The class Mammals has about 4,000 species. Representatives of the class reached the most progressive development in the process of evolution and are distributed almost everywhere, with the exception of the Antarctic continent. They inhabit a wide variety of living environments.
The external structure of mammals is determined by the conditions of the habitat and it is very diverse. Mammals include creatures as diverse as mice and fish-like huge whales, graceful tall giraffes and obese hippos, horn-covered armadillos, and spiny echidnas and hedgehogs.
The body of a mammal is divided into a head, neck, torso, paired limbs and a tail.
The head in mammals is usually mobile. On it are the mouth, nostrils, eyes, outer ears (auricles).
The mouth has upper and lower lips. Lips are needed for capturing food, and in cubs for sucking milk. Above the mouth is an elongated nose with two nostrils.
The eyes are protected by eyelids and eyelashes. The blinking membrane (third eyelid) is underdeveloped in mammals.
The most common type of terrestrial mammals, whose limbs are located under the body (and not on the sides of the body, as in reptiles). Due to this, mammals raise their bodies quite high above the ground.
The skin of mammals is involved in thermoregulation of the body. The skin is abundantly permeated with blood vessels and is supplied with blood. Heat transfer increases with vasodilatation or decreases with vasoconstriction.
Various glands are located in the skin of mammals:
- sebaceous glands – secrete sebum, which lubricates the skin and hair, gives them elasticity and prevents moisture;
- sweat glands – secrete sweat, which cools the body and excretes harmful substances from the body;
- odor glands – needed to scare off enemies, attract a female and mark their territory;
- lactiferous (mammary) glands – needed for feeding the cubs with milk.
Horny derivatives of the epidermis of the skin are hair, nails, claws, hooves, “hollow” horns (but! – the horns of deer consist of bone substance!).
The hairline consists of different types of hair:
- spine (guard hair) – long, thin, but strong hair.
- undercoat – short and fluffy hair, located under the guard hair.
- vibrissae – hard and long hair that form eyebrows and mustache (serve as the organ of touch).
In some animals, the coat consists of an awn (deer); for others – from the undercoat (mole); in the third – from awn and undercoat (nutria, marten).
All animals shed periodically. At the same time, the density of their fur changes (winter fur has more undercoat), and in some, the color (the common squirrel is silvery-gray in winter, and red in summer).
There are animals whose guard hairs are modified into needles (hedgehogs, porcupines).
Hair loss in mammals may be related to adaptation to aquatic life (in Cetaceans and Pinnipeds) or to existence in hot climates (in elephants).