activity only during hours of maximum air humidity (in clear weather - at dusk, as well as at night), they go to rest in minks, where high humidity is maintained due to soil moisture.

The duality of the respiratory organs makes it impossible to completely separate the large and small circles of blood circulation. But the specific features in the structure of the heart and the blood trunks extending from it provide some separation of the blood flow, despite the fact that there is only one ventricle in the heart of amphibians, and there is an admixture of arterial blood in the superior vena cava. The development of muscular outgrowths of the walls of the ventricle reduces the mixing of blood, and the departure of the arterial cone from the right (more venous) side of the ventricle and the details of its internal structure (the sequence of the discharge of arterial arches, the device of the spiral valve) allow directing more venous blood to the skin and lungs, more arterial to the brain and sense organs.

Large, compared with fish, differentiation of the digestive tract leads to some increase in the intensity of food use. However, the rate of digestion in amphibians is low and depends on the ambient temperature. Food connections are fairly simple; the range of feed used is small (only animals of relatively small sizes).

Poikilothermicity causes a pronounced seasonal change in activity in amphibians of temperate and northern latitudes: when the air temperature drops to +5 - +8 ° C, all amphibians go into shelters (some species of frogs go into pits at the bottom of reservoirs; most species of anurans and tailed amphibians hide in rodent burrows, rotten tree roots, heaps of moss, etc.) and fall into a state of stupor. The geographic distribution of amphibians is also connected with this: most of the species of these animals are characteristic of the tropical zone. In the tropics, under relatively stable temperature conditions throughout the year, in a number of amphibian species, the state of torpor is caused by a sharp decrease in air humidity (“hibernation” during the dry season).

A very large dependence of amphibians on humidity and environmental temperature is reflected in the fact that weather conditions (in our latitudes - severe droughts in summer, very coldy without snow in winter) are often the main cause of mortality and determine sharp fluctuations in the number of amphibians over the years.

additional literature

Bannikov A. G., Denisova M. N. Essays on the biology of amphibians. M., 1956
Vorontsova M. A., Liozner L. D., Markelova I. V., Pukhelskaya E. Ch. Triton and axolotl. M., 1952.
Gurtovoy N. N., Matveev B. S., Dzerzhinsky F. Ya. Practical zootomy of vertebrates. Amphibians, reptiles. M., 1978.
Terentiev P. V. Frog. M., 1950.
Terentiev P. V. Herpetology. M., 1961.
Shmalgauzen I. I. Fundamentals of comparative anatomy. M., 1947.
Shmalgauzen II Origin of terrestrial vertebrates. M., 1964.

SYSTEMATIC POSITION OF THE OBJECT

Subtype Vertebrates, Vertebrata Class Amphibians, Amphibia Detachment Tailless, Anura (Ecaudata) Representative - Lake Frog, Rana ridibunda Pall.

MATERIAL AND EQUIPMENT

For one or two students you need: 1. A disassembled frog skeleton mounted on cardboard tablets. 2. Dissecting needles - 2.

EXERCISE

Understand the structural features of the skeleton of amphibians. Make the following drawings: 1. Frog skull from above. 2. Skull from below. 3. The spinal column and the pelvic girdle attached to it from above. 4. Belt of the forelimbs (straightened) from below. 5. Skeleton of the forelimb. 6. Pelvic girdle on the side. 7. Skeleton of the hind limb.

Additional task

Compare, without sketching, the skeletons of an anuran (frog) and a tailed amphibian (wet preparation).

DESCRIPTION OF THE SKELETON

The skeleton of amphibians, like other vertebrates, is divided into the axial skeleton (vertebral column), the skull (cerebral and visceral), paired limbs and their belts. In almost all parts of the skeleton, cartilage plays a rather important role.

1. Cervical region (pars cervicalis; Fig. 54, 1 ) in all amphibians is represented by one cervical vertebra, which, with the help of two articular platforms, is movably articulated with the skull. 2. Trunk (pars thoracalis; Fig. 54, 2 ) the spine of frogs consists of 7 vertebrae (in tailed amphibians - from 14 to 63). 3. Sacral department (pars sacralis; Fig. 54, 3 ) in all amphibians is represented by one sacral vertebra, to the massive transverse processes of which the ilium bones of the pelvic girdle are attached (Fig. 54, 5 ). 4. The tail section (pars caudalis) in the larvae of tailless amphibians consists of a fairly large number of individual vertebrae, which during metamorphosis merge into one tail bone - the urostyle (urostyl; Fig. 54, 4 ). In tailed amphibians, 26-36 individual vertebrae are preserved in the tail.

Rice. 54. Axial skeleton and pelvic girdle of a frog (top view): 1 - cervical vertebra, 2 - trunk vertebrae, 3 - sacral vertebra. 4 - urostyle (fused tail vertebrae), 5 - pelvic girdle, 6 - acetabulum

Rice. 55. Trunk vertebra of a frog. A - general view; B - longitudinal section: 1 - vertebral body, 2 - superior arch, 3 - canal for the spinal cord, 4 - spinous process, 5 - transverse process, 6 - articular process

The trunk vertebrae of most frogs are of the procoelous type: the vertebral body is concave anteriorly and convex posteriorly (Fig. 55), but the last trunk vertebra has an amphicoelous (biconcave) type of structure. Above the vertebral bodies are the upper arches (arcus neuralis; Fig. 55, 2 ) that form a canal for the spinal cord. On the dorsal side of the arc there is a small spinous process (processus spinosus; Fig. 55, 4 ). Paired transverse processes depart from the upper lateral surface of the vertebral body (processus transversus; Fig. 55, 5 ); in tailed amphibians, short ribs are attached to their ends, in tailless amphibians there are no ribs. The vertebrae are connected to each other by the articulation of the vertebral bodies themselves (which is ensured by the procoelous type of their structure) and the connection of special paired articular processes (processus atricularis; Fig. 55, 6 ), located in front and behind on the basis of the upper arc. Compared with fish, amphibians are characterized by a greater differentiation of the spinal column into sections, a change in the shape of the vertebral bodies, and a stronger development of articular processes. These transformations are associated with a terrestrial lifestyle and provide greater strength of the axial skeleton while maintaining its mobility, a strong connection of the pelvic girdle with it, and allow some mobility of the skull in a vertical plane relative to the body (the ability to raise and lower the head).

Rice. 56. Frog Skull. A - from above; B - from below; B - behind; G - the lower jaw from above, the dotted line shows the cartilaginous parts of the skull 1 - lateral occipital bone, 2 - occipital condyle, 3 - anterior bone, 4 - sphenoid-olfactory bone, 5 - nasal bone, 6 - fronto-parietal bone, 7 - squamous bone , 8 - parasphenoid, 9 - palatine bone, 10 - vomer, 11 - choana, 12 - palatine-square cartilage, 13 - premaxillary bone, 14 - maxillary bone, 15 - square-zygomatic bone, 16 - pterygoid bone, 17 - Meckel cartilage, 18 - chin-jaw bone, 19 - dentary, 20 - angular bone, 21 - foramen magnum

In the cartilage of the occipital part of the brain skull, only paired lateral occipital bones (occipitale laterale; Fig. 56, 1 ), bordering the large occipital foramen (foramen occipitale magnum; Fig. 56, 21 ). Each of them forms a condyle (condylus occipitalis; Fig. 56, 2 ) for articulation with the cervical vertebrae. In the region of the auditory capsule, instead of five pairs of ear bones characteristic of bony fish, only one pair appears in amphibians - the anterior bones (prooticum; Fig. 56, 3 ). In the anterior part of the brain skull, during ossification of the cartilage, an unpaired sphenoid-olfactory bone (sphenethmoideum, Fig. 56, 4 ), having the form of a bone ring girdle.

Rice. 57. Cartilaginous skull of a tadpole: 1 - cerebral skull, 2 - palatine-square cartilage, 3 - Meckel's cartilage, 4 - gill arches, 5 - jaw joint

The rest of the brain skull remains cartilaginous. It is strengthened by integumentary (skin) bones. Above in the anterior part of the skull are paired nasal bones with an elongated triangular shape (nasale; Fig. 56, 5 ), then paired fronto-parietal bones merged from the frontal and parietal bones (frontoparietale, Fig. 56, 6 ), and outward from the ear bones - squamous bones having a complex shape (squamosum; Fig. 56, 7 ). The bottom of the brain skull is covered by a powerful cruciform integumentary bone - parasphenoid (parasphenoideum; Fig. 56, 8 ). In front of it also lie the integumentary paired palatine bones (palatinum; Fig. 56, 9 ) and paired openers (vomer; fig. 56, 10 ); small teeth sit on the coulters. In front of the openers are paired internal nostrils - choanas (Fig. 56, 11 ). The visceral region of the amphibian skull also retains much cartilage. Throughout life, the palatine-square cartilage (cartilago palatoquadratum; Fig. 56, 12 ), growing with the anterior end to the olfactory region of the brain skull, and with the posterior end - to the base of the skull in front of the auditory capsule (Fig. 57, 2 ). Therefore, the skull of amphibians, as well as other terrestrial vertebrates, is autostylic according to the type of attachment of the jaw arch. The bones of the secondary upper jaw that arise in the skin are adjacent to the palatine-square cartilage: paired premaxillary bones (intermaxillare or praemaxillare; Fig. 56, 13 ), bearing teeth and maxillary bones (maxillare; Fig. 56, 14 ). Behind them, strengthening the back of the palatine-square cartilage, an integumentary square-zygomatic bone (quadratojugale; Fig. 56, 15 ), and from below also the integumentary - pterygoid bone (pterygoideum; Fig. 56, 16 ). Primary lower jaw - Meckel's cartilage (cartilago Meckeli; Fig. 56, 17 ) also remains cartilaginous, only its very anterior end ossifies into small paired chin-jaw bones (mento-mandibulare, Fig. 56, 18 ). They are joined by integumentary dentitions (dentale; Fig. 56, 19 ), in amphibians devoid of teeth. The back of the Meckel cartilage is overgrown with a long integumentary angular bone (angulare; Fig. 56, 20 ) and a few more small integumentary bones. Through the articular process of Meckel's cartilage, the lower jaw is movably articulated with the posterior part of the palatine-square cartilage (Fig. 57, 5 ).

Rice. 58. Schematic section through the auditory region of the head of a frog: 1 - brain, 2 - auditory capsule with semicircular canals, 3 - middle ear cavity, 4 - stirrup, 5 - tympanic membrane, 6 - Eustachian tube, 7 - oral cavity

The complete reduction of the operculum in amphibians and the change of the hyostylistic type of attachment of the jaws to the autostylistic ones lead to the loss of the main functions of the hyoid arch (strengthening the jaws, supporting the operculum). Even in the ancestors of modern amphibians, the hyoid arch began to decrease, and the cavity of the spiracle (the remnant of the gill gap between the jaw and hyoid arches) in connection with the transition to life in the air was transformed into the cavity of the middle ear (Fig. 58, 3 ). Located next to the splash, the upper element of the hyoid arch - pendants (hyomandibulare) turned into an auditory bone-column, or stirrup (columel1a, or stapes; Fig. 58, 4 ). In modern tailless amphibians, the stirrup has the appearance of a thin rod-shaped bone lying perpendicular to the brain skull under the squamosal and square-zygomatic bones. With one end, the stirrup rests against the center of the tympanic membrane (Fig. 58, 5 ), and others - in the oval window of the auditory capsule. This mechanism, which amplifies sound vibrations and provides the possibility of hearing in the air, is secondarily reduced in some modern amphibians to varying degrees. Additional mechanisms that ensure the perception of sound waves propagating along a solid substrate are their lower jaw, as well as the transmission of sound vibrations through the blood trunks.

Rice. 59. Hyoid apparatus of the frog: 1 - body, 2 - horns

Paired limbs and their belts. The limbs of amphibians, kyaks and limbs of other classes of terrestrial vertebrates represent in the diagram a system of levers movably connected to each other. The schemes of the structure of the fore and hind limbs are of the same type (Fig. 60):

Rice. 60. Scheme of the structure of paired limbs (left) of terrestrial vertebrates. A - forelimb; B - hind limb: a - shoulder - thigh, b - forearm - lower leg, a - hand-foot; 1 - humerus, 2 - ulna, 3 - radius, 4 - wrist, 5 - metacarpus, 6 - phalanges of fingers, 7 - femur, 8 - tibia, 9 - fibula, 10 - tarsus, 11 - metatarsus, 12 - patella, I - V - fingers

Proximal forelimb - shoulder (humerus; Fig. 61, 7 ) - tubular bone; its middle part is called the diaphysis, and the thickened ends are called the epiphyses. In amphibians, the epiphyses of the shoulder (and hip) remain cartilaginous. The proximal end has a rounded shoulder head (caput humeri; Fig. 61, 2 ), which enters the articular fossa of the girdle of the forelimbs; at the distal end - a hemispherical surface for articulation with the bones of the forearm. The surface of the shoulder has ridges to which muscles attach. In tailless amphibians, the ulna lying outside (ulna; Fig. 61, 4 ) and from the inside - radial (radius; Fig. 61, 5 ) the bones merge into a single bone of the forearm (antebrachium, Fig. 61, 3 ); longitudinal groove shows the border of their confluence. In tailed amphibians, these bones are independent. The proximal ends of both bones form an articular fossa for connection with the shoulder; behind this fossa is the olecranon (Fig. 61, 6 ) of the ulna, limiting the extension of the limb. Wrist (carpus, Fig. 61, 7 ) consists of two rows of small bones. Five elongated bones of the metacarpus (metacarpus; Fig. 61, 8 ). The phalanges of the fingers (phalanges digitorum; Fig. 61, 9 ). In amphibians, the first (thumb) finger is greatly reduced and the hand ends with only four well-developed fingers.

Rice. 61. Forelimb and shoulder girdle of a frog: 1 - humerus, 2 - head of the shoulder, 3 - forearm, 4 - ulna, 5 - radius, 6 - olecranon, 7 - wrist, 8 - metacarpus, 9 - phalanges of fingers, 10 - scapula, 11 - suprascapular cartilage, 12 - coracoid, 13 - articular cavity for the head of the shoulder, 14 - procoracoid cartilage, 15 - clavicle, 16 - sternum, 17 - presternum, I - reduced first finger, II - V - well developed fingers

The girdle of the forelimbs, or shoulder girdle, in amphibians, as well as in shark fish, lies in the thickness of the muscles of the body, connecting it with the axial skeleton. From the upper (dorsal) scapular part of the primary belt, a scapula (scapula; Fig. 61, 10 ); its uppermost part remains cartilaginous in the form of a wide suprascapular cartilage (cartilago suprascapularis, Fig. 61, 11 ). On the anterior-outer surface of the suprascapular cartilage, some anurans have a small ossification - the remnant of the kleytrum of fish-like ancestors. The ossified coracoid part of the belt turned into a powerful coracoid bone (coracoideum; Fig. 61, 12 ), together with the scapula forming the articular cavity for the head of the shoulder (Fig. 61, 13 ). Anterior to the coracoid behind a small hole lies a cartilaginous procoracoid (cartilago procoracoidea; Fig. 61, 14 ), which rests on a thin integumentary bone - the clavicle (clavicula; Fig. 61, 15 ). The non-ossified cartilaginous inner ends of the coracoids and procoracoids of the right and left sides fuse together in the midline. Behind the coracoids is the bone sternum (sternum, Fig. 62, 16 ) with a cartilaginous posterior end. Ahead of the procoracoids, the presternum (praesternum; Fig. 61, 17 ) also with a cartilaginous end. In the girdle of the forelimbs of caudate amphibians, there is noticeably more cartilage, and the ossifications are smaller; clavicles often do not develop. The shoulder girdle serves as a support for the forelimbs and an attachment point for the muscles that control them. The chest in amphibians does not develop: the sternum does not articulate with the ribs.

Rice. 62. Hind limb (A) and pelvic girdle (B) of a frog from the side: 1 - femur, 2 - femoral head, 3 - lower leg, 4 - tibia, 5 - fibula, 6 - tarsus, 7 - tibiale, 8 - fibulare, 9 - metatarsus, 10 - phalanges of fingers, 11 - rudiment of the VI finger, 12 - ilium, 13 - ischium, 14 - pubic cartilage, 15 - acetabulum, I - V - fingers

The hind limb has an elongated tubular bone - the thigh (femur; Fig. 62, 1 ), the proximal part of which ends with a head (Fig. 62, 2 ), which enters the acetabulum (Fig. 62, 15 ) of the pelvic girdle. Tibia (tibia; Fig. 62, 4 ) and small tibia (fibula, Fig. 62, 5 ) bones of tailless amphibians merge into a single leg bone (crus, fig. 62, 3 ); in tailed amphibians they remain separated. The proximal row of tarsal bones (tarsus, Fig. 62, 6 ) tailless amphibians consists of two elongated bones that form an additional limb arm. The inner one is called tibiale (astragalus; fig. 62, 7 ; adjoins the tibial edge of the lower leg), external - fibulare (calcaneus, Fig. 62, 8 ). An ankle joint is formed between the lower leg and these bones. From the distal row of tarsal bones in amphibians, only 2-3 small bones are preserved. Metatarsus (metatarsus; Fig. 62, 9 ) is formed by five long bones, to which the phalanges of the fingers are attached (phalanges digitorum; Fig. 62, 10 ). The longest toe in frogs is IV. To the side of the I (inner) finger is a small rudiment of the VI ("pre-first") finger (praehallus; Fig. 62, 11 ). The belt of the hind limbs, or pelvic, in amphibians, as in all terrestrial vertebrates, consists of three paired elements; moreover, all of them together form the articular acetabulum (acetabulum; Fig. 62, 15 ) to connect to the femoral head. Long, forward-directed ilium (ilium; Fig. 62, 12 ) with their ends are attached to the transverse processes of the sacral vertebra (see Fig. 54). The lower part of the pelvic girdle in amphibians does not ossify and is represented by pubic cartilage (cartilago pubis, Fig. 62, 14 ). Behind him lie the paired ischial bones (ischium; Fig. 62, 13 ). In tailed amphibians, compared to anurans, there is much more cartilage in the pelvic girdle, and the formed bones are small.

CONCLUSION

Amphibians (Amphibia) are the first class of terrestrial vertebrates. However, representatives of the class still retain a constant connection with water. This duality is clearly manifested in the features of embryonic and postembryonic development. Oocytes (caviar) can only develop in water (or in rare cases in an extremely humid environment). A larva emerges from the egg - a tadpole, which has distinct signs of a typical aquatic animal: it has gills and gill arches supporting them, a two-chambered heart, one circle of blood circulation, paired limbs of a terrestrial type are absent, the main organ of movement is a powerful caudal fin, lateral line organs are developed etc. During metamorphosis (transformation), the larva loses some of the characteristics characteristic of aquatic animals, and acquires features that ensure the transition to a terrestrial (or rather, terrestrial-aquatic) lifestyle: paired limbs of a terrestrial type appear, lungs develop, gills are reduced and the skeletal apparatus that supports them, the circulatory system is rebuilt - two incompletely separated circles of blood circulation are formed, etc. The duality of organization as an adaptation to a terrestrial-aquatic lifestyle is also well expressed in adults.

With a relatively small surface of the lungs of amphibians, sufficiently powerful additional respiratory organs are needed. Always moist (due to the abundance of mucous glands), easily permeable to moisture and gases, the skin and partly the mucous membrane of the oral cavity becomes such an organ. In an active pond frog, the lungs absorb up to 50% of the oxygen consumed by the body and release only about 14% of carbon dioxide; through skin respiration, up to 50% of oxygen is absorbed and up to 86% of carbon dioxide is released. In the common frog, which leads a more terrestrial life, up to 67% of oxygen enters during pulmonary respiration and up to 26% of carbon dioxide is released, and 33% of oxygen is absorbed by skin respiration and 74% of carbon dioxide is released. With an increase in the level of metabolism (increase in overall activity and all metabolic processes with an increase in the temperature of the environment), the specific role of the lungs in providing the body with oxygen increases markedly. A decrease in the temperature of the environment causes a decrease in the level of metabolism. At the same time, skin respiration almost completely provides both the saturation of the body with oxygen and the release of carbon dioxide, and the relative importance of the lungs in respiration decreases sharply.

This dual nature of breathing is explained not only by the insufficient development of the surface of the lungs and the imperfection of pulmonary ventilation (“swallowing” air in the absence of a chest); it is necessary for the amphibious way of life of representatives of this class. It is this duality of the respiratory organs that provides amphibians with the possibility of a long stay in the water (up to wintering at the bottom of the reservoir of many species of anurans, when, with a sharp decrease in the level of metabolism, skin respiration completely provides all the body's needs for oxygen and carbon dioxide release).

The use of skin for breathing is possible only when it is easily permeable to moisture and gases. But such skin cannot protect the body from large losses of water (drying). Therefore, almost all species of amphibians inhabit only damp, damp areas where the body loses less moisture and can always make up for its loss. In relatively few toads associated with water (they winter on land, they go into water bodies only for spawning) the skin is thickened; this reduces the possibility of skin respiration, which is compensated by an increase in the inner surface of the lungs. However, even in them, despite the thickening of the skin, the body loses up to 15-30% of water during the night period of hunting. Some reduction in moisture loss (while maintaining the permeability of the skin) in amphibians helps mucus covering the skin. Extensive subcutaneous lymphatic cavities serve as reservoirs for reserve water. Moisture loss is also reduced due to the reabsorption of water in the bladder, posterior intestine and cloaca. Moisture loss is very sharply reduced due to the adaptive features of behavior: amphibians show increased activity only during hours of maximum air humidity (in clear weather - at dusk, as well as at night), they go to rest in minks, where high humidity is maintained due to soil moisture.

Amphibians, like fish, are characterized by variability in body temperature (poikilothermy): in amphibians it is usually only 0.5-1 ° C higher than the ambient temperature. Only during the period of the highest activity (pursuit of prey, avoidance of danger) the body temperature can exceed the temperature of the environment by 5-7°C. Poikilothermicity causes a pronounced seasonal change in activity in amphibians of temperate and northern latitudes: when the air temperature drops to +5 - +8 ° C, all amphibians go into shelters (some species of frogs go into pits at the bottom of reservoirs; most species of anurans and tailed amphibians hide in rodent burrows, rotten tree roots, heaps of moss, etc.) and fall into a state of stupor. The geographic distribution of amphibians is also connected with this: most of the species of these animals are characteristic of the tropical zone. In the tropics, under relatively stable temperature conditions throughout the year, in a number of amphibian species, the state of torpor is caused by a sharp decrease in air humidity (“hibernation” during the dry season). The very large dependence of amphibians on humidity and environmental temperature is reflected in the fact that weather conditions (in our latitudes - severe droughts in summer, severe frosts without snow in winter) often serve as the main cause of mortality and determine sharp fluctuations in the number of amphibians over the years.

additional literature

Bannikov A. G., Denisova M. N. Essays on the biology of amphibians. M., 1956 Vorontsova M. A., Liozner L. D., Markelova I. V., Pukhelskaya E. Ch. Triton and axolotl. M., 1952. Gurtovoy N. N., Matveev B. S., Dzerzhinsky F. Ya. Practical zootomy of vertebrates. Amphibians, reptiles. M., 1978. Terentiev P.V. Frog. M., 1950. Terentiev P.V.. Herpetology. M., 1961. Schmalhausen I. I. Fundamentals of comparative anatomy. M., 1947. Schmalhausen I. I. Origin of terrestrial vertebrates. M., 1964.

The main parts of the skeleton of amphibians are the same as those of fish: the vertebral column, the skull (cerebral and visceral), limbs and their belts. However, in connection with the release of amphibians to land, their skeleton is undergoing a number of key changes.

There are fewer bones in the skeleton of amphibians compared to fish. This is achieved, among other things, due to the fact that a number of cartilages do not ossify throughout life. As a result, the skeleton of amphibians is lighter than that of fish. This is important for living in a terrestrial environment, since air has a lower density than water, and therefore it has less buoyant force. It becomes harder for the animal to carry its body, so making it easier to land on land is an important evolutionary task.

The spine of amphibians consists of four sections, and not two, like in fish. In addition to the trunk and tail sections, amphibians have cervical and sacral sections.

The cervical spine consists of only one vertebra, movably attached to the skull. Thanks to this, amphibians can move their heads up and down (they cannot right and left).

The sacral region also consists of one vertebra, to which the ilium bones of the hind limb girdles are attached.

The trunk section includes 7 vertebrae in frogs and toads (they belong to the order Ansalis). However, in amphibians belonging to the Legless order, their number reaches 200. Small ribs are attached to the trunk vertebrae. However, in anurans they are completely reduced. The chest in the skeleton of amphibians is not formed.

The tail section of the spine in anurans consists of one flattened bone, which is formed by the fusion of the vertebrae of the tail section. In caudate and legless amphibians, individual vertebrae remain in the caudal region.

The skull is flattened and broad. The upper jaw fuses with the brain skull, the lower jaw remains mobile.

The limbs of amphibians are of the so-called lever type, when one part of the limb can move relative to the other, which plays a key role when moving along the ground. The paired limbs of amphibians not only participate in swimming like fish, but thanks to them amphibians walk, jump and even climb. In tailless amphibians (toads, frogs, tree frogs, etc.), the hind limbs are more powerful than the front ones.

The forelimbs of amphibians consist of the humerus, two bones of the forearm, and the bones of the hand. The hand is formed by the bones of the wrist, metacarpus and fingers.

The hind limbs consist of the femur, two lower leg bones, and foot bones. The foot consists of the bones of the tarsus, metatarsus and fingers.

The limbs in the skeleton of amphibians are attached to the limb belts, which are their support. The stronger the support, the more force can be applied.

The girdle of each forelimb (shoulder girdle) includes the clavicle, shoulder blade, and crow bone. The girdles of both forelimbs are attached to a common sternum. Since the ribs in amphibians are short or absent at all, the sternum is not attached to the spine by means of them. Thus, the shoulder girdle lies freely in the body of the frog.

The belt of the hind limbs (pelvic girdle) includes the ilium, ischium and pubic bones, which fuse together. The pelvic girdle is attached to the spine (sacrum) through the ilium.

STRUCTURE OF AMPHIBIANS

Skin. The skin of all amphibians is naked, devoid of scales. The epidermis is rich in multicellular glands (Fig. 68). The significance of the skin glands is diverse. They provide a liquid film on the surface of the skin, without which gas exchange is impossible during skin respiration. This film to some extent protects the body from drying out. The mucus helps coagulate foreign particles on the surface of the body. The secretions of some skin glands have bactericidal properties and protect the body from the penetration of pathogenic microbes through the skin. Poisonous skin glands largely protect amphibians from predators. Often poisonous forms are brightly colored,

Rice. 68. Incision through the skin of a salamander:
1 - protruding secret of the gland; 2 - pigment layer; 3 - mucous skin glands; 4 - poisonous skin gland; 5 - cut blood vessels, 6 - epidermis; 7 - fibrous layer of the skin

warning predators of danger when grasping prey. It has been established that in some tailless amphibians the upper layer of the epidermis becomes keratinized. This is most strongly developed in toads, in which the stratum corneum on the back makes up approximately 60% of the entire surface of the epidermis. In most amphibians, weak keratinization of the epidermis does not prevent water from penetrating through the skin, and, being in a pond, amphibians constantly "drink water with their skin."

Skeleton amphibians compared with fish has a number of changes. in the axial skeleton spine amphibians in connection with the semi-terrestrial way of life is more dissected. It includes cervical, trunk, sacral and caudal departments (Fig. 69). The cervical region is represented by one vertebra, its body is small and has two articular fossae, with the help of which the vertebra is articulated with the skull. The number of trunk vertebrae varies. The smallest number of them in anurans (usually 7), the largest - in legless (more than 100). The only sacral vertebra (missing in legless ones) bears long transverse processes, to which the iliac bones of the pelvis are attached. The tail section is most typically expressed in caudates, in legless it is very small, and in anurans it is represented by a bone - urostyle: during embryonic development, it is laid in the form of a series of individual vertebrae, subsequently fused.

The shape of the vertebrae in different representatives within the class of amphibians is extremely variable. In lower amphibians (legless, lower tailed) it happens amphicoelous in this case, the chord is preserved between the vertebrae for life. Anurans have vertebrae procellar, i.e. concave in front and curved behind; at

higher caudate - opisthocoelous, i.e. curved in front and concave in the back. There are many options for deviating from this characteristic, for example, in an extremely primitive New Zealand frog leopelma(Leopelma) vertebrae are amphicoelous. True ribs do not develop in anurans; legless amphibians have very short ones; caudates develop short "upper" ribs.

Brain skull. The brain box in a significant part remains cartilaginous for life (Fig. 70). This is due to the weak development of chondral and superimposed ossifications. The following chondral bones develop in the primary brain skull. In the occipital region, only two lateral occipital bones places corresponding to the main and upper occipital bones of fish remain cartilaginous. In the area of the auditory capsule, one small ear bone, while most of the capsule remains cartilaginous. In the anterior part of the eye socket, anurans develop one cunei-olfactory bone, in caudates this bone is paired. The olfactory capsule is cartilaginous.

There are also few integumentary bones. The roof of the skull is made up of the parietal and frontal bones, which in anurans fuse into frontoparietal bones. Ahead of them are nasal bones, in the legless they fuse with the premaxillae. On the sides of the back of the skull are located squamosal bones, especially strongly developed in legless ones. The bottom of the skull is lined with a large parasphenoid, and in front of him are paired vomer bones.

The bones of the visceral skeleton also take part in the formation of the bottom of the skull - palatine and pterygoid. The first are adjacent to

Rice. 69. Vertebral column of a frog along with the pelvic girdle (from the dorsal side):
I- cervical region(from one vertebra), II - trunk section; III - sacrum, IV - urostyle, 1 - spinous process of the 3rd trunk vertebra, 2 - transverse process of the same vertebra, 3 - articular surfaces on the first cervical vertebra

Rice. 70. Frog Skull:
I - top view; II - ventral view (false bones on one side removed). Designation of bones: 1 lateral occipital, 2 - fronto-parietal; 3 - nasal; 4 - intermaxillary, 5 - anterior; 6 - scaly; 7 - parasphenoid (its left half); 8 - coulter; 9 - palatine; 10 - pterygoid; 11 - wedge-olfactory; 12 - maxillary; 13 - quadrangular; 14 - hole for the exit of the optic nerve, 15 - hole for the trigeminal nerve

vomers, the second - to squamous bones. They develop on the lower surface of the palatine-square cartilage. The functions of the upper jaws are performed, as in bony fish, premaxillary(or intermaxillary) and maxillary bones.

The lower jaw is represented by Meckel's cartilage, which is covered from the outside. dental and angular bones.

Amphibian skull autostylic, i.e. palatine-square cartilage directly adheres to the cerebral skull. Due to the auto-style of the skull, the hyoid arch does not participate in the attachment of the jaw apparatus to the skull. The upper element of this arc - pendants (hyomandibular) - turned into a small bone - stirrup, which, with its proximal end, rests against the auditory capsule, and with its outer (distal) end, against the tympanic membrane. In connection with the formation of the middle ear cavity, this bone is located inside this cavity and plays the role of the auditory ossicle. Thus, the hyomandibular (suspension) leaves the system of the fourth (hyoid) visceral arch (Fig. 70).

The lower elements of the hyoid arch and gill arches are modified into sublingual plate and her horns. This plate is located between the branches of the lower jaw. Her front horns

Rice. 71. Frog shoulder girdle in front:
1 - sternum; 2 - cartilaginous anterior and posterior parts of the sternum; 3 - coracoid; 4 - clavicle lies on the procoracoid; 5 - presternum; 6 - scapula; 7 - articular cavity on the shoulder blade for the shoulder (cartilage is covered with dots)

bending upwards and covering the intestinal tube from the sides, they are attached to the auditory capsules. Changes in the visceral skeleton are accompanied by the loss of gill covers.

Thus, the skull of amphibians differs from the skull of most bony fishes: 1) by the weak development of chondral and skin ossifications; 2) autostyle; 3) modification of the hyoid and gill arches, partly converted into auditory, partly into the hyoid apparatus; 4) reduction of the gill cover.

Limb belts. Shoulder girdle has the form of an arc, the apex facing the abdominal surface of the animal (Fig. 71). Each half of the arc (left and right) consists of the following main elements. The upper (dorsal) part is presented spatula with wide suprascapular cartilage. The lower (abdominal) part includes coracoid and lying in front of him procoracoid. In anurans between the presternum and the scapula there is a thin rod-shaped collarbone. The listed elements of the belt converge at the point of attachment of the humerus and form an articular fossa. Anterior to the junction of the left and right coracoids is presternum, and behind - sternum. Both of these bones end in cartilage. The shoulder girdle, unlike bony fish, lies freely in the thickness of the muscles and is not connected to the skull. Due to the absence or incomplete development of the chest ribs, amphibians do not have.

Pelvic girdle(Fig. 72) is formed by three paired elements converging in the region of the acetabulum, which they form. Long iliac bones with their proximal (anterior) ends are attached to the transverse processes of a single sacral vertebra. Forward and downward pubic the element of the girdle in frogs remains cartilaginous. Behind him is ischial bone. This arrangement of the elements of the pelvic girdle is characteristic of all terrestrial vertebrates.

Skeleton of free limbs typical of terrestrial vertebrates and significantly different from the skeleton of the limbs of fish. While the limbs of fish represent simple

Rice. 72. Frog Skeleton:
I - whole skeleton; II - vertebra from above; III - vertebra in front: 1 - cervical vertebra; 2 - sacral vertebra; 3 - urostyle; 4 - sternum; 5 - cartilaginous back of the sternum; 6 - presternum; 7 - coracoid; 8 - procoracoid; 9 - scapula; 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 - lower leg (large and small tibia); 23 - tarsus; 24 - metatarsus; 25 - rudiment of an additional finger; 26-1 finger; 27 - vertebral body; 28 - spinal canal; 29 - articulating platform; 30 - spinous process; 31 - transverse process

single-membered levers that move only relative to the body and do not carry muscles, the limbs of terrestrial vertebrates are multi-membered levers with sufficiently powerful muscles. In this case, not only the entire limb moves relative to the body, but individual elements of the limb also move relative to each other.

In the scheme, the five-fingered limb consists of three main sections (Fig. 72).

I- shoulder in the anterior limb hip- in the back; this section always consists of one bone, which is attached to the girdle with its proximal end;

II- forearm in the anterior limb shin in the back. In a typical case, the department consists of two parallel

bones: forearm - from ulnar and ray, lower leg - from tibia and small tibia,

III- brush in the forelimb and foot in the hind; The department consists of three subdivisions:

1) wrist- in the forelimb tarsus- in the back; this subsection is represented in a typical case by 9-10 small bones arranged in three rows;

2) metacarpus- in the forelimb metatarsus- in the back; in a typical case, the subdivision consists of 5 elongated bones arranged in one row, as if in a fan, from the wrist or tarsus;

3) phalanges of four to five fingers represent, as it were, a continuation of the metacarpus or metatarsus and include three to five rows of bones in each.

The skeleton of the limbs of tailed amphibians almost completely corresponds to the above diagram. There are some deviations in frogs. The main ones are as follows: both elements of the forearm and lower leg are fused into one bone, most of the bones of the carpus and tarsus are fused together, in front of the first finger of the hind limb there is a rudiment of an additional finger. These features are of a secondary nature and are apparently associated with the adaptation of frogs to locomotion by jumping.

Muscular system differs significantly from the muscular system of fish in two main features associated with the movement of animals with the help of five-fingered limbs and, to a certain extent, on a solid substrate. Firstly, powerful and complexly organized muscles develop on free limbs. Secondly, in connection with complex movements, the musculature of the body is more differentiated, and its segmentation, characteristic of fish, is disturbed in amphibians. The metamerism of the muscular system is more pronounced in caudate and legless animals. In turn, in anurans it can be seen only in a few places on the body in adult forms and in the larval state.

Digestive organs. The oral fissure leads to a large oropharyngeal cavity, which, narrowing, passes into the esophagus. Open into the oropharyngeal cavity choanae, eustachian foramen(middle ear cavity) and laryngeal fissure. The ducts absent in fish also open here. salivary glands. Their secret serves to wet the food bolus and does not chemically affect the food. At the bottom of the oropharyngeal region is located real language having its own musculature. The form of the language is varied. In some caudates, it is fixedly attached, in others it looks like a mushroom sitting on a thin stalk. In frogs, the tongue is attached at one end to the bottom of the mouth, and its free part, in a calm state, is turned inward. All amphibians have a language

secretes a sticky substance and is used to catch small animals. Few amphibians do not have a language.

On the premaxillary and maxillary bones, on the vomer, and in some of them even on the lower jaw teeth. They look like small monotonous cones, the tops of which are somewhat bent back. Some species, such as toads, do not have teeth on the jaw bones. When swallowing, pushing the food bolus from the oropharyngeal region into the esophagus is helped by the eyeballs, which are separated from this cavity only by a thin mucous membrane and, with the help of special muscles, can be somewhat drawn into the oropharynx.

The short esophagus empties into a relatively poorly demarcated stomach. Actually intestines relatively longer than in fish. In the front loop ( thin) department lies pancreas. Large liver has gallbladder, its duct flows into the anterior part of the small intestine (duodenum). The ducts of the pancreas also flow into the bile duct, which has no independent communication with the intestines (Fig. 73). The second section of the intestine thick- delimited from the thin section indistinctly. On the contrary, the third straight department is well separated. It ends cloaca.

Rice. 73. General location viscera of a female frog:
1 - esophagus; 2 - stomach; 3 - lobes of the liver; 4 - pancreas; 5 - small intestine; 6 - large (rectum) intestine; 7 - cloaca (opened); 8 - ventricle of the heart; 9 - left atrium; 10 - right atrium; 11 - carotid artery (right); 12 - left aortic arch; 13 - pulmonary-skin arc (right); 14 - posterior (lower) vena cava; 15 - abdominal vein; 16 - lungs; 17 - left kidney (posterior end); 18 - right ovary; 19 - left oviduct; 20 - its mouth (funnel); 21 - bladder; 22 - gallbladder; 23 - spleen; 24 - anterior vena cava (right)

Respiratory system amphibians are varied. As an adult, most species breathe through the lungs and through the skin. Lungs are paired bags with thin cellular walls. With a relatively small surface of the lungs, the value is very large skin respiration. The ratio of the surface of the lungs to the surface of the skin in amphibians is 2:3 (while in mammals, the inner surface of the lungs is 50-100 times the surface of the skin). In a green frog, 51% of oxygen enters through the skin. The role of the skin in the excretion of carbon dioxide from the body is important: 86% is excreted through the skin, and 14% through the lungs.

Skin respiration is of great functional importance not only in connection with the imperfection of the lungs, but also as a device that ensures the oxidation of the blood when the animal is in the water for a long time, for example, during hibernation or hiding in a reservoir when pursued by land-based predators. In these cases, only skin breathing is carried out, and the right atrium (where the oxidized blood of the skin vein flows through the vena cava - see below) becomes arterial, and the left - venous.

In the American lungless salamanders and in the Far Eastern newt, the lungs are completely atrophied, and gas exchange in them takes place entirely through the skin and oral mucosa.

The ability of the skin and lungs to absorb oxygen (O 2) and release carbon dioxide (CO 2) in amphibians depends on the temperature of the environment. Table 5 presents data for the green frog.

As can be seen, at an ambient temperature of +5°C, 1.5 times more oxygen enters through the skin than through the lungs. At a temperature of +25°C, on the contrary, 2 times more oxygen enters through the lungs than through the skin. The situation is different with the release of CO 2 . With an increase in temperature from +5 to +25 ° C, the value of the skin in the release of carbon dioxide increases only 3.3 times, and the lungs - 7 times. At any temperature, the role of the skin in removing CO2 is noticeably greater than that of the lungs.

Table 5

Dependence of O 2 consumption and CO 2 release on environmental temperature in a green frog
(after Prosser, 1977)

Amphibian larvae breathe with the help of branched external gills, which subsequently disappear in the vast majority of species, while in Proteus and Sirens they remain for life. Amphiums in adulthood, along with lungs, also have internal gills.

Due to the absence of the chest, the mechanism of pulmonary respiration is very peculiar. The role of the pump is performed by the oropharyngeal cavity, the bottom of which either descends (air is sucked in with open nostrils), then rises (air is pushed into the lungs with closed nostrils). Therefore, the skull of amphibians at a low height is extremely wide: the efficiency of pulmonary respiration is the greater, the greater the distance between the branches of the lower jaw. For example, it reaches its greatest width in toads, whose keratinized skin has a small respiratory load.

Circulatory system. Heart all amphibians have three chambers, consists of two atrial and one ventricle(Fig. 74). In lower forms (legless and caudate), the left and right atria are not completely separated. In anurans, the septum between the atria is complete, but in all amphibians, both atria communicate with the ventricle by one common opening. In addition to these main parts of the heart, there is venous sinus. It receives venous blood and communicates with the right atrium. Adjacent to the heart arterial cone, into which blood flows from the ventricle. The arterial cone has spiral valve involved in the distribution of blood into three pairs of vessels emerging from it. Cardiac index (the ratio of heart mass to body weight in percent) varies and depends on the motor activity of the animal. So, in relatively little moving grass and green frogs, it is 0.35-0.55%, and in a completely terrestrial (except for the breeding season) and active green toad, it is 0.99%.

In higher - tailless - amphibians, originate from the arterial cone three pairs of arterial arches.

The first pair (counting from head to tail) carries blood to the head, this is - carotid arteries. They are homologous to the first pair of gill arteries in fish. The second pair, also extending from the ventral side of the arterial cone, is homologous to the second pair of gill vessels of fish and is called systemic aortic arches. They depart from her subclavian arteries that carry blood to the shoulder girdle and forelimbs. The right and left system arcs, having described a semicircle, are connected together and form dorsal aorta, located under the spine and giving rise to arteries going to the internal organs. The last, third, pair, homologous to the fourth pair of gill arteries of fish, departs not from the ventral, but from the dorsal side of the arterial cone. It carries blood to the lungs and is pulmonary arteries. From each lung

artery leaves a vessel that carries venous blood to the skin: this cutaneous arteries.

In tailed amphibians, which have lungs, the arrangement of arterial vessels is basically the same. But, unlike the anurans, they retain a pair of arteries corresponding to the third pair of gill vessels, and thus the total number of paired arterial trunks in them is four, and not three, as in the anurans. In addition, the pulmonary arteries maintain communication with the systemic aortic arches through the so-called botal ducts(see fig. 77 on p. 137).

In caudate amphibians, in which gills are preserved for life, the circulatory scheme is very close to that of fish and larvae of higher amphibians. They have four pairs of arterial arches departing from the abdominal aorta. The carotid arteries depart from the efferent part of the first arc and remain carotid ducts. With the advent of the lungs, the pulmonary arteries are formed, originating from the fourth arterial arch. The circulatory scheme in this case is almost identical to that lungfish(see fig. 44 on p. 83).

The venous system of lower amphibians is similar to that of lungfish. tail vein divided into two portal veins kidneys. From them, blood enters the unpaired posterior vena cava and in pairs posterior cardinal veins. The latter at the level of the heart merge with paired jugular, subclavian and skin veins and form the Cuvier ducts, pouring blood into the venous sinus. Blood is collected from the intestines subintestinal and abdominal veins, which merge to form portal vein of the liver. From the liver, blood enters the vena cava through the hepatic vein.

In anurans, the cardinal veins are not preserved, and all blood from the trunk region is ultimately collected in posterior vena cava flowing into the venous sinus. As in the previous case, there are abdominal and axillary veins that form the portal circulatory system in the liver. Due to the lack

Rice. 74. Scheme of the structure of the opened heart of a frog:
1 - right atrium; 2 - left atrium; 3 - ventricle; 4 - a valve that closes the common opening leading from both atria to the ventricle; 5 - arterial cone; 6 - common arterial trunk; 7 - skin-pulmonary artery; 8 - aortic arch; 9 - common carotid artery; 10 - carotid gland; 11 - spiral valve arterial cone

cardinal veins in anurans are not formed and the Cuvier ducts. jugular veins, merging with the subclavian, in this case form paired anterior vena cava, flowing into the venous sinus, or sinus. into the superior vena cava and cutaneous veins of the corresponding side, which carry not venous, but arterial blood.

Pulmonary veins carry blood directly to the left atrium. Venous blood (with a fairly significant admixture of oxidized blood coming from the skin veins into the anterior vena cava) flows into the venous sinus (sinus), and from there into the right atrium. When the atria contract, venous and arterial blood is poured out through the opening common to both atria into the ventricle. The arterial cone adjoins its right side, into which more venous blood first enters, which goes further into the open opening of the skin-pulmonary arteries. The openings of the remaining arterial arches are covered at this time cone arterial valve. With further contraction of the ventricle, the pressure in the arterial cone increases, the spiral valve shifts and the openings of the systemic aortic arches open, through which mixed blood flows from the central part of the ventricle. Further shifting of the spiral valve frees the mouths of the carotid arteries, where the most oxidized blood passes, leaving the arterial cone last from the left side of the ventricle. With all this, there is still no complete separation of arterial and venous blood flows.

The speed of blood movement (an important indicator of the intensity of metabolism) in amphibians is low. This is indirectly evidenced by the pulse rate. In a common frog with a body weight of 50 g, it is equal to 40-50 beats per minute. For comparison, we point out that in a bird similar in size, this indicator is approximately 500. In aquatic forms, bradycardia is observed. Level blood pressure amphibians are also low. In caudates, it is approximately 22/12-30/25, in tailless ones - 30/20. For comparison: in scaly reptiles, this figure is about 80/60 (Prosser, 1978).

Nervous system. Brain(Fig. 75) is characterized by a number of progressive features. This is expressed in the relatively larger forebrain than in fish, in the complete separation of its hemispheres, and in the fact that not only the bottom of the lateral ventricles, but also their sides and roof contain nerve cells. Thus, amphibians have a real brain vault - archipallium, which among bony fish is characteristic only of lungfish. midbrain relatively small sizes. Cerebellum very small, and in some caudates (in Proteus) it is practically

Rice. 75. Frog Brain:
I - above, II - below; III - on the side; IV - in a longitudinal section; 1 - hemispheres of the forebrain; 2 - olfactory lobe, 3 - olfactory nerve; 4 - diencephalon; 5 - visual chiasm; 6 - funnel, 7 - pituitary gland, 8 - midbrain; 9 - cerebellum; 10 - medulla oblongata; 11 - fourth ventricle, 12 - spinal cord 13 - third ventricle; 14 - Sylvian plumbing; 15 - head nerves

invisible. The weak development of this part of the brain is associated with extremely monotonous, uncomplicated movements of amphibians. Completes everything medulla. Ten pairs leave the brain head nerves(1-X).

spinal nerves in caudate and tailless form well-defined brachial and lumbar plexuses. well developed sympathetic nervous system, represented mainly by two nerve trunks located on the sides of the spine.

sense organs amphibians are more developed than fish. Olfactory organs amphibians represent paired olfactory capsules, whose inner surface is lined with olfactory epithelium. They communicate with the external environment with paired external nostrils; depart from the olfactory capsules internal nostrils (choanas), communicating with the oropharyngeal cavity. In amphibians, as in all terrestrial vertebrates, this system serves for the perception of odors and for respiration.

Lateral line organs

organs of taste. Located in the oral cavity. It is assumed that the frog perceives only bitter and salty.

organs of vision. The eyes of amphibians have a number of features associated with a semi-terrestrial lifestyle: 1) moving eyelids protect the eyes from drying and pollution; at the same time, in addition to the upper and lower eyelids, there is also a third eyelid, or nictitating membrane located in the anterior corner of the eye; 2) yes lacrimal gland, the secret of which washes the eyeball; 3) convex (and not flat, like fish) cornea and lenticular (not round like fish) lens; both of the latter features determine the farsighted vision of amphibians (it is interesting that in water the cornea of amphibians becomes flat); four) accommodation of vision achieved, as in sharks, by displacement of the lens under the action of the ciliary muscle. There is no information about the color vision of amphibians.

hearing organ it is much more complex than that of fish, and is adapted to the perception of sound stimuli in the air. This is most fully expressed in tailless amphibians. Except inner ear, presented, as in fish, membranous labyrinth, amphibians also have middle ear. The latter is a cavity, one end of which opens into the oropharynx, and the other comes to the very surface of the head and is covered with a thin membrane called tympanic. This cavity forms a bend, the top of which is located at the membranous labyrinth. Top part cavity from the tympanic membrane to the membranous labyrinth is called tympanic cavity. It contains a rod-shaped bone - stirrup, which at one end rests on oval window inner ear, others - in the eardrum. The lower part of the middle ear cavity that opens into the oropharynx is called eustachian tube.

Comparative anatomy and embryological data show that the middle ear cavity is homologous to the fish spatter, i.e. rudimentary gill slit lying between the jaw and hyoid arches, and the auditory ossicle is homologous to the upper section of the hyoid arch - the hyomandibular. This example shows that an important change in an organ can be achieved by modifying and changing the functions of formations that previously existed in primitive forms.

In the legless and caudate, the tympanic membrane and tympanic cavity are absent, but the auditory ossicle is well developed. The reduction of the middle ear in these groups is apparently a secondary phenomenon.

Lateral line organs characteristic of the larvae of all amphibians. In the adult state, they are preserved only in aquatic forms of tailed amphibians and a few, also aquatic, tailless. Unlike fish, the sensory cells of this organ are not located in a recessed channel, but lie superficially in the skin.

excretory organs(Fig. 76) are arranged according to the type of their organization in cartilaginous fish. In the embryonic state, the excretory organ

serves pronephros, in adults - mesonephros with its typical output path - wolf channel. The ureters open into the cloaca. Here at the higher land amphibians opens bladder. After filling it, urine is discharged through the same opening into the cloaca and then expelled out.

The number of nephrons in lower (tailed) amphibians is about 500, in higher (tailless) - about 2000. Such a noticeable difference, apparently, is determined by the fact that in caudates, which are more closely associated with water bodies, there is also an extrarenal (through the skin and gills) path excretion of products of nitrogen metabolism. In caudates, unlike anurans, nephrons (or at least part of them) have nephrostomes, i.e. funnels communicating them with the body cavity (primitive feature). Vascular tangles in Bowman's capsules are well developed, and amphibians excrete a lot of liquid urine. For example, we point out that in frogs of the genus Rana, the blood filtration rate is about 35 ml/(kg*h).

The main product of protein metabolism in amphibians is urea, which is not very toxic, but requires a large amount of water to be excreted from the body, in which it dissolves. Physiologically, this is quite justified, since the absorption of water by the body of amphibians in the overwhelming case does not encounter difficulties.

The connection between the type of protein metabolism and environmental conditions is also proved by the following two examples. In the newt in autumn on land, the share of ammonia in the total products of nitrogen metabolism is 13%, and during the summer water existence it increases to 26%. In a tadpole, the proportion of ammonia is 75%, while in a frog that has lost its tail and with developed legs, it is only 16%.

Reproductive organs. In males, paired testicles do not have independent excretory tracts. vas deferens

Rice. 76. genitourinary system male frog:
1 - kidney; 2 - ureter; 3 - cavity of the cloaca; 4 - urogenital opening; 5 - bladder; 6 - opening of the bladder; 7 - testis; 8 - vas deferens, 9 - seminal vesicle; 10 - fat body, 11 - adrenal gland

pass through the anterior part of the kidney and flow into the wolffian canal, which, therefore, serves not only as an ureter, but also as a vas deferens. Each wolf canal in males forms an extension before flowing into the cloaca - seminal vesicle The in which the seed is temporarily reserved.

Above the testes lie fat bodies- irregularly shaped formations yellow color. They serve to nourish the testicles and the spermatozoa developing in them. The size of the fat bodies varies with the seasons. In autumn they are great; in the spring, during intensive spermatogenesis, their substance is energetically consumed and the size of the fat bodies is sharply reduced. The vast majority of amphibians do not have copulatory organs.

Females develop paired ovaries, fat bodies also lie above them. Ripe eggs enter the body cavity, from where they enter the funnel-shaped extensions of the steam rooms. oviducts - müllerian canals. The oviducts are long, highly convoluted tubes, the posterior section of which opens into the cloaca.

From the foregoing, it can be seen that, like in cartilaginous fish, in male amphibians, the urinary and genital ducts are combined and represent a single wolf canal, while in the female wolf, the canal performs the function of only the ureter, and the reproductive products are excreted through an independent genital duct - the oviduct, or muller channel.

Spine. And in the axial skeleton of amphibians, a genetic connection with fish is found. This is most clearly seen during the development of amphibians. The chord of embryos and larvae of amphibians is very close to that of fish both in structure and development. In some cases, that primary diplospondyly, the duality of the arches, which is characteristic of lower fish, is still found. So, in Siredon, the upper cartilaginous arches are laid double, while in the lower ones there is a gap indicating their initial duality, the composition of the cranial and caudal arches. In the rest of Urodela, only the caudal arches are laid, and from the remains of the cranial, the intervertebral cartilage is laid, as is observed in Lepidosteus, from which, by forming a gap in it, the articulation is outlined. According to the shape of the bodies, the vertebrae of amphibians are amphicoelous, opisthocoelous and procoelous (Fig. 233), the spine is well developed, completely ossified, cartilage is rare in adults. The shape of the vertebral bodies corresponds to the way of movement in a different environment.

The vertebrae of the stegocephalians show a very great resemblance to the vertebrae of the ancient Teleostomi; it was among the more ancient representatives of the stegocephalic vertebrae that consisted of several elements that formed biconcave bodies (Fig. 234). In the Rhachitomi order, there were 4 pairs of these elements in each vertebra: basidorsale, basiventrale, interdorsale and interventrale; in Embolomeri, the vertebrae had a double body, as in the caudal part of Amia. Spine from individual elements could not, of course, be preserved in the Tetrapoda, who had completely landed, because it was too weak for them. Segmentation, and hence primary diplospondylia, evolved to facilitate flexion of the body when moving through the water. In terrestrial animals, movement is not accomplished with the help of body bends, and therefore diplospondylia is not only unnecessary, but also harmful. Fusion of parts of the vertebrae already began in stegocephals in the orders Stereospondyli (Labyrinthodontia), Branchiosauria (Phyllospondyli). From the vertebrae of the latter type, the spine of modern amphibians developed, development proceeding along two different paths in caudates and anurans. Another type of formation of a whole vertebra, which led to the formation of vertebrae of the reptile type, we see in representatives of the stegocephalus - Ceraterpetomorpha, although the latter were not the ancestors of reptiles; the similarity here is convergent.

Amphibian vertebrae have strongly developed transverse processes, as well as anterior and posterior articular processes - pre- and post-zygapophyses, which are connected to each other with the help of real joints - diarthroses. Ribs in modern amphibians are greatly reduced, sometimes rudimentary or even completely absent (in Anura), but were more developed in stegocephals. In Anura, instead of the ribs, there are only strongly developed transverse processes. The ribs are more developed in Apoda and Urodela, but in them they are limited only to the body, while in the stegocephalus they developed up to half of the tail. At the proximal end, the amphibian ribs are bifurcated in a fork-like manner and are connected both to the upper arch and to the process extending from the body. This type of attachment of the ribs first appears in amphibians, since in fishes the articulation takes place only with the lower arch, and is repeated later in higher vertebrates.
In amphibians, there is already some dissection of the spine into sections. The first vertebra in the form of a ring articulates with the skull with the help of two articular fossae, corresponding to two articular tubercles on the last one. The second vertebra is normal. Atlas and epistropheus, characteristic of higher vertebrates, are not yet here. Amphibians have a sacral vertebra, to which the pelvic girdle is connected and followed by the tail section. In Anura, the latter is represented by a long rod-shaped bone - the urostyle (Fig. 235). Traces of segmentation in some amphibians (nerve foramina, rudimentary transverse processes, and rudimentary superior arches) show that the urostyle was formed from the fusion of 12 vertebrae. Triassic Protobatrachus had vertebrae in its tail.

The number of vertebrae in amphibians varies: Anura usually has 8 trunk and 1 sacral vertebrae, followed by the urostyle; in Urodela, the number of trunk vertebrae varies from 14 (Triturus) to 63 (Amplxiuma), and the caudal vertebrae range from 22 (Cryptobranchus) to 36 (Triturus); Apoda has 200 to 300 vertebrae, of which 25-30 are in the tail.
Scull. The skull of modern amphibians differs significantly from the skull of fish, but is easily associated with the latter through the stegocephalic skull. As mentioned above, the occipital part of the skull is short,
the trunk vertebrae are not part of the skull. The part of the skull built from trabeculae is wide, the skull is platybasal like in lampreys and in the most ancient fishes: "gapoids", crossopterygii and lungfishes. The cranial cavity extends far forward between the eye sockets.
The palatine cartilage is firmly connected to the skull (autostylic skull). Hyomandibulare turns into an auditory ossicle - columella auris. Chondrocranium (Fig. 236) is still largely preserved in modern amphibians; bones replacing cartilage are few in the same way as we see it in lungfish, bone halides and crossopterans.
A feature of the amphibian skull in comparison with fish is the formation in the auditory capsule of an oval window (fenestra ovalis) covered with a membrane, against which columella auris or stapes rests.
The Anura larvae also have peculiar labial cartilages in the skull, upper lower, possibly homologous to the labial cartilages of sharks. The skull of the Anura larva bears a significant resemblance to that of the lamprey, but there is no genetic connection between them (Fig. 237). This similarity is in both cases an adaptation to the formation of a sucking mouth.

In the hyobranchial skeleton of amphibian larvae, in addition to the hyoid arch, four more anterior branchial arches are laid. From the fifth, the skeleton of the larynx develops. In adults, the hyobranchial skeleton undergoes some reduction.
Thus, in the chondrocranium and the visceral apparatus, the connection of amphibians with fish, their ancestors, is revealed.
In the skull of modern amphibians, there are few ossifications, both replacement and integumentary. In this respect, they differ significantly from fish. In the occipital region there are only two lateral occipital bones (exoccipitalia), each with an articular tubercle for articulating the skull with the spine. In the auditory region, which forms significant lateral protrusions for connecting the palatine-square section, only one anterior auditory bone (prooticum) develops (Fig. 238), and only sometimes there are traces of the external occipital (epioticum).

In the orbital region, tailless amphibians develop a characteristic unpaired cingulate bone (sphetbmoideum), covering the cranial cavity: in caudate amphibians, paired bones (orbitosphenoidea) develop in this region, occupying only the side walls of the skull. The olfactory, ethmoid region remains cartilaginous. Of the skin ossifications, there are paired parietal (parictalia) and frontal (frontalia) ossifications on the roof of the skull; In front of the frontal bones, above the nasal capsules, there are paired nasal bones (nasalia), and in the caudate ones, on the sides behind the latter, there are also prefrontal bones (praefrontalia) (Fig. 239). From below, the skull is covered with parasphenoid (rarasphenoideum) and vomer (vomer). The last pair surrounds the posterior nasal openings (choanae). The palate-square apparatus in anurans fuses with the skull posteriorly and in the auditory region and anteriorly in the ethmondal region, while in the middle it bends around the orbits in the form of a free arc. In caudate palatine square cartilage does not reach the ethmoid region, ending freely. Only a small area, the quadratum, ossifies in the palatine-square cartilage. In Anura, a thin quadratojugale departs from here, closing the arc formed by the premaxillary, maxillary, and quadratojugal bones. On the outer side of the quadratum, an integumentary bone develops - para quadratum, homologous to squamosum, and on the arc going forward - the pterygoid bone (pterygoideum), with its front end reaching the palatine bone (palatinum); in the form of integumentary ossifications on the upper jaw, there are intermaxillary and jaw bones (praemaxillaria and maxillaria).
In the lower jaw on Meckel's cartilage develop: dentary (dentale), skin articular (gonial) and lamellar (spleniale). The latter is in Apoda and in the water-dwelling Urodela. Ahead, another small element, the mentomandibulare, may ossify.

The visceral skeleton of modern amphibians consists of a well-developed hyoid arch, homologous to the fish hyoid arch, and 4-2 gill arches. Since the larvae, as well as the tailed ones living in the water, breathe with gills, their visceral skeleton is well developed, very clearly indicating the fish ancestors of amphibians. About metamorphosis from the visceral skeleton, only the first or hyoid arch is completely preserved and connects both halves of the copula, leading to the formation of the skeleton of a highly developed tongue. In caudates, lines remain after metamorphosis, the remains of two anterior arches, while the posterior ones completely disappear. Anurans in the adult state have only a wide hyoid ossifying cartilage - the body of the hyoid bone (corpus hyoideum) and two pairs of "horns" extending from it, which are the remnants of the hyoid and branchial arches (Fig. 240). The remains of the hyoid and 3-4 gill arches are preserved in Apoda. The larynx develops from the fifth arch.

The skull of the stegocephalus looked much more like the skull of ancient fish. First of all, there was much more ossification in it. There were 4 occipital bones in the Embolomeri order, in the auditory, in addition to prooticum, also opistoticum, etc. On the other hand, the skull of primitive stegocephals had some features that brought them closer to reptiles: it was less platybasal than that of modern amphibians, the occipital region primitive stegocephalus included several trunk segments, and the anterior spinal nerves (n. hypoglossus) exited through the holes in the lateral occipital bones, there was ossification - epipterygoideum (= columella cranii), characteristic of reptiles (between pterygoideum and skull), and transversum (between first and maxillare).
Stegocephalus also had much more integumentary ossifications. These bones converged among themselves, were wide and formed the vault of the skull. Columella auris was already there, having evolved from the suspension released due to the development of autostyle. The similarity of the skull of the stegocephalus with the skull of Crossopterygii is exceptionally great and therefore convincingly speaks for their relationship (Fig. 241).

The main feature of the stegocephalic skull is its stegality. The integumentary bones form a strong roof of the skull, descend along the sides around the orbits and through the temporal region, comma with the muscles of the jaws, and the quadratum. The skull is completely covered with bones; there are openings only for the external nostrils, for the orbits and for the parietal eye. The rough sculpture on the bones shows that the bones lay very superficially. This is also indicated by the presence of lateral line canals located on the same bones, as in fish (Fig. 242). The shape of the skull in different stegocephals varied greatly. Modern amphibians are closest to the skull of Branchiosauria (Fig. 243), which had large orbits and significantly reduced endochondral ossifications.

Stegocephalians also show similarities with Crossopterygii in the structure of the lower jaw, rich in ossifications, as in Crossopterygii.
In later forms of stegocephalus, there is a tendency to reduce both integumentary and endochondral ossifications, leading (although intermediate links we do not have) to the skull of modern amphibians.
Some stegocephalians have been found to have gills.
The development of the auditory apparatus in stegocephals is interesting. In the oldest representative of Stegocephali Embolomeri, columella auris, which originated from the upper part of the hyoid arch, which lost its functional significance due to the development of autostyly, simply abuts with its proximal end against the auditory capsule. Ho, already early in the stegocephalus, an oval window (fenestra ovalis) appears, into which columella now rests. Probably, the stegocephalus already had a tympanic membrane, which can be judged by the shape of the parts surrounding the tympanic cavity. This cavity was formed from the gap between the jaw and hyoid arches. Of modern amphibians, the most primitive device of the auditory apparatus is in Anura, where the columella is represented by a firebox stick freely sticking out in the tympanic cavity. Urodela has neither a tympanic membrane nor a rod-shaped columella. In Apoda, the short columella is articulated with the quadratum. It is worth it in connection with the underwater and underground lifestyle.

Limbs and their girdle. The limb type of hand (cheiropterygium) is the most feature terrestrial, including amphibians, which distinguishes them from fish. Limbs of this type should have developed due to the fact that the paired limbs in terrestrial animals have a much greater load than the fins of fish: the paired limbs in terrestrial animals bear the entire weight of the animal's body and move it, which animals do not have to do in water. Hence the need for the development of the system of levers and elongation of the limbs. Terrestrial limb - cheiropterygium - developed from a fin. This is mainly supported by embryology, which shows that the terrestrial limb develops in the embryo from the same rudiment from which the fin develops; the initial stages of their development are similar. Paleontology, albeit weakly, testifies in favor of this view: in some fossil representatives from Crossopterygyii (Eustenopteron, Sauripterus) from the Upper Devonian, we see in the fin skeleton an approximation to the type of a terrestrial limb (Fig. 244). The number of rays of this shape of the fin, which served as the initial basis for the development of the hand, was apparently 7. This was a limb, partly close to the limb of the ganoids, partly to that of the selachians, part of the Dipnoi.

Ho, a real terrestrial-type limb could not develop immediately. It has gone through a well-known history, the first stage of which we know is the stegocephalic limb. Their limbs already consisted of those departments that we see in all terrestrial ones. The latter consists of the shoulder (humerus) in the anterior and thigh (femur) in the hind limb, followed by the forearm of the ulna (ulna) and radius (radius) or lower leg of the tibia (tibia) and fibula (fibula) bones. The presence of two bones in the forearm and lower leg contributes to greater mobility of the hand and foot following the forearm and lower leg departments. The hand consists of several bones, the wrist (carpus), namely the radiocarpal (radiale), intermediate (intermedium), elbow (ulnare), one or two central (centralia) and several carpal (carpalia), corresponding to the number of fingers. With the latter, the radiantly diverging bones of the metacarpus (metacarpalia) are connected, followed by the phalanges (phalangae) of the fingers. In the hind limb, the foot (metatarsus) consists of the tarsal bones going in the same order: tibiale, intermedium, fitulare, centralia, tarsal ia, metatarsalia and phalangae.
The limbs of the stegocephalus were heavy, roughly built, unsuitable for fast travel. They were located transversely to the axis of the body, the shoulder and thigh occupied a horizontal position, the paws stood at an angle. A lot of work was required to keep the body at some distance above the ground, which explains the rough shape of the limbs, the ridges and outgrowths on which served as the attachment point for strong muscles. This applies equally to the free limbs and to the belts.

The shoulder girdle of the stegocephalus was very similar to the shoulder girdle of the primitive Osteichthyes, it always ossified sufficiently and consisted of the scapula (scapula), coracoid, procoracoid (proci racoideum), suprascapular cartilage (niprascapulare), cleithrum (cleithrum) and clavicles (claviculae) (Fig. 246 ). The last two bones are overhead and leaned on the front edge of the shoulder primary girdle. On the middle line there was an integumentary ossification - the interclavicle (interclavicula), which is apparently an expanded scale of the abdominal series and corresponding to the episternum (episternum) of reptiles.
The pelvic girdle of the stegocephalus consisted (Fig. 246) of a cartilaginous semi-arc on each side, in which the following ossifications can be distinguished: the ilium (os ilem), connected to one sacral rib; pubic bone - (os pubis) and ischium (os ischium). All three bones formed an articular fossa (acetabulum). In the oldest representatives of the stegocephalus (Eogyrinus, Cricotus), the ilium was connected to the long, slightly modified sacral ribs not with the help of articulation, but loosely, with the help of muscles. From this we can conclude that the first tetrapods arose in the water.
In the free limbs of primitive stegocephalians, the number of carpus elements was said to be significant: radiale, ulnare, intermedium, fundus centralia, five distal capralia, and an additional, so-called pisiform bone (os pisiforme). The oldest stegocephalians (Embolonieri) had 5 fingers. The higher representatives of the class often had fewer elements.

In modern amphibians, the limb belts and the limbs themselves show a reduction rather than a progressive development in comparison with stegocephals (Fig. 247). The shoulder girdle of Urodela is larval in nature and consists of a cartilaginous plate on each side, an articular fossa for the limb, which is divided into the coracoid part and the scapular part. In the coracoid part, an anterior outgrowth is isolated, called the procoracoid (procoracoideum). Ossification is observed only near the articular fossa. Coracoid plates overlap each other, connected by loose connective tissue. Behind them lies a small cartilaginous sternum (sternum). In Anura, the coracoid ossifies, and a superimposed bone, the clavicula, develops on the procoracoid cartilage. The scapula also ossifies, leaving at the top, like in Urodela, the suprascapular cartilage. In some Anura both halves of the shoulder girdle still overlap each other, as in Urodela, in others they fuse in the middle. Based on this feature, tailless amphibians are divided into mobile-thoracic and immobile-thoracic (Arcifera and Firmisternia). Some have transitional features between the one and the other type. In Anura larvae, the shoulder girdle is movable.
Apoda, having no limbs, have neither a shoulder nor a pelvic girdle. The pelvic girdle of the caudate and anurans basically repeats what the stegocephalus had. On each side of the body, it is represented by a plate. The right and left plates fuse together along the midline. The Anura pelvis, in connection with its adaptation to jumping, has very long iliac bones, merging posteriorly and with the ischiopubic bones into one vertical circle. The latter bears the articular fossa for the hind limbs. In Urodela, the iliac bones are narrow, their connection with the sacral ribs is loose, from the anterior edge of the pelvis along the mid-abdominal line, a forked-branched suprapubic cartilage (epipubis) extends forward. Epipubis is also present in some of the most primitive Amira (Ascaphus, Xenopus) (Fig. 248).

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