Mammals (Mammalia) are a class of vertebrate animals whose main distinguishing features are live births (except for the cloaca infraclass) and milk-feeding of their young. There are about 4500 species of mammals in the world.

Apart from live births and nursing, mammals are characterized by a number of signs; some of them are also found in other vertebrate groups, some are not characteristic of every mammal species, and only a few such signs are unique. Such features include:

The presence of hair (fur), sweat and sebaceous glands
A special type of brain structure (including a powerful formation of the final brain, taking over the functions of the main visual centre and the centre controlling difficult behaviours)
The presence of three auditory ossicles of the middle ear, the external ear canal and the auricle
Seven vertebrae in the cervical spine
Thermal blood
Four-chambered heart. One (left) aortic arch
Alveolar structure of the lungs.
Teeth seated in the alveoli of the jaws; heterodontics (dentition)
Nuclear-free erythrocytes
In mammals the spine is divided into five divisions: cervical, thoracic, lumbar, sacral and caudal. Only cetaceans have no sacrum. The cervical spine is approximately invariably composed of seven vertebrae. Thoracic consists of 10-24 vertebrae, lumbar consists of 2-9 vertebrae, and sacral consists of 1-9 vertebrae. Only in the caudal division their number varies powerfully: from 4 (in some monkeys and humans) to 46.

The true ribs articulate only with the thoracic vertebrae (rudimentary may be on other vertebrae). In front, they are joined by the breastbone to form the rib cage. The shoulder girdle consists of 2 shoulder blades and 2 clavicles. Some mammals do not have clavicles (ungulates), others are poorly developed or replaced by ligaments (rodents, some carnivores).

The pelvis consists of three pairs of bones: the iliac, pubic and sciatic bones, which are tightly fused together. Cetaceans do not have a true pelvis.

The forelimbs are used by mammals for locomotion on land, swimming, flight and grasping. The humerus is powerfully shortened. The ulna is weaker than the radius and serves to articulate the hand with the shoulder. The forearm is made up of the wrist, metacarpal and fingers. The wrist consists of 7 bones arranged in two rows. The number of bones in the metacarpal corresponds to the number of fingers (no more than five). The thumb consists of 2 joints and the others 3. Cetaceans have a greater number of joints.

In the hind limbs, the femur in most mammals is shorter than the tibia.

Respiratory system of mammals consists of larynx and lungs. The lungs are distinguished by a huge branching of bronchi. The thinnest of these are the bronchioles. At the ends of the bronchioles are thin-walled vesicles (alveoli) densely braided with capillaries. The diaphragm is a characteristic anatomical sign of mammals. It plays a significant role in the respiratory process.

Kidneys of mammals are bean-shaped and located in the lumbar region, on the sides of the spine. The kidneys, as a result of blood filtration, produce urine, after which it flows through the ureters into the bladder. From there, the urine exits through the urethra.

In mammals, the forebrain and cerebellum are particularly developed. The cerebral cortex is formed by several layers of nerve cell bodies and covers each forebrain. It forms folds and convolutions with large furrows in most mammalian species. The greater the number of folds and gyrus, the more challenging and varied the behaviour of an animal. The peripheral nervous system of mammals is also well developed, which provides them with the highest reflex speed. The sensory organs include: the visual organs, the hearing organs, the olfactory organs. Visual organs are of great importance in life of mammals. In contrast to birds, each eye of which sees objects separately, mammals have binocular vision. The organs of hearing include the external auditory canal and the auricle. The organs of smell are located in the anterior and posterior regions of the nasal cavity.

The digestive system of mammals is the gastrointestinal tract, a tube connecting the mouth to the anal opening. The digestive system includes the oral cavity, salivary glands, pharynx, esophagus, stomach, intestines, and anus.

Most mammals (other than monotremes, some cetaceans, lizards and anteaters) have teeth. They are located in the cells of the jaw bones. Four types of teeth are distinguished: incisors, canines, false roots and true radicals.

After entering the oral cavity, food is chewed with the teeth. The food is then moistened with saliva through the ducts of the salivary glands. This makes it easier to swallow and move down the oesophagus. Under the influence of saliva, difficult carbohydrates (starch, sugar) contained in food are converted into less difficult ones. Salivary glands are powerfully developed in herbivorous animals. A cow, for example, excretes 60 liters of saliva per day. Most animals have saliva with pronounced antiseptic properties.
In the small intestine, food is digested under the influence of digestive juices. These are secreted by glands in the intestinal wall and by the liver and pancreas, which open into the original small intestine, the duodenum. Nutrients in the small intestine are absorbed into the blood and the remains of undigested food enter the large intestine.

At the junction of the small intestine and large intestine there is an ileocecal valve which prevents the feces being formed to flow back into the small intestine. In the cecum, the metamorphosis of difficult-to-digest food substances takes place under the influence of bacteria. Also in most mammals there is a huge amount of lymphatic tissue in the wall of the cecum, which makes it a significant organ of the immune system. Many animals (e.g. rabbits, beavers) have a large cecum. Some animals have an appendix. In the colon, the faeces dehydrate, accumulate in the rectum and are then removed to the outside through the anus.