The cervical spine of amphibians consists of. Amphibians
The skeleton of amphibians has a number of changes compared to fish. In the axial skeleton, the spine of amphibians is more articulated due to their semi-terrestrial lifestyle. It includes the cervical, trunk, sacral and caudal sections (Fig. 69). The cervical region is represented by one vertebra; its body is small and bears two articular fossae, with the help of which the vertebra articulates with the skull. The number of trunk vertebrae varies. The smallest number is in tailless animals (usually 7), the largest in legless animals (more than 100). The only sacral vertebra (absent in legless animals) bears long transverse processes to which the ilia of the pelvis are articulated. The caudal region is most typically expressed in caudates; in legless animals it is very small, and in tailless animals it is represented by a bone - the urostyle: during embryonic development it is formed in the form of a number of individual vertebrae, which subsequently fuse together.
The shape of the vertebrae is extremely variable among different representatives within the class of amphibians. In lower amphibians (legless, lower tailed) it is amphicoelous; in this case, the chord remains between the vertebrae for life. In anurans, the vertebrae are urocoelous, i.e. concave in front and curved in back; in higher caudates - opisthocoelous, i.e. curved in front and concave in back. There are many ways to deviate from this characteristic; for example, the extremely primitive New Zealand frog Leopelma has amphicoelous vertebrae. True ribs do not develop in tailless amphibians; legless amphibians have very short ones; caudates develop short upper ones. ribs
Brain skull. Much of the braincase 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 cranium. In the occipital region there are only two lateral occipital bones; the places corresponding to the main and upper occipital bones of fish remain cartilaginous. In the area of the auditory capsule, one small ear bone is formed, but most of the capsule remains cartilaginous. In the anterior part of the orbit, the dangtlin-olfactory bone develops in anurans; 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 tailless animals are fused into the frontoparietal bones. In front of them are the nasal bones; in legless animals they are fused with the premaxillary bones. On the sides of the back of the skull there are scaly bones, especially highly developed in legless ones. The bottom of the skull is lined by a large parasphenoid, and in front of it lie paired vomer bones. The bones of the visceral skeleton - the palatine and pterygoid - also take part in the formation of the bottom of the skull. The former are adjacent to the vomers, the latter to the squamosal bones. They develop on the lower surface of the palatoquadrate cartilage. The functions of the upper jaws are performed, like in bony fish, by the premaxillary, (or premaxillary) and maxillary bones. The lower jaw is represented by Meckel's cartilage, which is covered externally by the dental and angular bones. Amphibian skull, i.e. The palatoquadrate cartilage is directly attached to the cranium. Due to the autostyly of the skull, the hyoid arch does not take part in attaching the maxillary apparatus to the skull. The upper element of this arch - the pendant (hyomandibular) - is turned into a small bone - the stapes, the proximal end of which rests on the auditory capsule, and the outer (distal) end - on the eardrum. In connection with the formation of the cavity of the middle ear, this bone is located inside this cavity and acts as an auditory ossicle. Thus, the hyomandibular (suspensory) emerges from the system of the fourth (hyoid) visceral arch (Fig. 70) The lower elements of the hyoid arch and branchial arches are modified into the hyoid plate and her horns. This plate is located between the branches of the lower jaw. Its anterior horns, curving upward and enclosing the intestinal tube from the sides, 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 that of most bony fish:
1) weak development of chondral and cutaneous ossifications;
2) autostyle;
3) modification of the hyoid and gill arches, transformed partly into the auditory apparatus, partly into the hyoid apparatus;
4) reduction of the operculum.
Limb belts. The shoulder girdle has the shape of an arch, with its apex facing the ventral surface of the animal (Fig. 71). Each half of the arc (left and right) consists of the following basic elements. The upper (dorsal) part is represented by a scapula with a wide suprascapular cartilage. The lower (abdominal) part includes the coracoid and the procoracoid lying in front of it. In anurans, between the presternum and the scapula there is a thin rod-shaped clavicle. The listed elements of the belt converge at the point of attachment of the humerus and form the articular fossa. Anterior to the junction of the left and right coracoids is the presternum and behind is the 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 ribs, amphibians do not have a rib cage.
The pelvic girdle (Fig. 72) is formed by three paired elements. converging in the area of the acetabulum, which they form. The long iliac bones with their proximal (anterior) ends are attached to the transverse processes of the single sacral vertebra. The pubic element of the girdle directed forward and downward in frogs remains cartilaginous. Behind it is the ischium. This arrangement of the elements of the pelvic girdle is characteristic of all terrestrial vertebrates. The skeleton of free limbs is typical of terrestrial vertebrates and is significantly different from the skeleton of fish limbs. While the limbs of fish represent simple single-membered levers in the diagram, moving only relative to the body body and not bearing muscles, the limbs of terrestrial vertebrates are multi-membered levers with fairly powerful muscles. In this case, not only does the entire limb move relative to the body, but also the individual elements of the limb move relative to each other. In the diagram, the five-fingered limb consists of three main sections (Fig. 72). I - shoulder in the forelimb, thigh - in the hindlimb; this section always consists of one bone, which is attached to the girdle at its proximal end; II - forearm in the forelimb, shin in the hindlimb. In a typical case, the section consists of two parallel bones: the forearm - from the ulna and radius, the lower leg - from the tibia and fibula; III - hand in the forelimb and foot in the hindlimb; the department consists of three subsections: 1) the carpus - in the forelimb, the tarsus - in the hindlimb; this subsection is typically represented by 9-10 small bones arranged in three rows; 2) metacarpus - in the forelimb, metatarsus - in the hindlimb; in a typical case, the subsection consists of 5 elongated bones located in one row, like a fan, from the wrist or tarsus; 3) the phalanges of the four fingers are like 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 that shown in Fig. 72 scheme. Frogs exhibit some deviations. The main ones are the following: both elements of the forearm and shin fuse into one bone, most of the bones of the wrist and tarsus fuse together, and in front of the first finger of the hind limb there is a rudiment of an additional finger. These features are secondary in nature and are apparently associated with the adaptation of frogs to move by jumping.
Amphibians(they are amphibians) - the first terrestrial vertebrates to appear in the process of evolution. However, they still maintain a close connection with the aquatic environment, usually living in it at the larval stage. Typical representatives of amphibians are frogs, toads, newts, and salamanders. They are most diverse in tropical forests, as they are warm and damp. There are no marine species among amphibians.
General characteristics of amphibians
Amphibians are a small group of animals, numbering about 5,000 species (according to other sources, about 3,000). They are divided into three groups: Tailed, Tailless, Legless. Frogs and toads familiar to us belong to the tailless ones, newts belong to the tailed ones.
Amphibians develop paired five-fingered limbs, which are multi-membered levers. The forelimb consists of the shoulder, forearm, and hand. Hind limb - from the thigh, lower leg, foot.
Most adult amphibians develop lungs as respiratory organs. However, they are not as perfect as in more highly organized groups of vertebrates. Therefore, skin respiration plays an important role in the life of amphibians.
The appearance of lungs in the process of evolution was accompanied by the appearance of a second circulation and a three-chambered heart. Although there is a second circuit of blood circulation, due to the three-chambered heart there is no complete separation of venous and arterial blood. Therefore, most organs receive mixed blood.
The eyes not only have eyelids, but also lacrimal glands for wetting and cleansing.
The middle ear with the eardrum appears. (In fish, only internal.) The eardrums are visible, located on the sides of the head behind the eyes.
The skin is bare, covered with mucus, and contains many glands. It does not protect against water loss, so they live near bodies of water. Mucus protects the skin from drying out and bacteria. The skin consists of epidermis and dermis. Water is also absorbed through the skin. Skin glands are multicellular, while in fish they are unicellular.
Due to the incomplete separation of arterial and venous blood, as well as imperfect pulmonary respiration, the metabolism of amphibians is slow, like that of fish. They are also cold-blooded animals.
Amphibians breed in water. Individual development proceeds with transformation (metamorphosis). The frog larva is called tadpole.
Amphibians appeared about 350 million years ago (at the end of the Devonian period) from ancient lobe-finned fish. Their heyday occurred 200 million years ago, when the Earth was covered with huge swamps.
Musculoskeletal system of amphibians
Amphibians have fewer bones in their skeletons than fish, as many bones are fused while others remain cartilage. Thus, their skeleton is lighter than that of fish, which is important for living in the air, which is less dense than water.
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The brain skull is fused with the upper jaws. Only the lower jaw remains mobile. The skull retains a lot of cartilage that does not ossify.
The musculoskeletal system of amphibians is similar to that of fish, but has a number of key progressive differences. So, unlike fish, the skull and spine are movably articulated, which ensures the mobility of the head relative to the neck. For the first time, the cervical spine appears, consisting of one vertebra. However, the mobility of the head is not great; frogs can only tilt their heads. Although they have a cervical vertebra, there is no neck in the external appearance of the body.
In amphibians, the spine consists of more sections than in fish. If fish have only two of them (trunk and caudal), then amphibians have four sections of the spine: cervical (1 vertebra), trunk (7), sacral (1), caudal (one tail bone in tailless amphibians or a number of separate vertebrae in tailed amphibians) . In tailless amphibians, the caudal vertebrae fuse into one bone.
The limbs of amphibians are complex. The anterior ones consist of the shoulder, forearm and hand. The hand consists of the wrist, metacarpus and phalanges of the fingers. The hind limbs consist of the thigh, lower leg and foot. The foot consists of the tarsus, metatarsus and phalanges.
The limb girdles serve as support for the skeleton of the limbs. The girdle of the forelimb of an amphibian consists of a scapula, clavicle, and crow bone (coracoid), common to the girdles of both forelimbs of the sternum. The clavicles and coracoids are fused to the sternum. Due to the absence or underdevelopment of the ribs, the belts lie deep in the muscles and are in no way indirectly attached to the spine.
The hind limb girdles consist of the ischial and ilium bones, as well as pubic cartilage. Fusing together, they articulate with the lateral processes of the sacral vertebra.
The ribs, if present, are short and do not form a rib cage. Tailed amphibians have short ribs, while tailless amphibians do not.
In tailless amphibians, the ulna and radius bones are fused, and the bones of the lower leg are also fused.
The muscles of amphibians have a more complex structure than those of fish. The muscles of the limbs and head are specialized. Muscle layers break down into individual muscles, which provide movement of some parts of the body relative to others. Amphibians not only swim, but also jump, walk, and crawl.
Digestive system of amphibians
The general structure of the digestive system of amphibians is similar to that of fish. However, some innovations are emerging.
The anterior tip of the tongue of frogs grows to the lower jaw, while the posterior one remains free. This structure of the tongue allows them to catch prey.
Amphibians develop salivary glands. Their secretion moistens food, but does not digest it in any way, since it does not contain digestive enzymes. The jaws have conical teeth. They serve to hold food.
Behind the oropharyngeal cavity is a short esophagus that opens into the stomach. Here the food is partially digested. The first section of the small intestine is the duodenum. A single duct opens into it, into which the secretions of the liver, gallbladder and pancreas enter. In the small intestine, food digestion is completed and nutrients are absorbed into the blood.
Undigested food remains enter the large intestine, from where it moves to the cloaca, which is an extension of the intestine. The ducts of the excretory and reproductive systems also open into the cloaca. From it, undigested residues enter the external environment. Fish do not have a cloaca.
Adult amphibians feed on animal food, most often various insects. Tadpoles feed on plankton and plant matter.
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Respiratory system of amphibians
Amphibian larvae (tadpoles) have gills and one circulation (like fish).
In adult amphibians, lungs appear, which are elongated sacs with thin elastic walls that have a cellular structure. The walls contain a network of capillaries. The respiratory surface of the lungs is small, so the bare skin of amphibians also participates in the breathing process. Up to 50% of oxygen enters through it.
The mechanism of inhalation and exhalation is ensured by the raising and lowering of the floor of the oral cavity. When lowering, inhalation occurs through the nostrils; when raising, air is pushed into the lungs, while the nostrils are closed. Exhalation is also carried out by raising the bottom of the mouth, but at the same time the nostrils are open and the air comes out through them. Also, when you exhale, the abdominal muscles contract.
Gas exchange occurs in the lungs due to the difference in gas concentrations in the blood and air.
The lungs of amphibians are not well developed enough to fully ensure gas exchange. Therefore, skin breathing is important. Drying out amphibians can cause them to suffocate. Oxygen first dissolves in the fluid covering the skin and then diffuses into the blood. Carbon dioxide also first appears in the liquid.
In amphibians, unlike fish, the nasal cavity has become through and is used for breathing.
Underwater, frogs breathe only through their skin.
Circulatory system of amphibians
A second circle of blood circulation appears. It passes through the lungs and is called the pulmonary circulation, as well as the pulmonary circulation. The first circle of blood circulation, passing through all organs of the body, is called major.
The heart of amphibians is three-chambered, consisting of two atria and one ventricle.
The right atrium receives venous blood from the organs of the body, as well as arterial blood from the skin. The left atrium receives arterial blood from the lungs. The vessel entering the left atrium is called pulmonary vein.
Contraction of the atria pushes blood into the common ventricle of the heart. Here the blood is partially mixed.
From the ventricle, blood is sent through separate vessels to the lungs, body tissues, and head. The most venous blood from the ventricle enters the lungs through the pulmonary arteries. Almost pure arterial blood flows to the head. The most mixed blood entering the body flows from the ventricle into the aorta.
This division of blood is achieved by a special arrangement of vessels emerging from the distribution chamber of the heart, where blood enters from the ventricle. When the first portion of blood is pushed out, it fills the closest vessels. And this is the most venous blood, which enters the pulmonary arteries, goes to the lungs and skin, where it is enriched with oxygen. From the lungs, blood returns to the left atrium. The next portion of blood - mixed - enters the aortic arches, going to the organs of the body. The most arterial blood enters the distant pair of vessels (carotid arteries) and is directed to the head.
Excretory system of amphibians
The kidneys of amphibians are trunk and oblong in shape. Urine enters the ureters, then flows along the wall of the cloaca into the bladder. When the bladder contracts, urine flows into the cloaca and then out.
The excretion product is urea. Its removal requires less water than the removal of ammonia (which is produced by fish).
Reabsorption of water occurs in the renal tubules of the kidneys, which is important for its conservation in air conditions.
Nervous system and sensory organs of amphibians
There were no key changes in the amphibian nervous system compared to fish. However, the forebrain of amphibians is more developed and divided into two hemispheres. But their cerebellum is less developed, since amphibians do not need to maintain balance in water.
Air is clearer than water, so vision plays a leading role in amphibians. They see further than fish, their lens is flatter. There are eyelids and nictitating membranes (or an upper fixed eyelid and a lower transparent movable one).
Sound waves travel worse in air than in water. Therefore, there is a need for a middle ear, which is a tube with an eardrum (visible as a pair of thin round films behind the eyes of a frog). From the eardrum, sound vibrations are transmitted through the auditory bone to the inner ear. The Eustachian tube connects the middle ear cavity to the oral cavity. This allows you to reduce pressure drops on the eardrum.
Reproduction and development of amphibians
Frogs begin to reproduce at about 3 years of age. Fertilization is external.
Males secrete seminal fluid. In many frogs, males attach themselves to the backs of females and, while the female spawns eggs over several days, waters them with seminal fluid.
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Amphibians spawn less eggs than fish. Clusters of eggs are attached to aquatic plants or float.
The mucous membrane of the egg in water swells greatly, refracts sunlight and heats up, which contributes to faster development of the embryo.
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An embryo develops in each egg (in frogs it usually takes about 10 days). The larva that emerges from the egg is called a tadpole. It has many features similar to fish (two-chambered heart and one circulation, breathing with gills, lateral line organ). At first, the tadpole has external gills, which later become internal. The hind limbs appear, then the forelimbs. The lungs and the second circle of blood circulation appear. At the end of metamorphosis, the tail resolves.
The tadpole stage usually lasts several months. Tadpoles feed on plant matter.
TOPIC 10. Amphibian SKELETON
SYSTEMATIC POSITION OF THE OBJECT
Subphylum Vertebrates, Vertebrata
Class Amphibians, Amphibia
Order Anurans, 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. Preparation needles - 2.
EXERCISE
Understand the structural features of the amphibian skeleton. 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 forelimbs (straightened) from below.
5. Skeleton of the forelimb.
6. Side view of the pelvic girdle.
7. Skeleton of the hind limb.
Additional task
Compare, without sketching, the skeletons of a tailless amphibian (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 girdles.
In almost all parts of the skeleton, cartilage still plays a fairly 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 is usually reduced in adulthood. 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 caudal 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 movably articulates with the skull using two articular platforms.
2. The trunk section (pars thoracalis; Fig. 54, 2) of the spine of frogs consists of 7 vertebrae (in tailed amphibians - from 14 to 63).
3. The sacral region (pars sacralis; Fig. 54, 3) in all amphibians is represented by one sacral vertebra, to the massive transverse processes of which the iliac bones of the pelvic girdle are attached (Fig. 54, 5).
4. The caudal region (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 - urostyle (urostyl; Fig. 54, 4). In tailed amphibians, 26-36 individual vertebrae are retained in the tail.
Rice. 55. Body 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 in front, convex in the back (Fig. 55), however, the last trunk vertebra has an amphicoelous (biconcave) type of structure. Above the vertebral bodies are the upper arches (arcus neuralis; Fig. 55, 2), forming a canal for the spinal cord. On the dorsal side of the arch there is a small spinous process (processus spinosus; Fig. 55, 4). Paired transverse processes (processus transversus; Fig. 55, 5) extend from the superolateral surface of the vertebral body; 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 anteriorly and posteriorly at the base of the upper arch.
Compared to fish, amphibians are characterized by greater differentiation of the vertebral column into sections, a change in the shape of the vertebral bodies, and a stronger development of the articular processes. These transformations are associated with a terrestrial lifestyle and provide greater strength to the axial skeleton while maintaining its mobility, a strong connection with it of the pelvic girdle and allow some mobility of the skull in the vertical plane relative to the body (the ability to raise and lower the head).
Scull. The axial, or cerebral, skull of amphibians, like the skull of cartilaginous fish, is of the platybasal type: with a wide base and widely spaced orbits, between which the anterior end of the brain is located. The skull, compared to bony fish, retains a lot of cartilage, and the number of ossifications is relatively small.
Rice. 56. Frog Skull
A - from above; B - bottom; B - behind; D - lower jaw from above, the dotted line shows the cartilaginous areas of the skull
1 - lateral occipital bone, 2 - occipital condyle, 3 - anterior auricular bone, 4 - sphenoid-olfactory bone, 5 - nasal bone, 6 - frontoparietal bone, 7 - squamosal bone, 8 - parasphenoid, 9 - palatine bone, 10 - vomer, 11 - choana, 12 - palatoquadrate cartilage, 13 - premaxillary bone, 14 - maxillary bone, 15 - quadratozygomatic bone, 16 - pterygoid bone, 17 - Meckel's cartilage, 18 - geniomaxillary bone, 19 - dentary, 20 - angular bone, 21 - foramen magnum
In the cartilage of the occipital part of the skull, only paired lateral occipital bones (occipitale laterale; Fig. 56, 1) are formed, bordering the foramen magnum (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 area of the auditory capsule, instead of five pairs of ear bones, characteristic of bony fish, amphibians have only one pair - the anterior ear 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 has the appearance of a bone ring girdle.
Rice. 57. Cartilaginous tadpole skull:
1 - brain skull, 2 - palatoquadrate cartilage, 3 - Meckel's cartilage, 4 - branchial arches, 5 - jaw joint
The rest of the skull remains cartilaginous. It is strengthened by the integumentary (skin) bones. On top of the front part of the skull lie the paired nasal bones (nasale; Fig. 56, 5), which have an elongated triangular shape, then the paired frontoparietale bones (frontoparietale, Fig. 56, 6), fused from the frontal and parietal bones, and outward from the ear bones - scaly bones having a complex shape (squamosum; Fig. 56, 7). The bottom of the skull is covered by a powerful cruciform integumentary bone - the parasphenoid (parasphenoideum; Fig. 56, 8). In front of it also lie the paired integumentary palatine bones (palatinum; Fig. 56, 9) and paired vomers (vomer; Fig. 56, 10); Small teeth sit on the vomers. In front of the vomers there are paired internal nostrils - choanae (Fig. 56, 11).
The visceral region of the amphibian skull also retains a lot of cartilage. Throughout life, the palatoquadrate cartilage (cartilago palatoquadratum; Fig. 56, 12) is preserved, growing with its anterior end to the olfactory region of the cerebral 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, like other terrestrial vertebrates, is autostylish according to the type of attachment of the jaw arch.
Adjacent to the palatoquadrate cartilage are the bones of the secondary upper jaw that arise in the skin: paired intermaxillare or praemaxillare; Fig. 56, 13), load-bearing teeth and maxillary bones (maxillare; Fig. 56, 14). Behind them, strengthening the posterior part of the palatoquadrate cartilage, the integumentary quadratojugal bone (quadratojugale; Fig. 56, 15) is formed on top, and the integumentary pterygoid bone (pterygoideum; Fig. 56, 16) is also formed below.
The primary lower jaw - Meckel's cartilage (cartilago Meckeli; Fig. 56, 17) also remains cartilaginous, only its very anterior end ossifies into small paired chin-maxillary bones (mento-mandibulare, Fig. 56, 18). They are joined by integumentary dental bones (dentale; Fig. 56, 19), which in amphibians lack teeth. The posterior part of the Meckel's cartilage is overgrown with a long integumentary angular bone (angulare; Fig. 56, 20) and several more small integumentary bones. Through the articular process of Meckel's cartilage, the lower jaw movably articulates with the posterior part of the palatoquadrate cartilage (Fig. 57, 5).
Rice. 58. Schematic section through the auditory region of the frog's head:
1 - brain, 2 - auditory capsule with semicircular canals, 3 - middle ear cavity, 4 - stapes, 5 - eardrum, 6 - eustachian tube, 7 - oral cavity
Complete reduction of the operculum in amphibians and the replacement of the hyostylistic type of attachment of the jaws with an autostylistic one 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 be reduced, and the cavity of the sputter (the remnant of the gill slits between the maxillary and hyoid arches) due to the transition to life in the air was transformed into the cavity of the middle ear (Fig. 58, 3). The upper element of the hyoid arch, the pendant (hyomandibulare), located next to the splatter, has turned into an auditory column-bone, or stapes (columellaa, or stapes; Fig. 58, 4). In modern tailless amphibians, the stapes has the appearance of a thin rod-shaped bone lying perpendicular to the brain skull under the squamosal and quadratojugal bones. One end of the stapes rests against the center of the eardrum (Fig. 58, 5), and the other against the oval window of the auditory capsule. This mechanism, which amplifies sound vibrations and provides the possibility of hearing in the air, is secondary reduced to varying degrees in some modern amphibians. Additional mechanisms that ensure the perception of sound waves propagating along a solid substrate are the lower jaw, as well as the transmission of sound vibrations along the blood trunks.
Rice. 59. Sublingual apparatus of the frog:
1 - body, 2 - horns
The lower element of the hyoid arch is the hyoid (hyoideum) and the gill arches that function in amphibian larvae transform into the hyoid apparatus during metamorphosis (Fig. 59). In tailless amphibians, it is a cartilaginous plate with two main layers of processes - horns. The anterior, longer horns (modified hyoids) are directed backward and upward and are attached to the walls of the auditory capsules of the brain skull. The hyoid 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 are also transformed remains of the branchial arches.
Paired limbs and their belts. The limbs of amphibians, kak and the limbs of other classes of terrestrial vertebrates, represent in the diagram a system of levers movably connected to each other. The structural diagrams of the front and rear 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, 7 - femur, 8 - tibia, 9 - fibula, 10 - tarsus, 11 - metatarsus, 12 - patella, I - V - fingers
Forelimb: Hindlimb:
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 the fingers (phalanges digitorum; Fig. 60, 6). I. Thigh (femur; Fig. 60, 7).
II. Tibia (crus): tibia (tibia; Fig. 60, 5),
fibula (fibula; Fig. 60, 9).
III. Foot (pes): tarsus (tarsus; Fig. 60, 10),
metatarsus (fig. 60, 11),
phalanges of fingers (phalanges digitorum; Fig. 60, 6)
The proximal part of the forelimb is the shoulder (humerus; Fig. 61, 7) - tubular bone; its middle part is called the diaphysis, and its thickened ends are called epiphyses. In amphibians, the epiphyses of the shoulder (and femur) remain cartilaginous.
The proximal end has a rounded head of the humerus (caput humeri; Fig. 61, 2), which enters the articular fossa of the girdle of the forelimbs; at the distal end there is a hemispherical surface for articulation with the bones of the forearm. The surface of the shoulder has ridges to which muscles are attached.
In tailless amphibians, the ulna (ulna; Fig. 61, 4) on the outside and the radius (radius; Fig. 61, 5) on the inside merge into a single forearm bone (antebrachium, Fig. 61, 3); the longitudinal groove shows the border of their fusion. 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 there is the olecranon process (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. Adjacent to the distal row of carpal bones are five elongated metacarpus bones (metacarpus; Fig. 61, 8). The phalanges of the fingers (phalanges digitorum; Fig. 61, 9) articulate 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 humerus, 3 - forearm, 4 - ulna,
5 - radius, 6 - olecranon, 7 - wrist, 8 - metacarpus, 9 - phalanges, 10 - scapula, 11 - suprascapular cartilage, 12 - coracoid, 13 - articular cavity for the head of the humerus, 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. The scapula (scapula; Fig. 61, 10) is formed from the upper (dorsal) scapular part of the primary girdle; 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 in some tailless amphibians there is a small ossification - a remnant of the cleithrum of fish-like ancestors. The ossified coracoid part of the girdle turned into a powerful coracoid bone (coracoideum; Fig. 61,12), together with the scapula, forming an articular cavity for the head of the humerus (Fig. 61, 13). Anterior to the coracoid, behind a small opening, lies a cartilaginous procoracoid (cartilago procoracoidea; Fig. 61, 14), on which lies a thin covering bone - the clavicle (clavicula; Fig. 61, 15). The unossified cartilaginous inner ends of the coracoids and procoracoids of the right and left sides fuse together along the midline. Behind the coracoids there is a bony sternum (sternum, Fig. 62, 16) with a cartilaginous posterior end. Anterior to the procoracoids protrudes 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; The collarbones often do not develop.
The shoulder girdle serves as a support for the forelimbs and the attachment point for the muscles that control them.
The rib cage 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 - tibia, 4 - tibia, 5 - fibula, 6 - tarsus, 7 - tibiale, 8 - fibulare, 9 - metatarsus, 10 - phalanges, 11 - rudiment VI finger, 12 - ilium, 13 - ischium, 14 - pubic cartilage, 15 - acetabulum, I - V - fingers
The hind limb has an elongated tubular bone - the femur (femur; Fig. 62, 1), the proximal part of which ends with the head (Fig. 62, 2), which enters the acetabulum (Fig. 62, 15) of the pelvic girdle. The tibia (tibia; Fig. 62, 4) and fibula (fibula, Fig. 62, 5) bones of tailless amphibians merge into a single tibia bone (crus, Fig. 62, 3); in tailed amphibians they remain separated.
The proximal row of tarsal bones (tarsus, Fig. 62, 6) of tailless amphibians consists of two elongated bones that form an additional lever of the limb. The inner one is called tibiale (astragalus; Fig. 62, 7; adjacent to the tibial edge of the tibia), the outer one is called fibulare (calcaneus, Fig. 62, 8). The 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 finger in frogs is IV. On the side of the I (inner) finger there is a small rudiment of the VI (“pre-first”) finger (praehallus; Fig. 62, 11).
The hind limb girdle, or pelvic girdle, in amphibians, as in all terrestrial vertebrates, consists of three paired elements; and all of them together form the articular acetabulum (acetabulum; Fig. 62, 15) for connection with the femoral head. The long, forward-directed iliac bones (ilium; Fig. 62, 12) are articulated at 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 it lie the paired ischiums (ischium; Fig. 62, 13).
In tailed amphibians, compared to tailless amphibians, 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 maintain a constant connection with water. This duality is clearly manifested in the characteristics of embryonic and postembryonic development. Eggs (spawn) can only develop in water (or in rare cases in an extremely humid environment). A larva emerges from the egg - a tadpole, which has clearly expressed 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 the 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) way of life: paired limbs of the 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 adult individuals.
The terrestrial lifestyle is ensured by a number of structural features: greater differentiation of the spinal column into sections and a stronger connection of the vertebral bodies with each other (replacement of amphicoelous vertebrae with procoelous or opisthocoelous); the appearance of paired terrestrial limbs; increased complexity of the structure and greater strength of the limb girdles (in this case, a sufficiently strong connection of the pelvic girdle with the axial skeleton is already established); strong reduction of metameric muscles and its replacement by a fairly powerful complex complex of muscles; the appearance of eyelids (protecting the eyes from mechanical damage, preventing the cornea from drying out, etc.); formation of the middle ear cavity with the eardrum and the auditory ossicle - the stapes (ensuring the possibility of hearing in the air). A major role was played by the disappearance of the gills and the development of the lungs, larynx and choanae, which create the possibility of air breathing; the emergence of two circles of blood circulation; greater differentiation of the digestive system (high energy costs when maintaining the body in the air), etc.
The general lengthening of the hind limbs, the isolation of an additional lever in them (due to the sharp lengthening of the two proximal tarsal bones) and the possibility of a strong bend in the middle of the body at the junction of the branches of the iliac bones with the transverse processes of the sacral vertebrae - adaptations for jumping movement in tailless amphibians. Crawling tailed amphibians do not have these features. The fusion of two bones of the forearm and two bones of the lower leg into a single whole is associated with a sharp decrease in the need for rotational movements of the foot and hand when moving by jumping. In tailed 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.
“Aquatic” structural features are manifested in a number of features: the relatively weak development of skeletal ossifications, the abundance of mucous glands in the skin (the mucus covering the skin reduces friction when moving in water, prevents the penetration of bacteria and fungi into the skin, etc.), preservation of the tail, often flattened laterally and bordered by a leathery fold (newts and other tailed amphibians), great similarity of the genitourinary system with most groups of fish, external fertilization characteristic of the vast majority of amphibian species, etc.
With a relatively small surface area of the lungs of amphibians, fairly powerful additional respiratory organs are required. Such an organ becomes the skin that is always moist (due to the abundance of mucous glands), easily permeable to moisture and gases, and partly the mucous membrane of the oral cavity. 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 grass frog, which leads a more terrestrial life, pulmonary respiration takes in up to 67% of oxygen and releases up to 26% of carbon dioxide, and through skin respiration, 33% of oxygen is absorbed and 74% of carbon dioxide is released. With an increase in the level of metabolism (increased general activity and all metabolic processes with increasing environmental temperature), the specific role of the lungs in providing the body with oxygen increases markedly. A decrease in environmental temperature causes a decrease in the level of metabolism. At the same time, skin respiration almost completely ensures both the saturation of the body with oxygen and the release of carbon dioxide, and the relative importance of the lungs in breathing sharply decreases.
This duality in the 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 a chest); it is necessary for the amphibian lifestyle of representatives of this class. It is this duality of the respiratory organs that provides amphibians with the opportunity to stay in water for a long time (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 meets all the body’s needs for oxygen and the release of carbon dioxide).
Using 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 types of amphibians inhabit only damp, damp areas, where the body loses less moisture and can always replenish its loss. Relatively few toads associated with water (overwinter on land and go into water bodies only to spawn) have thickened skin; this reduces the possibility of cutaneous 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 hunting period. Some reduction in moisture loss (while maintaining skin permeability) in amphibians is helped by the mucus covering the skin.
Extensive subcutaneous lymphatic cavities serve as reservoirs of reserve water. Moisture loss is also reduced due to the reabsorption of water in the bladder, hind intestine and cloaca. Moisture loss decreases very sharply due to adaptive behavioral features: 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 burrows, where high humidity is maintained due to soil moisture.
The duality of the respiratory organs makes it impossible to completely separate the systemic and pulmonary circulations. But 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 origin of the arterial arches, the device of the spiral valve) allow more venous blood to be directed into the skin and lungs, more arterial - to the brain and sensory organs.
Greater differentiation of the digestive tract, compared to fish, leads to a slight 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 quite simple; the range of feed used is small (only animals of relatively small size).
Amphibians, like fish, are characterized by variability in body temperature (poikilothermia): in amphibians it is usually only 0.5-1°C higher than the ambient temperature. Only during the period of highest activity (pursuing prey, avoiding danger) can body temperature exceed the ambient temperature by 5-7°C.
Poikilothermy 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 shelter (some species of frogs - into holes at the bottom of reservoirs; most species of tailless and tailed amphibians hide in rodent burrows, rotten tree roots, heaps of moss, etc.) and fall into a state of stupor. The geographical distribution of amphibians is also connected with this: most species of these animals are characteristic of the tropical zone. In the tropics, under relatively stable temperature conditions throughout the year, a state of torpor in a number of amphibian species is caused by a sharp decrease in air humidity (“hibernation” during the dry period of the year).
The very high dependence of amphibians on environmental humidity and 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 from year to year.
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 the axolotl. M., 1952.
Gurtovoy N. N., Matveev B. S., Dzerzhinsky F. Ya. Practical zootomy of vertebrates. Amphibians, reptiles. M., 1978.
Terentyev P.V. Frog. M., 1950.
Terentyev P.V. Herpetology. M., 1961.
Shmalgauzen I.I. Fundamentals of comparative anatomy. M., 1947.
Shmalgauzen I. I. Origin of terrestrial vertebrates. M., 1964.
Legless Squad - Apoda. Includes the only family of caecilians - Caecilidae, uniting about 60 species that externally resemble large worms or snakes (length 30-120 cm). Superficial transverse constrictions seem to divide the worm-like body into separate “segments”. The limbs and their belts are missing; there is no tail, and the cloaca opens outward at the end of the body. The skin glands secrete copious acrid mucus; the skin contains small bone scales. The integumentary bones of the skull are well developed; vertebrae are amphicoelous. The septum between the atria is incomplete. Distributed in the humid tropics of Africa, Asia, and America.
Most species lead an underground lifestyle: they move slowly, mining loose soil and forest litter and eating its inhabitants - soil insects and their larvae, worms, and mollusks. Some species settle in termite mounds and anthills, feeding on their inhabitants. Fertilization is internal; eggs are laid in moist soil or in burrows dug along the banks. In many species, females protect the clutch by wrapping their body around it. Several species of caecilians lead an aquatic lifestyle; these species are viviparous.
Features of the organization of amphibians.
Body Shape. Variations in the body shape of modern amphibians are small: a short, flattened dorsoventral body with a reduced tail, the hind limbs are longer and more powerful than the forelimbs (order anurans); ridged, elongated, sometimes slightly flattened or laterally compressed body with a small head, long tail and short limbs (order caudate); a limbless worm-like body with a small head (neg. legless). The sizes of modern species are small: tailless ones are 3-25 cm long, tailed ones are 10-30 cm long, and only a few are larger (giant salamander - up to 1.6 m); legless (caecilians) reach a length of 30-120 cm. Integument. Leather and its derivatives. The epidermis is multilayered, the corium is thin, but richly saturated with capillaries. The skin of amphibians is rich in multicellular glands. The mucus they secrete in a thin layer covers the entire body, moisturizing the skin and protecting it from drying out, which ensures that the skin participates in gas exchange. In toads living in relatively dry habitats, thickened mucus forms a dense film on the skin that reduces moisture loss. The secretion secreted by the skin glands may contain irritating or toxic substances (toads, toads, some salamanders). The secretion also contains substances that have signaling value; they influence the behavior of other individuals. In the lower layers of the epidermis and in the corium there are pigment cells that determine the species-specific color. The coloration of amphibians performs various functions: camouflage (cryptic, or protective, coloration); warnings and repellents in species with poisonous glands (aposematic coloration with bright colored spots); sexual differences - in males, the coloring often becomes brighter at the beginning of reproduction, facilitating the meeting of sexually mature individuals and stimulating mating. Few species are able to change the intensity of color depending on the background color; this ability is best expressed in some tree frogs.
In terrestrial species, flat cells of the outer layer of the epithelium undergo more or less pronounced keratinization. In a few species, the thickened skin at the ends of the fingers becomes keratinized, forming claws (the clawed frog, the clawed newt, which is also found in Primorye - Onychodactylus fischeri). In legless animals, small bone scales are scattered in the corium, the remains of the dermal bone cover of Paleozoic amphibians. Tailless amphibians have extensive lymphatic lacunae under the skin - peculiar reservoirs that, under favorable conditions, allow them to accumulate a supply of water. By strips of connective tissue that form bridges between the lacunae, the skin is connected to the muscles of the body only in a few areas.
Propulsion system and main types of movement. The pattern of movement of amphibians is quite uniform and can be reduced to two main types. Fossil and modern tailed amphibians retained the basic type of movement characteristic of fish - using strong lateral bends of the entire body, but relying on short legs when moving on the ground. With short limbs, the lateral bends of the body increase the length of the step, and the bends of the tail help maintain balance. When moving in water, the limbs do not play any noticeable role. Legless animals also move using the bends of the entire body.
Tailless amphibians move on land by jumping, lifting their body into the air with a sharp push of both hind limbs. Short-legged species, such as toads, in addition to jumping, can slowly walk, sequentially rearranging their limbs. Anurans swim in water, vigorously working with their hind limbs (breaststroke style, but without the participation of the forelimbs). It is believed that powerful hind limbs developed as an adaptation to swimming, and only later were used for jumping on land. Ancestors of tailless amphibians ( Protobatrachus), apparently led an aquatic lifestyle (Griffith, 1963). Jumping movement led to shortening and dorso-ventral flattening of the body, disappearance of the tail, lengthening of the hind limbs and the development of a number of specific features in the structure of the skeleton (reduction in the number of vertebrae, their strong connection, lengthening of the ilium, etc.).
Skeleton. The axial skeleton consists of vertebrae and is divided into 4 sections: cervical, trunk, sacral and caudal. The cervical and sacral sections have only one vertebra. The first provides some mobility of the head relative to the body, and the sacral one serves for articulation with the pelvic girdle. Anurans usually have 7 trunk vertebrae, and all caudal vertebrae (approximately 12) merge into a single bone - the urostyle. Caudates have 13-62 trunk and 22-36 caudal vertebrae; in legless animals the total number of vertebrae reaches 200-300.
In more primitive amphibians (legless, some tailed, smooth-legged among the tailless), the vertebrae, like those of fish, are amphicoelous; Remnants of the notochord are preserved between the vertebral bodies and within them. In true salamanders, the majority of lungless salamanders and parts of anurans (round-tongued, pipiform) the vertebrae are opisthocoelous (the vertebral bodies are convex in front, concave in back), and in the rest of the anurans they are procoelous (concave in front, convex in back). Above the vertebral bodies, the superior arches are well developed, forming a canal in which the spinal cord lies. At the base of the upper arch of each vertebra, articular processes develop, articulating with the corresponding processes of neighboring vertebrae. The development of articular processes and the acquisition of opisthocoelous or procoelous increases the strength of the vertebral connection without reducing the flexibility of the spinal column. The trunk vertebrae have well-developed transverse processes, to which very short ribs are attached in caudates; in most anurans, the ribs merge with the transverse processes. On the cervical vertebra the transverse processes are poorly developed. The iliac bones of the pelvic girdle are attached to the free ends of the well-developed transverse processes of the sacral vertebra. In caudates, the caudal vertebrae bear lower arches, forming, like in fish, a hemal canal.
The skull of amphibians retains quite a lot of cartilage in adulthood. Compared to bony fish, modern amphibians have fewer bones, while ancient extinct species had more integumentary bones. In the occipital region of the axial skull, paired lateral occipital bones (occipitale laterale) develop, bordering the foramen magnum; each of them has a condyle. Two occipital condyles articulating with a cervical vertebra are a characteristic feature of amphibians (and mammals; reptiles and birds have only one condyle). In the auditory region there is one pair of bones - the anterior ear (prooticum). In the orbital region of caudate amphibians there are paired oculo-sphenoid bones (orbitosphenoideum); in tailless animals they merge into one ring-shaped sphenoid-olfactory bone (sphenethmoideum). The rest of the braincase remains cartilaginous. The number of integumentary bones is also small. The roof of the skull is formed by paired parietal (parietale) and frontal (frontale) bones, which in tailless animals merge into paired frontoparietale bones (frontoparietale). In front lie paired nasal bones (nasale), and in caudates there are also 1-2 pairs of prefrontal bones (praefrontale). In the auditory region, the covering scaly bone (squamosum) is formed. The bottom of the skull is covered by a large parasphenoid (parasphenoideum), in front of which lie paired integumentary bones - palatine (palatinum) and vomer (vomer); in caudates they merge into paired velar bones (vomeropalatinum). Small teeth sit on the vomers, and on the caudate and palatine bones.
In the visceral part of the skull, the nonquadrate cartilage is preserved throughout life; with its anterior and posterior ends it adheres to the skull (autostyly). Adjacent to the palatoquadrate cartilage are paired integumentary bones - the premaxilla (praemaxillare) and the maxilla (maxillare). Small teeth sit on the premaxillary and maxillary bones; in some species, such as toads, they are reduced. The posterior portion of the quadrate palatine cartilage is covered from above by the integumentary quadratojugal bone (quadratojugale) and the already mentioned squamosal bone, and from below by the pterygoideum. In some caudate amphibians, the posterior portion of the palatoquadrate cartilage ossifies, forming a small square bone (quadratum). The primary lower jaw - Meckel's cartilage - remains cartilaginous; only its anterior end ossifies into small paired chin-maxillary bones (mentomandibulare). Behind them, covering the Meckel's cartilage, lie the integumentary dental bones (dentale), which in modern amphibians lack teeth. The posterior part of the Meckel's cartilage is overgrown with a long integumentary angular bone (angulare) and several small additional integumentary bones. The articular process of Meckel's cartilage articulates with the posterior end of the palatoquadrate cartilage, forming the jaw joint.
Differentiation of the trunk skeleton of fish, amphibians and reptiles
In the skullless subtype there is only an axial skeleton in the form of a chord. It is built of highly vacuolated cells, tightly adjacent to each other and covered on the outside with common elastic and fibrous membranes. The elasticity of the chord is given by the turgor pressure of its cells and the strength of the membranes. The notochord is formed in the ontogenesis of all chordates and in more highly organized animals performs not so much a supporting, but a morphogenetic function, being an organ that carries out embryonic induction. Throughout life in vertebrates, the notochord is retained only in cyclostomes and some lower fish. In all other animals it is reduced. The formation of vertebrae in phylogeny begins with the development of their arches, covering the neural tube and becoming sites of muscle attachment. Beginning with cartilaginous fish, cartilagination of the shell of the notochord and the growth of the bases of the vertebral arches are detected, as a result of which the vertebral bodies are formed.
The fusion of the upper vertebral arches above the neural tube forms the spinous processes and the spinal canal, which encloses the neural tube. Replacing the notochord with the spinal column - a more powerful support organ with a segmental structure - allows you to increase the overall size of the body and activates motor function. Further progressive changes in the spinal column are associated with tissue substitution - the replacement of cartilaginous tissue with bone, which is found in bony fish, as well as with its differentiation into sections. Differentiation of its sections in connection with the transition to a terrestrial lifestyle and movement of the body on the ground with the help of limbs. In aquatic animals (fish), only the body and tail sections differ. The trunk section is divided into typical vertebrae - amphicoelous, which distinguish the body, the upper arch with the upper (neural) spinous processes (protecting the spinal cord) and the large lower arches with the lower processes. In the trunk region, the ribs are attached to the spine (to the transverse processes or to the vertebral body).
In the caudal region, the transverse processes, closing, form the lower (hemal) arch, which ends in the lower spinous process. The hemal canal contains the caudal artery and vein. The last caudal vertebra is flattened and serves to attach the rays of the caudal fin; it often changes its usual shape: it lengthens and bends upward, forming a urostyle. The skeleton of amphibians, like other vertebrates, is divided into an axial skeleton (vertebral column), a skull (cerebral and visceral), paired limbs and their girdles. The axial skeleton in amphibians is represented by the vertebral column, consisting of ossified vertebrae; the notochord is usually reduced in adulthood. Compared to fish, the axial skeleton of amphibians consists of a larger number of sections.
1. The cervical region in all amphibians is represented by one cervical vertebra, which movably articulates with the skull using two articular platforms.
2. The trunk section of the spine of frogs consists of 7 vertebrae (in tailed amphibians - from 14 to 63).
3. The sacral region in all amphibians is represented by one sacral vertebra, to the massive transverse processes of which the iliac bones of the pelvic girdle are attached.
4. The caudal region (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.
In tailed amphibians, 26-36 individual vertebrae are retained in the tail. The trunk vertebrae of most frogs are of the procoelous type: the vertebral body is concave in front, convex in the back, but the last trunk vertebra has an amphicoelous (biconcave) type of structure. Above the vertebral bodies are the upper arches, forming a canal for the spinal cord. On the dorsal side of the arch there is a small spinous process. Paired transverse processes extend from the upper lateral surface of the vertebral body; 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 located anteriorly and posteriorly at the base of the upper arch. Compared to fish, amphibians are characterized by greater differentiation of the vertebral column into sections, a change in the shape of the vertebral bodies, and a stronger development of the articular processes. These transformations are associated with a terrestrial lifestyle and provide greater strength to the axial skeleton while maintaining its mobility, a strong connection with it of the pelvic girdle and allow some mobility of the skull in the vertical plane relative to the body (the ability to raise and lower the head). The differentiation of the axial skeleton, or spine, into sections is expressed much more clearly in reptiles than in amphibians.
The cervical region (pars cervicalis) is always composed of several vertebrae, of which the two anterior ones have a special structure. The first cervical vertebra is called the atlas or atlas. It lacks a vertebral body and has the shape of a ring divided into two parts. On the lower anterior surface of this vertebra there is an articular cavity that movably connects to the condyle of the skull (see below). The second cervical vertebra, epistrophaeus, has a large odontoid process (processus odontoideus) in front, which is the body of the first cervical vertebra, fused with the epistrophaeus. The odontoid process fits freely into the lower opening of the atlas. This structure of the first cervical vertebrae provides greater mobility of the head. The remaining cervical vertebrae have the usual structure (see below); many of them bear short cervical ribs.
What types of vertebrae are characteristic of fish, amphibians, reptiles, birds, mammals
The vast majority of the largest species of animals, including humans, belong to the group - Vertebrates. Among the invertebrates there are also huge animals, such as giant squids, but their comparative paucity only emphasizes the dominant position of vertebrates: mammals, birds, reptiles, amphibians and fish. On land, in water and even in the air, vertebrates constitute the main and most important component of natural communities. All vertebrates have an internal skeleton, the basis of which is a strong axial rod - the spine. The spine is made up of individual vertebrae and contains the spinal cord. But vertebrae made of strong bone tissue, for example, in humans, are characteristic only of the most advanced vertebrates, which appeared relatively recently in the history of the Earth. These include bony fish, amphibians, reptiles, birds, and mammals. In cartilaginous fish (sharks and rays), the vertebrae are formed by cartilaginous tissue, and in even more primitive vertebrates, such as lampreys, instead of a spine, a notochord stretches along the back - a flexible cord of cells enclosed in a durable fibrous cover. When compared with various groups of invertebrates - worms, mollusks, crustaceans, spiders, insects, etc. - it is thanks to the internal skeleton that vertebrates achieved such a complex organization and high level of development. Today there is a diversity of modern vertebrates, numbering about 45 thousand species.
Features of the structure of the shoulder and pelvic girdles and the skeleton of the free limbs of vertebrates
The skeleton of the shoulder girdle consists of two paired bones - the clavicle and the scapula. The clavicle is an S-shaped bone, has a body and two ends, which through joints are connected to the manubrium of the sternum and the acromial process of the scapula. The scapula is a flat, triangular-shaped bone. It distinguishes the anterior and posterior surfaces of the coracoid and acromial processes and the articular cavity for connection with the humerus. There is a ridge on the back surface of the scapula. The skeleton of the free upper limb includes the humerus, bones of the forearm and hand and their joints.
The humerus consists of a diaphysis and two epiphyses. On the upper epiphysis there is a head, anatomical and surgical neck, greater and lesser tubercles, on the lower epiphysis there are lateral and medial epicondyles (serve for muscle attachment) and articular surfaces for connection with the bones of the forearm. There are two to the spine of the forearm: the ulnar (located on the side of the little finger) and the radial. The bones of the hand are divided into the bones of the wrist (8 small bones lying in two rows), the bones of the metacarpus (5 bones) and the phalanges of the fingers (the thumb has 2, the remaining fingers have 3 phalanges). The skeleton of the pelvic girdle consists of two pelvic bones, which, connecting with each other, the sacrum and the coccyx, form the pelvis. The pelvic bone grows together (by the age of 16) from 3 bones: the ilium, the pubis and the ischium. At the site of their fusion there is an acetabulum, into which the head of the femur enters. On the pelvic bone, the iliac wing, crest and fossa, ischial tubercle, obturator foramen, etc. are distinguished.
The following pelvic connections are available:
1) paired sacroiliac joint, flat in shape;
2) pubic fusion, or symphysis - the connection of two pubic bones with the help of cartilage, inside of which there is a cavity;
3) own ligaments of the pelvis (obturator membrane, sacrospinous and sacrotuberous ligaments).
It is customary to distinguish between the large and small pelvis; there is a border line between them. The large pelvis is formed primarily by the wings of the ilium, and the small pelvis by the ischial and pubic bones with the sacrum and coccyx. In the pelvic region there is the cecum in the right iliac fossa, and the sigmoid colon in the left iliac fossa. The pelvic cavity contains the bladder, rectum and part of the genitals.
The structure of the head skeleton of vertebrate fish
Like cartilaginous fish, the skull of bony fishes consists of two sections: the axial skull, or braincase (neurocranium), and the facial, or visceral, skull (splanchnocranium). But unlike cartilaginous fish, the skull of bony fish is almost entirely formed by bone tissue and consists of numerous individual bones.
The axial skull of bony fishes is divided into the same sections as those of cartilaginous fishes. Each section produces several bones. The occipital region of the skull is composed of four bones: the main, or lower, occipital. Two lateral occipital and superior occipital. These bones border the foramen magnum, through which the brain connects to the spinal cord. In the auditory section of the skull there are five ear bones on each side. In the orbital region there are paired lateral wedge-shaped and oculosphenoid (orbitosphenoideum) bones.
The latter are located on the thin interorbital septum and are usually destroyed during preparation; pike perch don't have them. At the base of the braincase the main sphenoid bone (basisphenoideum) is formed; When viewing the skull from the side, only its anterior end is visible. In front, in the region of the olfactory region, there are paired lateral olfactory bones and an unpaired middle olfactory bone. All these bones are replacement (primary) in origin. The bones covering the skull from above and below are of integumentary (secondary) bones in origin. They are laid in the connective tissue layer of the skin and later, plunging under the skin, grow to the cartilaginous base of the skull. Among them, in the roof of the skull, large flat frontal bones are striking, covering most of the skull. Behind them, on either side of the superior occipital crest, are relatively small parietal bones. In front of the frontals, in the region of the olfactory region, there are paired nasal bones, separated by the already mentioned middle olfactory bone.
The bottom of the skull is covered by a large unpaired bone called the parasphenoid. An unpaired vomer is located in front of the parasphenoid, equipped with teeth in its anterior part. Scull. The axial, or cerebral, skull of amphibians, like the skull of cartilaginous fish, is of the platybasal type: with a wide base and widely spaced orbits, between which the anterior end of the brain is located. The skull, compared to bony fish, retains a lot of cartilage, and the number of ossifications is relatively small. In the cartilage of the occipital part of the skull, only paired lateral occipital bones are formed, bordering the foramen magnum. Each of them forms a condyle for articulation with a cervical vertebra. In the area of the auditory capsule, instead of five pairs of ear bones characteristic of bony fish, amphibians have only one pair - the anterior ear bones.
In the anterior part of the brain skull, during ossification of the cartilage, an unpaired sphenoid-olfactory bone is formed, which has the appearance of a bone ring girdle. The rest of the skull remains cartilaginous. It is strengthened by the integumentary (skin) bones. On top, in the front part of the skull, lie paired nasal bones with an elongated triangular shape, then paired frontal-parietal bones fused from the frontal and parietal bones, and outward from the ear bones - squamosal bones with a complex shape. The bottom of the brain skull is covered by a powerful cruciform integumentary bone - the parasphenoid. In front of it also lie the integumentary paired palatine bones and paired vomers; Small teeth sit on the vomers. In front of the vomers there are paired internal nostrils - choanae. The visceral region of the amphibian skull also retains a lot of cartilage.
Adjacent to the palatoquadrate cartilage are the bones of the secondary upper jaw that arise in the skin: paired premaxillary bones, load-bearing teeth and maxillary bones. Behind them, strengthening the posterior part of the quadrate palatine cartilage, the integumentary quadratozygomatic bone is formed on top, and the integumentary pterygoid bone is also formed below. The primary lower jaw - Meckel's cartilage - also remains cartilaginous, only its very anterior end ossifies into small paired chin-maxillary bones. They are joined by integumentary dental bones, which in amphibians lack teeth. The posterior part of the Meckel's cartilage is overgrown with a long integumentary angular bone and several more small integumentary bones. Through the articular process of Meckel's cartilage, the lower jaw movably articulates with the posterior part of the palatoquadrate cartilage.
The brain section of the skull in dogs of all breeds is formed by unpaired (occipital, sphenoid, ethmoid) and paired (parietal, temporal, frontal bones). The interparietal bone fuses early with the occipital bone. In short-headed dog breeds, there may be one to three irregularly shaped sutural bones in front of the interparietal bone. The skull is light, with a developed brain part, and varies greatly in shape between different breeds. On this basis, long-headed - dolichocephalic (greyhound, collie), medium-sized - mesocephalic (shepherd dogs, pointers) and short-headed - brachycephalic (bulldog, boxer, Pekingese) dog breeds are classified.
In long-headed dogs, the cerebral part of the skull at the level of the zygomatic arches is narrower than in short-headed dogs, the external sagittal ridge is more developed, and the profile of the forehead and nose is slightly concave. There are other breed and intrabreed characteristics. The body of the occipital bone in all representatives of the breeds is flattened, the jugular processes are short and wide, the scales are triangular in shape and, together with the interparietal bone, form the posterior portion of the outer sagittal crest.
The body of the sphenoid bone is flattened, relatively thin, without a sinus (sinus), the fossa of the sella turcica is deep, especially in long-headed dog breeds. The frontal bone is weakly (in long-headed) or strongly (in short-headed) curved. Its frontal part is better developed than the nasal part and contains a large frontal sinus. There is no supraorbital foramen, the zygomatic process is small and does not reach the frontal process of the zygomatic bone. The facial part of the dog’s skull is formed by paired nasal, zygomatic, lacrimal, maxillary, incisive, palatine, pterygoid, and mandibular bones. It also includes the vomer, turbinates and hyoid bone. There are significant breed variations in the facial region.