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| | Mammals (Latin: Mammalia) is a class of vertebrate animals, basic distinctive features of which are viviparous (with an exception of cloaca class) and feeding of babies with milk. Birds, fishes, reptiles and amphibians belong to class of mammals amongst the vertebrates.
In some classifications, mammals and wild animals (Latin: Theria) are studied as identical and in other classifications, wild animals were associated to subclass in class mammals (overwhelming majority of modern kinds belong to this class), referred to sub class Prototheria.
Anatomy
Respiratory system:
Mammals breathe with the help of lungs, which differ with huge branching of branchi. Very thin bronchi — bronchioles. Thin-walled micro sacks are located at the ends of bronchioles (alveoli), which are densely braided with capillaries.
Circulatory system:
Mammals have four chambered heart. It consists of right and left ventricles and also right and left atria. Chambers of heart communicate between themselves and with the great vessels with the help of valves. Heart supplies oxygen and nutrients to the tissues of the body and also frees them from disintegration products.
Excretory system:
In mammals, kidneys possess bean-shape and are located in lumbar region on each side of backbone (vertebra). As a result of blood filtration, urine is formed in kidneys and then, urine flows down along the ureters into urinary bladder. From urinary bladdr, the urine comes out through the urethra.
Nervous system:
Fore-rain and its cortex are especially developed in mammals. Cortex is formed by several layers of bodies of nervous cells and covers the entire forebrain. It forms folds and convulsions with deep furrows in majority of kinds of mammals. The more folds and convulsions, the more complex and variety would be the behavior of an animal. Peripheral nervous system is also very well developed in mammals, which provides the mammals with high-speed reflexes.
Digestive system:
After taken into mouth, the food is chewed by teeth. Then, the food is wetted with saliva, released along the ducts from salivary glands. It facilitates in swallowing and movement of food along the esophagus. Under the influence of saliva, complex carbohydrates (starch, sugar), contained in food, turn to less complex. Salivary glands are well developed in herbivores. Cow, for example, releases 60 liters of saliva in a day. In majority of animals, saliva possesses pronounced antiseptic properties.
Single chambered stomach is found in majority of mammals. Glands are present on the walls of stomach, which release digestive juice. But, stomach is multi chambered in herbivore mammals, such as deer, cow, goat, sheep etc.
Intestines are divided into small and large. Duodenal, jejunum and ileum belong to small intestines. Blind gut, colon and rectum belong to large intestines.
Food is digested in small intestines under the influence of juices. Juices are released by glands, present on the walls of guts and also liver and pancreas, opening in initial section of small intestines - duodenum. In small intestines, all the food nutrients are sucked into blood and the rest of undigested food enters into large intestines.
Ileocecal valve is present at the joint of small and large intestines and it prevents the backward advancement of formed fecal substances into small intestines. Changes in hard-to-digest food substances take place in blind gut under the influence of bacteria. Also,in majority of mammals, considerable quantity of lymphatic tissues is present on the walls of blind gut which makes the blind gut an important body of immune system. In many animals (for example, rabbits, beavers), the blind gut possesses huge dimensions. In some animals, blind gut is found with an appendix.
Origin:
Mammals, apparently, originated from cynodonts at the end of Triassic period. The first mammals were single-pass, retaining some resemblance to reptiles (some kinds exist till now) and multitubercular (bacome extinct at the end of Cretaceous period) and triconodonts (also become completely extinct in Cretaceous period) appeared a little later. Pantotheria appeared in Jurassic period ( also become extinct at the end of Cretaceous period), from which the present animals evolved, i.e viviparous mammals.
Interesting is the occurrence of first dinosaurs — archosaurus, in due time, was marked with mass (but not completely) extinction of terapcyds, highest form according to the its organisation were very similar to single-pass mammals and according to some assumptions, probably had mammary glands and wool.
Inhabitation:
Mammals are prevalant worldwide and are met on all continents, in all oceans and on majority of existing islands. Natual habitat of koala is limited to Australia and New Guinea, marsupials — space of Australia, Oceania and both American continents. Wide prevalence of placentary mammals, which as on today are present on all the continents, notably bats and mice inhabited in Australia before the arrival of Europeans. Only limited fauna was found on remote islands before the inhabitation of human being and only bats were found in many of the islands including New Zealand.
Mammals are found in almost all biospheres of the Earth and are found both in deserts and tropical forests and also in high mountains and in polar regions. Pacific Ocean and interiors of Antarctica belong to few regions and biospheres, in which mammals are not found (except for rare inhabitation of people).
Examples:
The following are the frequently encountered animals, which belong to mammals: Cats, dogs, cows, bears, wolf, hares and also human beings. |
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This is the fifth contest in the series of contests "Best of Freaking News pictures of 2007". Today's category is the "best animal picture". All the entries were nominated by the Freaking News staff, so this contest is for voting only. Authors, please feel free to edit your authors comments, but please don't change the titles or otherwise edit the entries. 
