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Mammals. Read and translate the text.
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Семейство ХОМЯКОВЫЕ (Cricetidae)
Семейство хомяковых самое большое и смешанное. В семейство объединяют более 580 видов и около 100 родов. Размеры этих животных колеблются от очень мелких до средних: длина тела от —6 до 35—50 см, масса от 7—8 г до 3 кг. Хвост — от едва заметного до превышающего длину тела. Он покрыт или роговыми чешуйками с примесью редких волос, или волосами разной густоты и длины, иногда с кисточкой на конце. Ноги либо нормального «бегающего» (наземного) типа, либо приспособленные к передвижению прыжками, либо к рытью земли (лапы с мощными когтями), либо к плаванию (перепонки между пальцами).
Mammal - any member of the class Mammalia, a group of vertebrate animals in which the young are nourished with milk from special secreting glands of the mother. In addition to these characteristic glands, mammals are distinguished by several other unique features. Hair is a typical mammalian feature. The mammalian lower jaw is hinged directly to the skull, instead of through a separate bone as in all other vertebrates. A chain of three tiny bones transmits sound waves across the middle ear. A muscular diaphragm separates the heart and the lungs from the abdominal cavity. Only the left aortic arch of the primitive pair persists (in birds the right arch persists; in reptiles, amphibians, and fishes both arches are retained). Mature red blood cells in all mammals lack a nucleus; all other vertebrates have nucleated red blood cells.
Except for the monotremes (echidnas and duck-billed platypuses), which lay eggs, all mammals are viviparous (i.e. bear live young). In the placental mammals (including humans) the young are carried within the mother's womb, reaching a relatively advanced stage of development before being born. In the marsupials (kangaroos, opossums, and allies) the newborn, incompletely developed at birth, continue to develop outside the womb, attaching themselves to the female's body in the area of her mammary glands.
The class Mammalia is worldwide in distribution. It has been said that mammals have a wider distribution and are more adaptable than any other single class of animals, with the exception of certain less complex forms such as the arachnids and insects. This versatility in exploiting the Earth is attributed in large part to the ability of mammals to regulate their body temperatures and internal environment both in excessive heat and aridity and in severe cold.
The evolution of the Mammalia has produced tremendous diversity in form and habits. Living kinds range in size from a bat weighing less than a gram, to the largest animal that has ever lived, the blue whale, which reaches a length of more than 30 meters and a weight of 136,000kilograms . Every major habitat has been invaded by mammals that swim, fly, run, burrow, glide, or climb.
There are approximately 4,625 species of living mammals, arranged in about 125 families and 24 orders. The rodents (order Rodentia) are the most numerous of existing mammals, both in number of species and number of individuals, and are one of the most diverse of living lineages.
Wild and domesticated mammals are so interlocked with our political and social history that it is impractical to attempt to assess the relationship in precise economic terms. Throughout our own evolution, for example, humans have been dependent on other mammals for food and clothing. Domestication of mammals helped to provide a source of protein for ever-increasing human populations and provided means of transportation and heavy work as well. Today, domesticated strains of the house mouse, European rabbit, guinea pig, hamster, gerbil and other species provide much-needed laboratory subjects for the study of human-related physiology, psychology, and a variety of diseases from dental caries to cancer. The study of nonhuman primates (monkeys and apes) has opened broad, new areas of research relevant to human welfare. The care of domestic and captive mammals is, of course, the basis for the practice of veterinary medicine.
Wild mammals are a major source of food in some parts of the world, and many different kinds (from fruit bats and armadillos to whales) regularly are captured and eaten. In addition, hunting, primarily for sport, of various rodents, lagomorphs, carnivores, and ungulates is a multibillion-dollar enterprise. In the United States alone, for example, it is estimated that more than 2,000,000 deer are harvested annually by licensed hunters.
Aside from pelts and meat, special parts of some mammals regularly have been sought for their special attributes. The horns of rhinoceroses are used in the Orient; ivory from elephants and walruses is highly prized; and ambergris, a substance regurgitated by sperm whales, was once widely used as a base for perfumes.
Some mammals are directly detrimental to human activities. Murid rodents (house rats and mice of Old World origin) now occur virtually throughout the world and each year cause substantial damage and economic loss. Herbivorous mammals may eat or trample crops and compete with livestock for food, and native carnivores sometimes prey on domestic herds. Large sums are spent annually to control populations of “undesirable” wild mammals, a practice long deplored by conservationists. In addition to their impact on food resources, mammals are important reservoirs or agents of transmission of a variety of diseases that afflict man, such as plague, tularemia, yellow fever, rabies, leptospirosis, hemorrhagic fever, Lyme disease, and Rocky Mountain spotted fever. The annual “economic debt” resulting from mammal-borne diseases that affect humans and domestic animals is incalculable.
Many large mammals have been extinct entirely or exist today only in parks and zoos; others are in danger of extinction. Perhaps at least some can be saved. One of the most noteworthy cases of direct man interference is the Steller's sea cow (Hydrodamalis gigas). These large (up to 12 meters, or 40 feet, long), inoffensive, marine mammals evidently lived in Recent times only along the coasts and shallow bays of the Commander Islands in the Bering Sea. Discovered in 1741, they were easily killed by Russian sealers and traders for food, their meat being highly prized, and the last known live individual was taken in 1768.