Features of the structure of mammals. General features and structure of mammals What are the structural features of mammalian organs

External structure

Animals have well expressed: head, neck, torso and tail. On the head usually distinguish between the cranial region, located behind the eyes, and the facial, or muzzle, located in front. Eyes equipped with upper, lower and third eyelids. Unlike birds, the nictitating membrane (third eyelid) covers only half of the mammalian eye. On the sides of the head are large ears, at the end of the muzzle are paired nostrils. Mouth bordered by fleshy lips characteristic of mammals. Very hard hairs usually sit on the upper lip - vibrissae. Several of them are located above the eyes. They play the role of additional organs of touch. Under the root of the tail is the anal opening, and a little anterior to it - the urogenital. In females, 4-5 pairs of nipples are located on the sides of the body on the ventral side. The limbs are five- or four-fingered, the fingers are armed with claws.

Skin

Wool, covering the body of mammals, is a derivative of the skin. There are two types of hair - guard and soft - downy. The skin is made up of two main layers - epidermis And corium. The first is a thin stratum corneum, and the second is very thick, dense. The lower part of it forms the subcutaneous tissue.

Skeleton

The spine consists of five sections: cervical, thoracic, lumbar, sacral and caudal. The vertebrae have flat articular surfaces characteristic of mammals and are separated from each other by round cartilaginous discs - menisci.

The cervical region in all mammals (with very rare exceptions) contains 7 vertebrae. (Both the mouse and the giraffe have 7 cervical vertebrae). These vertebrae lack free ribs. The thoracic region contains 12-13 vertebrae, all of which are equipped with ribs. The anterior seven pairs of ribs are connected to the sternum and are called "true ribs". The next five pairs do not reach the sternum. The lumbar ribs are devoid of and usually contain 6-7 vertebrae. The sacral region is formed in most mammals by four fused vertebrae. The anterior ones usually bear two processes, with the help of which the pelvis is attached. The caudal region is very variable in the number of vertebrae.

Scull It is divided into axial, consisting of the bones surrounding the brain, and visceral (facial), which includes the bones surrounding the mouth opening - the sky, the bones of the upper and lower jaws.

Shoulder girdle represented only by the scapula and clavicle, and there is no crow bone (coracoid) in mammals. In fast runners, the clavicle (ungulates) usually also disappears. The pelvic region consists of a pair of innominate bones, each of which was formed by the fusion of the ilium, ischium and pubis. The skeleton of paired limbs has three typical sections. In the forelimbs, this is the shoulder, forearm and hand, and in the hind limbs, the thigh, lower leg and foot. In mammals, on the hind limbs, a rounded tendon bone appears in the knee joint - the patella.

Muscular system

This system in animals reaches exceptional development and complexity. They have several hundred separate striated muscles. A feature of the muscular system of mammals is the presence of a diaphragm and the appearance of subcutaneous muscles. Diaphragm- This is a domed muscular septum that separates the thoracic region from the abdominal region. In the center it is perforated by the esophagus. The diaphragm takes part in the acts of respiration and excrement of animals. The subcutaneous musculature is a continuous subcutaneous layer. With its help, animals can move parts of the skin. The same muscles take part in the formation of the lips and cheeks. In monkeys, it has almost disappeared and is preserved only on the face. There she received an unusually strong development - this is the so-called mimic muscles.

Nervous system

Brain The animal has powerfully developed hemispheres of the forebrain and cerebellum. They cover all other parts of the brain from above.

forebrain consists of the cerebral hemispheres, covered with a gray medulla - the cerebral cortex. The olfactory lobes extend forward from the hemispheres. Between the hemispheres is a wide jumper of white nerve fibers.

diencephalon has a funnel and optic chiasm, like other classes of vertebrates. The pituitary gland is attached to the funnel of the diencephalon, while the epiphysis is located above the cerebellum on a long stalk. midbrain differs in very small sizes, in addition to the longitudinal furrow, it also has a transverse one, which is characteristic only of mammals. Cerebellum consists of an unpaired part - the worm and two lateral parts, which are very large and are usually referred to as the cerebellar hemispheres. Medulla has a feature that is also characteristic only of mammals. On the sides of this brain, bundles of nerve fibers leading to the cerebellum are isolated. They are called the posterior cerebellar peduncles. The medulla oblongata passes into the spinal cord.

sense organs

They are very highly developed in mammals, and, in accordance with the ecological specialization of a particular group, either smell, or sight, or hearing, or touch is of leading importance. The organs of hearing in animals are especially well developed. They have bony auditory drums and large mobile external ears.

Digestive organs

Oral cavity limited in animals by lips. Lips take part in grasping and holding prey. The oral cavity is bounded from above by a hard bony palate. Due to this, the choanae (internal nostrils) are pushed back towards the pharynx. This allows the animals to breathe while the food is in the mouth. The sides of the oral cavity are limited by soft muscular cheeks, and at the bottom of it there is a large muscular tongue. Its functions are to perceive taste sensations and push food during chewing under the teeth and into the throat during swallowing. The ducts of the salivary glands open into the mouth (4 paired glands - parotid, infraorbital, submandibular and sublingual). Teeth do not grow to the surface of the bone, as in the previous classes, but sit in independent cells. The teeth are differentiated into incisors, canines and molars. The tooth itself consists of such parts as a crown with a working surface, the body of the tooth and its root. throat of beasts short, the windpipe and choanae open into it. Thus, in mammals, the pharynx is the crossroads of two pathways - food and respiratory. Esophagus is a simple, highly extensible muscular tube. After passing through the diaphragm, it connects to the stomach. Stomach has the appearance of a large horseshoe-shaped curved bag that lies across the body. A fat-filled peritoneum hangs from the stomach, which covers all the internal organs with an apron. Liver located under the diaphragm, its flows open into the duodenum, in the loop of which lies the pancreas. Most mammals have a gallbladder. Intestines can be of various lengths, it depends on the composition of the feed. In a herbivorous rabbit, the intestines are very long - 15-16 times longer than the body. Its divisions are the small, large and rectum. At the beginning of the large intestine, mammals have an unpaired blind outgrowth - the caecum. The intestine opens to the outside with an independent anal opening.

Respiratory system

Larynx, as usual for mammals, has a cricoid cartilage, in front of which is a large thyroid cartilage. The larynx of a mammal is complex. The vocal cords are stretched on the inside of the larynx. These are paired elastic folds of the mucous membrane, stretched in the cavity of the larynx and limiting the glottis. Lungs represent a pair of spongy bodies hanging freely into the chest cavity. Their internal structure is characterized by great complexity. The trachea near the lungs divides into two bronchi. The bronchi, entering the lungs, are divided into secondary bronchi, which, in turn, are divided into bronchi of the third and fourth order. They end in bronchioles. The ends of the bronchioles are swollen and braided with blood vessels. These are the so-called alveoli, where gas exchange takes place.

Circulatory system

A heart animals, like birds, has four chambers, and the left ventricle drives blood through the systemic circulation and, like birds, has much thicker walls than the right one. A large vessel departs from the left ventricle - the aorta, which begins the systemic circulation. Arterial blood is supplied to all organs of the body, and venous blood is collected through the vein system. The largest of them - the posterior and two anterior vena cava - flow into the right atrium. From the right atrium, blood enters the right ventricle, from here the pulmonary circulation begins, or, as it is also called, the pulmonary circulation. Venous blood is ejected from the right ventricle into the great pulmonary artery. This artery divides into right and left, leading to the lungs. From each lung, blood is collected in the pulmonary vein (the blood in it is arterial), both veins merge and flow into the left atrium. Further, from the left atrium, the blood flows into the left ventricle and again goes through the systemic circulation.

Organs, secretions

At mammals is a pair of bean-shaped kidneys located in the lumbar region. From the inner concave side of each kidney departs along the ureter (thin tube), which flows directly into the bladder. The bladder opens into the urethra.

Sex organs

In mammals, these are paired testes (in males) or paired ovaries (in females). The testicles have a characteristic oval shape. Adjacent to them are the appendages of the testicles. Paired vas deferens open at the beginning of the urethra. The end parts of the vas deferens are expanded into the seminal vesicles. The paired ovaries of the female have an oval-flattened shape. Near each ovary is an oviduct. At one end, the oviduct opens into the body cavity, and at the opposite end, without a visible border, it passes into the uterus. The uterus in animals is bicornuate, the right and left horns of the uterus open independently into the vagina. It is unpaired. At its posterior end, it gradually passes into the urethra and the bladder opens into it. Outwardly, the vagina opens with the urogenital opening.

Embryo development

egg cells develop in the ovary, then mature cells, upon exiting the ovary into the body cavity, are caught there by the funnel of the oviduct. Thanks to the flickering movements of the cilia of the tube (oviduct), the egg moves along it, and if the female is fertilized, then in the tube (usually in its first third) the egg merges with the sperm. The fertilized egg continues to slowly descend into the uterus and at the same time its crushing (dividing the egg into many cells) begins. Having reached the uterus, the egg, which by that time has turned into a dense multicellular ball, is introduced into the wall. There, nutrients begin to flow to it. Pretty soon, a placenta forms around the implanted embryo. This is the shell of the fruit, very characteristic of mammals. The placenta is a spongy organ rich in blood vessels, in which children and maternal parts are distinguished. The nursery consists of the villi of the germinal membrane, and the maternal one consists of the wall of the uterus. During childbirth, the muscular layer of the uterus is greatly reduced and the baby placenta (chorion), by that time associated very slightly with the mucous membrane of the uterus, opens and exits along with the newborn in the form of a child's place.

Mammals are the most highly organized vertebrates. They differ in a wide variety of sizes and external structure, which depends on the conditions and lifestyle. For example, a baby shrew has an average weight of 1.5 g, an African elephant - 4-5 tons, and a blue whale up to 150 tons.

FEATURES OF THE EXTERNAL STRUCTURE

Consider them on the example of a dog. The body of mammals is divided into head, neck, trunk, tail and

two pairs of limbs. The head is elongated. It distinguishes the cranial and facial department or muzzle. On the head is a mouth bordered by movable fleshy lips, above which is a nose with a pair of nostrils. On the sides of the head are a pair of eyes protected by movable eyelids. The third eyelid (nictitating membrane) is reduced. Behind the eyes is a pair of movable auricles, characteristic only of mammals. The neck provides a movable connection of the head with an elongated body, raised high on the fore and hind limbs above the ground. On the ventral side of it (in females) there are several pairs of mammary glands, and under the root of the tail there is an anus. The limbs are five-fingered. All fingers end in claws.

BODY COVERS

The skin of mammals consists of two layers - epithelial and skin proper. The epithelium is keratinized. Due to it, sebaceous and sweat glands, hair, claws, nails, horns, hooves are formed. The presence of hair is a characteristic feature of mammals. The hair evenly covers the dog's body and is divided into awns (long and coarse), undercoat (short and fine) and downy hair. The awn protects the skin from damage, and the undercoat serves for thermal insulation. Hair is made up of a horny substance called keratin. In dogs, due to the seasons, they change 2 times a year.

The sebaceous glands located in the skin produce a secret that lubricates the surface of the skin and hair, helping to maintain its elasticity, and also protects against the penetration of microorganisms.

There are few sweat glands in dogs, tk. thermoregulation in them is carried out due to the evaporation of water from the surface of the tongue. The mammary glands are also derivatives of the epidermis, the secret of which the young feed on. In some mammals, sweat or sebaceous glands are modified into odorous ones: musky (muskrat, beaver), anal (predators). Their secret serves for species identification, protection, and marking of the occupied territory.

SKELETON AND MUSCLE

The skeleton has a structure typical of terrestrial vertebrates, but at the same time a number of features.

The skull is formed by several paired and unpaired fused bones. Its brain section has larger volumes than that of reptiles, which is determined by the significant development of the brain, especially the cortex. The facial section is characterized by the development of a secondary jaw and a bony hard palate.

The spinal column consists of 5 sections: cervical, thoracic, lumbar, sacral and caudal. The cervical region consists of 7 vertebrae, which is typical for almost all mammals. The number of thoracic vertebrae ranges from 12 to 15. Ribs are attached to them, which fuse with the sternum to form the chest. Massive lumbar vertebrae (6) are movably connected. The sacral vertebrae (3-4) fuse motionlessly between themselves and the pelvic bones, creating support for the hind limbs. The caudal region is characterized by great variability in the number of vertebrae.

The girdle of the forelimbs of the dog is formed by paired shoulder blades and crow bones fused with them. Clavicles are absent. The shoulder girdle is connected to the axial skeleton through muscles and ligaments.

The girdle of the hind limbs is formed by paired innominate bones. They are formed as a result of the fusion of the iliac, pubic and ischial bones. Fusing with the sacrum, they form a closed pelvis.

The free limbs are five-fingered, have a structure typical of terrestrial vertebrates. The hind limb is characterized by the development of a tendon bone-calyx.

The muscles of mammals are highly specialized. The masticatory muscles, which are involved in the capture and grinding of food, achieve significant development and differentiation. A characteristic feature of the muscular system is the development of the subcutaneous muscles and the diaphragm. The appearance of the diaphragm improves ventilation of the lungs, and also divides the body cavity into chest and abdominal. The subcutaneous musculature plays a role not only in thermoregulation, but also in the transmission of information. Good development of the muscles of the limbs provides a high speed of movement.

INTERIOR STRUCTURE

The digestive system is characterized by the development of specialized teeth, a clear division of the intestinal tube into sections and its considerable length, which ensures efficient digestion and absorption of nutrients.

The oral cavity begins with the vestibule of the mouth, the outer wall of which is fleshy lips, and the inner wall is well-developed jaws, equipped with specialized teeth.

Dogs have 42 teeth, divided into incisors (12), canines (4), anterior (16) and posterior (10). The teeth have a root, which is fixed in the hole of the jaw, and a crown, the shape of which depends

on the type of teeth. Dogs have small, chisel-shaped incisors. The fangs are large, conical, used to capture and kill prey. Indigenous have wide tuberous crowns with a sharp cutting edge. The last premolars of the upper jaw and the first molar of the lower form carnassial teeth. In the process of individual development, the milk teeth (incisors, canines and premolars) are replaced by permanent ones.

At the bottom of the mouth lies a muscular tongue, the surface of which is covered with taste buds. It is involved in mixing and swallowing food, as well as taste reception. The ducts of three pairs of salivary glands open into the oral cavity, the secret of which moisturizes food, and also contains enzymes that break down starch.

From the oral cavity through the pharynx and esophagus, food enters a well-developed simple stomach, and from it, after partial digestion, into the small intestine. The ducts of the liver and pancreas flow into its initial section, the duodenum. In the small intestine, food is hydrolyzed and absorbed. Undigested food remains in the large intestine, which is divided into the caecum and colon. In these sections of the intestine, the formation of feces occurs, which are removed outward through the rectum.

RESPIRATORY SYSTEM

Mammals breathe atmospheric air. The main role in gas exchange belongs to the lungs, which are connected with the external environment through the respiratory tract. The respiratory tract includes the nasal cavity, nasopharynx, pharynx, larynx, trachea and bronchi, which form numerous branches in the lungs. The smallest bronchi - bronchioles - end in pulmonary vesicles - alveoli. In the latter, gas exchange takes place. In the development of the respiratory organs of mammals, the appearance of the epiglottal cartilage, the larynx, and the alveolar structure of the lung should be noted.

CIRCULATORY SYSTEM

Mammals have a four-chambered heart, consisting of two atria and two ventricles. From the left ventricle, unlike birds, the left aortic arch departs. Blood moves through two circles of blood circulation. The systemic circulation starts from the left ventricle. The arterial blood contained in it is delivered to the tissues through the system of vessels that depart from the aorta. Venous blood is collected in the anterior and posterior vena cava, which empty into the right atrium, where the large circle ends.

The pulmonary circulation begins in the right ventricle. From it, venous blood flows through the pulmonary artery to the lungs. Oxygenated arterial blood flows from the lungs to the left atrium through the four pulmonary veins.

In mammals, due to the development of a four-chambered heart, arterial and venous blood do not mix. The supply of tissues with oxygenated arterial blood enhances the course of redox processes in cells, increasing the level of energy metabolism. As a result, most modern mammals are able to maintain a constant body temperature and remain active in conditions of sharp changes in environmental temperatures.

ORGANIZATIONS

In the water-salt metabolism in mammals, the main role belongs to the secondary kidneys. They are paired compact bean-shaped bodies located on the sides of the lumbar spine. A pair of ureters depart from the kidneys, opening into the bladder, from which urine is excreted through the urethra. The kidneys secrete urine that is hypertonic in relation to blood plasma, which saves water by removing metabolic products and salts from the body.

NERVOUS SYSTEM

It consists of the brain, spinal cord and peripheral nerves extending from them. The dog brain is divided into 5 sections, like all vertebrates, but has a number of features compared to other classes of vertebrates. The greatest size and development reach the hemisphere of the forebrain. The bulk of them is the bark, on the surface of which there are a large number of convolutions. The hemispheres are connected to each other by the corpus callosum.

The midbrain is divided by furrows into quadrigemina, unlike other vertebrates that have colliculus. Through the anterior colliculus, the pathways of the visual tract go to the cortex, and through the posterior colliculus, the auditory tract. The cerebellum is large. It consists of hemispheres and a worm located between them. It ensures the maintenance of muscle tone, balance, coordination of movements. 12 pairs of cranial nerves leave the brain.

SENSORS

Well developed. The organs of vision are represented by a pair of eyes. The cornea of ​​the eye is convex, the lens accommodates only due to a change in curvature. In connection with the development of the cortex, secondary associative visual centers are formed in it, located in its occipital lobe.

HEARING organ

Has a complex structure. In the process of evolution, three of its departments were formed: the inner, outer and middle ear. The outer ear is represented by a movable auricle and external auditory canal. Three auditory ossicles develop in the middle ear: the malleus, anvil, and stirrup. In the inner ear, the cochlea, in which the organ of Corti is located, reaches significant development.

Dogs, like many mammals, have a well-developed sense of smell. They are located in the upper-posterior part of the nasal cavity and represent a system of complexly branched shells, the surface of which is covered with olfactory epithelium. The sense of smell allows you to perceive various odors or combinations of them, characteristic of a group of individuals or an individual.

The organs of taste are represented by taste buds located on the tongue.

SKIN SENSITIVITY is represented by receptors that perceive temperature, pressure, touch.

REGENERAL SYSTEM

Dogs, like all mammals, are dioecious animals. Males have paired testes in which spermatozoa develop. The vas deferens from the testicles enter the urinary tract. Paired ovaries of females lie in the body cavity. The oviducts at one end face the body cavity, and at the other they open into the muscular organ inherent in higher mammals - the uterus, which opens outward with the vagina.

DEVELOPMENT

Fertilization is internal, occurring in the oviducts. Fertilized eggs, moving through the oviducts, begin crushing, turning into a multicellular embryo. When the embryo enters the uterine cavity, it attaches to its mucous membrane. At the site of contact of the embryo with the uterine mucosa, a child's place develops - the placenta. Through it, during embryonic development, the fetus receives nutrients and oxygen from the mother's blood, and at the same time removes metabolic products.

Dogs give birth to several blind, helpless cubs. Therefore, parents take care of their offspring. Mothers feed their cubs with milk, keep them warm, and protect them from enemies. And after the completion of feeding, the mother and father continue to protect the cubs, educate them, passing on individual experience to the offspring.

MAMMALIAN BEHAVIOR

The high level of development of the nervous system and sensory organs determine the complexity of mammalian behavior and its plasticity. It is based not only on a set of simple unconditioned reflexes that determine instinctive, innate behavior, but also on the ability to form conditioned reflexes and accumulate individual experience on their basis. In the process of interaction of the organism with the environment, its functional systems constantly adapt to changing conditions on the basis of the formation of new temporary connections in the cerebral cortex and the extinction of old ones. Therefore, the nervous activity of mammals is characterized by mobility, richness and complexity of connections with the environment. Mammals are able to anticipate the course of many recurring events and, in certain situations, make appropriate decisions.

ORIGIN OF MAMMALS

Mammals come from a group of ancient primitive reptiles - animal-toothed lizards. According to the remains of the skeletons of animal-toothed lizards, it was established that they lived 200-230 million years ago. Their legs were located under the body and raised it high above the ground. Their teeth had roots and were divided into incisors, canines and molars, and the hard palate was bony, secondary. The skin integuments retained the features of the organization of the skin of amphibians.

Mammals appeared on earth in the Triassic period of the Mesozoic era. Their origin from reptiles is evidenced by signs common to both classes: the presence of a keratinizing epithelium with homologues of horny scales - hair, the presence of claws on the fingers, the homology of the limbs and their belts, the division of the spinal column into 5 sections, the similarity of the early stages of development of embryos. However, mammals have a four-chambered heart and are warm-blooded. They are characterized by feeding the cubs with milk, live birth.

The origin of mammals from reptiles is also evidenced by the fact that representatives of the subclass of primitive animals (platypus, echidna) occupy an intermediate position between reptiles and mammals in structure and reproduction characteristics.

Modern taxonomy divides mammals into 2 subclasses:

1. First beasts and 2. Real beasts. One detachment belongs to the first subclass - Single-pass. The second subclass includes - infraclass - Lower animals with the order Marsupials and infraclass - Higher animals, uniting 19 modern and 12-14 extinct orders.

Both subclasses of mammals originate in the Triassic from the same original group of animal-like reptiles. In subsequent evolution, various adaptations of mammals contributed to their conquest not only of vast expanses of land, but also of air, soil, fresh and sea waters.

Outwardly, mammals are very diverse, their body structure depends on environmental conditions and lifestyle. Mammals have a head, neck, torso with two pairs of limbs, and a tail. The head has a mouth, nose, eyes, ears. The mouth in mammals is limited by soft mobile lips, which in childhood are involved in sucking milk, and later in capturing food. The eyes are protected by developed eyelids. Eyelashes are located along their edges. The nictitating membrane in mammals is underdeveloped.

Unlike amphibians and reptiles, the limbs of mammals are located under the body, so it is elevated above the ground.

The body of mammals is covered with strong and elastic skin. It contains the base of the hair. There are long thick guard hairs and short soft downy hairs. Hard long hair - vibrissae - is especially distinguished. As a rule, vibrissae are located on the head (the so-called "whiskers" of animals), on the lower part of the neck, on the chest. The structure of various mammalian systems is discussed in more detail in the table below.

The figure below shows the external structure of mammals (for example, a rabbit)

Features of the structure of mammals

The figure below shows the internal structure of mammals.

Zoology – a scientific discipline that studies the animal world, a major component of biology. According to the objectives of the study, zoology is divided into a number of disciplines: systematics, morphology, embryology, animal genetics, zoogeography, etc. According to the objects of study, protozoology, which studies protozoa, invertebrate zoology, and vertebrate zoology, is distinguished. The last object of study is T eriology, dealing with the study of mammals.

The emergence of mammals became possible as a result of the formation of a number of large aromorphoses, which reduced the dependence of animals on changes in the external environment. Mammals evolved from ancient reptiles at the very beginning of the Mesozoic era, i.e. earlier than birds, but the development that led to the modern wealth of forms of this class of vertebrates dates back to the Cenozoic era, after the extinction of large reptiles.

General features of mammals

Mammals are warm-blooded vertebrates from the group of amniotes. As I said, this is the most highly specialized group of land animals, which are distinguished by the following progressive features.

1. Highly developed central nervous system and sense organs. The cerebral cortex appears, formed by gray matter, which ensures a high level of nervous activity and complex adaptive behavior.

2. The system of thermoregulation, providing a relative constancy of body temperature.

3. Live birth (except for oviparous ones) and feeding of cubs with mother's milk, which ensures the best safety of offspring.

The height of the organization of mammals is also expressed in the fact that all organs in them reach the greatest differentiation, and the brain of the most perfect structure. The center of higher nervous activity is especially developed in it - the cerebral cortex, consisting of gray medulla. In this regard, the reactions and behavior of mammals reach exceptional perfection. This is facilitated by very complex sensory organs, especially hearing and smell. The differentiation of teeth into incisors, canines, and molars also contributed to the rapid progressive development of mammals.

A huge role in the development of this group was played by the acquisition of warm-bloodedness, that is, a constantly high body temperature. It arises due to: a) unmixed blood circulation, b) enhanced gas exchange, c) thermoregulatory devices. Unmixed circulation, as in birds, is achieved by a four-chambered heart and the preservation of only one (left) aortic arch in animals. The acquisition of the alveolar structure of the lungs and the appearance of the diaphragm led to increased gas exchange. Diaphragm- This is a muscular partition that completely divides the body into two parts - chest and abdominal. The diaphragm is involved in the act of inhalation and exhalation. thermoregulation achieved by the appearance of hair and skin glands.

Thanks to the perfection of the digestive, respiratory and circulatory systems, the entire metabolism of mammals proceeds very intensively, which, along with high body temperature, makes them less dependent on the climatic conditions of the environment than amphibians and reptiles. The rapid progressive development of animals is also due to the fact that the highest of them developed live birth. The nutrition of the embryo in the womb is carried out through a special organ - placenta. After birth, the baby is suckled with milk. It is secreted by special mammary glands. All this greatly increases the survival rate of offspring. Thanks to the height of organization and the perfect psyche, by the beginning of the Cenozoic era (65 million years ago), mammals were able to displace the reptiles that had dominated the Earth until then and occupy all the main habitats.

Features of the structure of mammals

External building. The animals have well-defined head, neck, torso and tail. On the head, a cranial region is usually distinguished, located behind the eyes, and a facial, or muzzle, located in front. The eyes are equipped with upper, lower and third eyelids. Unlike birds, the nictitating membrane (third eyelid) covers only half of the mammalian eye. On the sides of the head are large ears, at the end of the muzzle are paired nostrils.

Rice. 1. Scheme of the structure of mammals

1- skin; 2 - skull; 3 - spine; 4 - oral cavity; 5 - pharynx; 6 - esophagus; 7 - stomach; 8 - small intestines; 9 - large intestine; 10 - liver; 11 - kidneys; 12 - ureters; 13 - windpipe; 14 - lungs; 15 - heart; 16 - diaphragm; 17 - brain; 18 - spinal cord; 19 - gonad

The mouth is bordered by fleshy lips characteristic of mammals. On the upper lip usually sit very hard hairs - vibrissae. Several of them are located above the eyes. They play the role of additional organs of touch. Under the root of the tail is the anus, and a little anterior to it is the urogenital. In females, 4–5 pairs of nipples are located on the sides of the body on the ventral side. The limbs are five- or four-fingered, the fingers are armed with claws.

Skin covers. The wool that covers the body of mammals is a derivative of the skin. There are two types of hair - guard and soft - downy. The skin consists of two main layers - the epidermis and the corium. The first is a thin stratum corneum, and the second is very thick, dense. The lower part of it forms the subcutaneous tissue.

The hair represents a horn formation. It distinguishes between the lower expanded part - the bulb - and the long rod protruding outward; its lower part, together with the bulb, forms the root of the hair, sitting in the bag. In the rod under a microscope, 3 layers of cells are visible: cuticle, middle layer and core. Hair contains a pigment that determines its color. White hair color is sometimes associated with the presence of air inside the cells. In most animals, hair is divided into 2-3 main categories (Fig. 1).
Outside the fur, long guard hairs are visible, under them there is a thick and delicate underfur; often even longer guiding hairs are visible among the awns. Hair is not arranged randomly, but in certain groups. The shape of individual hairs and the type of their distribution are characteristic of each type of animal.

Rice. 2. The structure of the skin and hair types of mammals (according to Geiler, 1960)

1 - underfur; 2 - guard hair; 3 - stratum corneum of the epidermis; 4 - malpighian layer; 5 - corium; 6 - muscle of the hair follicle; 7 - sebaceous gland; 8 - hair root; 9 - hair papilla; 10 - blood vessel; 11 - sweat gland

A special modification of hair is represented by vibrissae, or tactile hair, located in groups on the muzzle ("whiskers", etc.), and sometimes on the paws and the ventral side of the body. The hairline modifications also include the stiff bristles of a wild boar, the quills of a porcupine, a hedgehog, etc. The hairline plays a very important role in the life of animals: it protects them from the adverse effects of the environment, helps regulate body temperature, and often masks the animal. The hairline (fur) reaches its best development in animals of a cold and temperate climate. The appearance of hair in the process of evolution turned out to be a very important adaptation that made it easier for animals to exist in the most unfavorable landscapes for life.

The hairline develops with the age of the animal and is periodically replaced during the year. Usually molting is seasonal, sometimes accompanied by a change in color. It is closely dependent on seasonal changes in meteorological conditions. In most of our land animals, winter hair is much thicker and more magnificent than summer. Thus, on the back of a squirrel on a skin area of ​​10 mm 2, there are 46 groups of hairs in summer, and 89 in winter, that is, almost twice as many. The length of the guard hairs increases from 11 to 20 mm, the length of the underfurs - from 7 to 12 mm. Seasonal hair dimorphism is weakly expressed in burrowing, hibernating, and aquatic animals.

Most species have 2 molts, but some have up to 3-4. The timing of the start and duration of molts depend on meteorological conditions, sex, age, fatness of the animal and therefore vary from year to year. But the order of the seasonal change of hair on certain parts of the body is natural and is basically preserved annually. In this case, usually spring and autumn molts occur in the reverse order (from head to tail and vice versa). The skin on the molting areas of the skin turns blue, which makes it easier to study the molting process. In terrestrial animals, the change of hairline occurs in a relatively short time, especially in spring, while in aquatic and semi-aquatic animals it is greatly extended in time. The hair coat of animals living in the water has much less sharp seasonal differences and even in summer remains relatively dense. This is due to weaker temperature fluctuations and increased thermal conductivity of water, which requires good protection against cooling throughout the year.

Some mammals (white hare, ermine, weasel, arctic fox) turn white in the winter. The timing of whitening generally coincides with the average long-term dates of the establishment of snow cover. But in some years this coincidence does not work out, and premature whitening of hares sometimes turns out to be disastrous for them. White coloration has a masking (cryptic) meaning. Assumptions about its role in thermoregulation were not confirmed by specially designed experiments.

Summer coloration sometimes also has a protective meaning, well masking the hidden animal; for example, the spotted pattern of young roe deer and deer, the striped pattern of young wild boars, the sandy coloration of many desert rodents, etc. In a number of cases, the nature of the coloration, apparently, is explained by the influence of temperature, air humidity, and other environmental factors. It is no coincidence that many fur-bearing animals of Eastern Siberia and Yakutia, where the climate is sharply continental, have not only the fluffiest, but also the darkest fur (sable, squirrel).

The hairline is closely related to the skin. It is composed of two main layers: the superficial epidermis and the deeper corium, which consists mainly of fibrous connective tissue. The cells of the epidermis, as they approach its surface, become more and more horny, die off and gradually exfoliate, being replaced by new cells coming from a deeper layer, called the Malpighian. The surface layer of the corium protrudes into the latter in the form of papillae. In these papillae, the smallest blood capillaries and tactile bodies develop. Deeper in the skin are blood vessels, nerves and fat is formed. The skin of mammals is very abundant in glands - tubular and alveolar. The former are predominantly sweat glands, while the latter are sebaceous. As mentioned above, the mammary glands are a kind of modification of the tubular glands.

Hair is a derivative of the epidermis, although its roots are located in deeply lying connective tissue layers. Derivatives of the epidermis also include such horny formations as claws, hooves, scales (for example, the shells of armadillos and pangolins; small scales on the tail of a beaver, muskrat, etc.), partly the horns of bovids, in which the horny substance in the form of a sheath covers the bone shaft. Claws, horns and others, like hair, undergo age and seasonal changes.

Skeleton. The spine consists of five sections: cervical, thoracic, lumbar, sacral and caudal. The vertebrae have flat articular surfaces characteristic of mammals and are separated from each other by round cartilaginous discs - menisci.

The cervical region in all mammals (with very rare exceptions) contains 7 vertebrae. (Both the mouse and the giraffe have 7 cervical vertebrae). These vertebrae lack free ribs. The thoracic region contains 12-13 vertebrae, all of which are equipped with ribs. The anterior seven pairs of ribs are connected to the sternum and are called "true ribs". The next five pairs do not reach the sternum. The lumbar ribs are devoid of and usually contain 6-7 vertebrae. The sacral region is formed in most mammals by four fused vertebrae. The anterior ones usually bear two processes, with the help of which the pelvis is attached. The caudal region is very variable in the number of vertebrae.

Fig.3. mammalian skeleton

1 - skull; 2 - lower jaw; 3 - cervical vertebrae; 4 - thoracic vertebrae; 5 - lumbar vertebrae; 6 - sacrum; 7 - tail vertebrae; 8 - ribs; 9 - sternum; 10 - scapula; 11 - humerus; 12 - ulna; 13 - radius; 14 - bones of the wrist; 15 - bones of the metacarpus; 16 - phalanges of the fingers of the forelimb; 17 - pelvis; 18 - femur; 19 - tibia; 20 - fibula; 21 - tarsal bones; 22 - bones of the metatarsus; 23 - phalanges of the fingers of the hind limb; 24 - patella

The skull is divided into axial, consisting of the bones surrounding the brain, and visceral (facial), including the bones surrounding the mouth opening - the sky, the bones of the upper and lower jaws. The shoulder girdle is represented only by the scapula and clavicle, and there is no crow bone (coracoid) in mammals. In fast runners, the clavicle (ungulates) usually also disappears. The pelvic region consists of a pair of innominate bones, each of which was formed by the fusion of the ilium, ischium and pubis. The skeleton of paired limbs has three typical sections. In the forelimbs, this is the shoulder, forearm and hand, and in the hind limbs, the thigh, lower leg and foot. In mammals, on the hind limbs, a rounded tendon bone appears in the knee joint - the patella.

Muscular system. This system in animals reaches exceptional development and complexity. They have several hundred separate striated muscles. A feature of the muscular system of mammals is the presence of a diaphragm and the appearance of subcutaneous muscles. The diaphragm is a domed muscular septum that separates the thoracic region from the abdominal region. In the center it is perforated by the esophagus. The diaphragm takes part in the acts of respiration and excrement of animals. The subcutaneous musculature is a continuous subcutaneous layer. With its help, animals can move parts of the skin. The same muscles take part in the formation of the lips and cheeks. In monkeys, it has almost disappeared and is preserved only on the face. There she received an unusually strong development - this is the so-called mimic muscles.

Nervous system. The animal's brain has powerfully developed hemispheres of the forebrain and cerebellum. They cover all other parts of the brain from above. The forebrain consists of cerebral hemispheres covered with a gray medulla - the cerebral cortex. The olfactory lobes extend forward from the hemispheres. Between the hemispheres is a wide jumper of white nerve fibers.

The diencephalon has a funnel and optic chiasm, as in other classes of vertebrates. The pituitary gland is attached to the funnel of the diencephalon, while the epiphysis is located above the cerebellum on a long stalk. The midbrain is very small in size, in addition to the longitudinal groove, it also has a transverse one, which is characteristic only of mammals. The cerebellum consists of an unpaired part - the vermis and two lateral parts, which are very large and are usually referred to as the cerebellar hemispheres. The medulla oblongata has a feature that is also unique to mammals. On the sides of this brain, bundles of nerve fibers leading to the cerebellum are isolated. They are called the posterior cerebellar peduncles. The medulla oblongata passes into the spinal cord.

Sense organs. They are very highly developed in mammals, and, in accordance with the ecological specialization of a particular group, either smell, or sight, or hearing, or touch is of leading importance. The organs of hearing in animals are especially well developed. They have bony auditory drums and large mobile external ears.

Digestive organs. The oral cavity is limited in animals by lips. Lips take part in grasping and holding prey. The oral cavity is bounded from above by a hard bony palate. Due to this, the choanae (internal nostrils) are pushed back towards the pharynx. This allows the animals to breathe while the food is in the mouth. The sides of the oral cavity are limited by soft muscular cheeks, and at the bottom of it there is a large muscular tongue. Its functions are to perceive taste sensations and push food during chewing under the teeth and into the throat during swallowing. The ducts of the salivary glands open into the mouth (4 paired glands - parotid, infraorbital, submandibular and sublingual). The teeth do not adhere to the surface of the bone, as in the previous classes, but sit in independent cells. The teeth are differentiated into incisors, canines and molars. The tooth itself consists of such parts as a crown with a working surface, the body of the tooth and its root. The pharynx of animals is short; the windpipe and choanae open into it. Thus, in mammals, the pharynx is the crossroads of two pathways - food and respiratory. The esophagus is a simple, highly extensible muscular tube. After passing through the diaphragm, it connects to the stomach. The stomach looks like a large horseshoe-shaped curved bag that lies across the body. A fat-filled peritoneum hangs from the stomach, which covers all the internal organs with an apron. The liver is located under the diaphragm, its streams open into the duodenum, in the loop of which lies the pancreas. Most mammals have a gallbladder. The intestine can be of various lengths, it depends on the composition of the feed. In a herbivorous rabbit, the intestines are very long - 15-16 times longer than the body. Its divisions are the small, large and rectum. At the beginning of the large intestine in mammals there is an unpaired blind outgrowth - the caecum. The intestine opens to the outside with an independent anal opening.

Respiratory system. The larynx, as usual for mammals, has a cricoid cartilage, in front of which is a large thyroid cartilage. The larynx of a mammal is complex. The vocal cords are stretched on the inside of the larynx. These are paired elastic folds of the mucous membrane, stretched in the cavity of the larynx and limiting the glottis. The lungs are a pair of spongy bodies hanging freely into the chest cavity. Their internal structure is characterized by great complexity. The trachea near the lungs divides into two bronchi. The bronchi, entering the lungs, are divided into secondary bronchi, which, in turn, are divided into bronchi of the third and fourth order. They end in bronchioles. The ends of the bronchioles are swollen and braided with blood vessels. These are the so-called alveoli, where gas exchange takes place.

Circulatory system. The heart of animals, like that of birds, is four-chambered, and the left ventricle drives blood through the systemic circulation and, like that of birds, has much thicker walls than the right one. A large vessel departs from the left ventricle - the aorta, which begins the systemic circulation. Arterial blood is supplied to all organs of the body, and venous blood is collected through the vein system. The largest of them - the posterior and two anterior vena cava - flow into the right atrium. From the right atrium, blood enters the right ventricle, from here the pulmonary circulation begins, or, as it is also called, the pulmonary circulation. Venous blood is ejected from the right ventricle into the great pulmonary artery. This artery divides into right and left, leading to the lungs. From each lung, blood is collected in the pulmonary vein (the blood in it is arterial), both veins merge and flow into the left atrium. Further, from the left atrium, the blood flows into the left ventricle and again goes through the systemic circulation.

Organs, secretions. In mammals, it is a pair of bean-shaped kidneys located in the lumbar region. From the inner concave side of each kidney departs along the ureter (thin tube), which flows directly into the bladder. The bladder opens into the urethra.

Sex organs. In mammals, these are paired testes (in males) or paired ovaries (in females). The testicles have a characteristic oval shape. Adjacent to them are the appendages of the testicles. Paired vas deferens open at the beginning of the urethra. The end parts of the vas deferens are expanded into the seminal vesicles. The paired ovaries of the female have an oval-flattened shape. Near each ovary is an oviduct. At one end, the oviduct opens into the body cavity, and at the opposite end, without a visible border, it passes into the uterus. The uterus in animals is bicornuate, the right and left horns of the uterus open independently into the vagina. It is unpaired. At its posterior end, it gradually passes into the urethra and the bladder opens into it. Outwardly, the vagina opens with the urogenital opening.

Embryo development. Egg cells develop in the ovary, then mature cells, upon exiting the ovary into the body cavity, are caught there by the funnel of the oviduct. Thanks to the flickering movements of the cilia of the tube (oviduct), the egg moves along it, and if the female is fertilized, then in the tube (usually in its first third) the egg merges with the sperm. The fertilized egg continues to slowly descend into the uterus and at the same time its crushing (dividing the egg into many cells) begins. Having reached the uterus, the egg, which by that time has turned into a dense multicellular ball, is introduced into the wall. There, nutrients begin to flow to it. Pretty soon, a placenta forms around the implanted embryo. This is the shell of the fruit, very characteristic of mammals. The placenta is a spongy organ rich in blood vessels, in which children and maternal parts are distinguished. The nursery consists of the villi of the germinal membrane, and the maternal one consists of the wall of the uterus. During childbirth, the muscular layer of the uterus is greatly reduced and the baby placenta (chorion), by that time associated very slightly with the mucous membrane of the uterus, opens and exits along with the newborn in the form of a child's place.

Mammals (animals) are a numerous and diverse class with 5,500 species. A characteristic feature of the class is the unique external structure of mammals, which differs from reptiles and birds.

Main features

Depending on the habitat (mountains, jungles, deserts) and the way they feed (predators, herbivores, scavengers), mammals have a different structure of the skull, limbs, teeth, and ears. However, all animals of the class Mammals are united general signs of external structure :

• warm-bloodedness;
• the location of the limbs under the body;
• wool cover or its modified forms (needles, shell, bristles);
• feeding babies with milk;
• live birth;
• the presence of a large number of glands (milk, sweat, sebaceous, odorous);
• differentiation of teeth into incisors, canines, molars;
• developed soft lips (an exception is the platypus).

The word "mammals" is derived from the Latin word mamma, i.e. "udder", "chest". The ability to feed young with milk is the main distinguishing feature of all mammals.

Rice. 1. Feeding cubs with milk.

Structural features

The body of any member of the class Mammal consists of five departments :

• head;
• torso;
• limbs;
• tail.

Table “External structure of mammals”

Rice. 2. Departments of the body of a mammal.

The coat consists of a soft short undercoat and a long cone-shaped hard awn. The awn may be missing. Some species have lost their woolen cover, but retained vibrissae - naked mole rat, elephant, walrus. Baby cetaceans have tufts of hair, but as they grow older, they completely lose their hair.

Species diversity

Depending on the habitat, the features of the external structure of mammals are clearly expressed. The table describes the types of animals in accordance with the way of life.