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| | Mammals (Latin: Mammalia) is a class of vertebrate animals, the basic distinctive features of which they are viviparous (with an exception of cloaca class) and feed their babies with milk. Birds, fish, reptiles and amphibians belong to the 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 a subclass in the class of mammals (the overwhelming majority of modern kinds belong to this class), referred to as the sub class Prototheria.
Anatomy:
Respiratory system:
Mammals breathe with the help of lungs, which differ with the huge branching of the branchi. Very thin bronchi, bronchioles, have thin-walled micro sacks that are located at the ends of bronchioles (alveoli), which are densely braided with capillaries.
Circulatory system:
Mammals have a four chambered heart. It consists of the right and left ventricles and also the right and left atria. The chambers of the heart communicate between themselves and with the great vessels with the help of valves. The heart supplies oxygen and nutrients to the tissues of the body and also frees them from disintegration products.
Excretory system:
In mammals, the kidneys possess a bean-shape look and are located in the lumbar region on each side of the backbone (vertebra). As a result of blood filtration, urine is formed in the kidneys and then, urine flows down along the ureters into the urinary bladder. From the urinary bladder, the urine comes out through the urethra.
Nervous system:
The forebrain and its cortex are especially developed in mammals. The 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 the majority of mammals. The more folds and convulsions, the more complex and the variety would be of the behavior of an animal. The peripheral nervous system is also very well developed in mammals, which provides the mammals with high-speed reflexes.
Digestive system:
After food is taken into the mouth, the food is chewed by the teeth. Then, the food is wetted with saliva, released along the ducts from salivary glands. It facilitates in swallowing and the movement of food along the esophagus. Under the influence of saliva, the complex carbohydrates (starch, sugar), contained in food, turn into less complex carbohydrates. The salivary glands are well developed in herbivores. The cow for example, releases 60 liters of saliva in a day. In the majority of animals, saliva possesses pronounced antiseptic properties.
A single chambered stomach is found in the majority of mammals. Glands are present on the walls of the stomach which release digestive juice. But, the stomach is multi chambered in herbivore mammals, such as the deer, cow, goat, sheep etc.
The intestines are divided into small and large. The duodenal, jejunum and ileum belong to the small intestines. The blind gut, colon and rectum belong to the large intestines.
Food is digested in the small intestines under the influence of juices. Juices are released by glands, present on the walls of the gut and also the liver and pancreas, opening in the initial section of the small intestines - duodenum. In small intestines, all the food nutrients are sucked into the blood and the rest of the undigested food enters into the large intestines.
The ileocecal valve is present at the joint of the small and large intestines and it prevents the backward advancement of formed fecal substances into the small intestines. Changes in hard-to-digest food substances take place in the blind gut under the influence of bacteria. Also, in the majority of mammals, a considerable quantity of lymphatic tissue is present on the walls of the blind gut which makes the blind gut an important body of the immune system. In many animals (for example, rabbits, beavers), the blind gut possesses huge dimensions. In some animals, the blind gut is found with an appendix.
Origin:
Mammals, apparently, originated from cynodonts at the end of the Triassic period. The first mammals were single-pass, retaining some resemblance to reptiles (some kinds exist till now) and multitubercular (they became extinct at the end of the Cretaceous period) and triconodonts (also become completely extinct in the Cretaceous period) appeared a little later. Pantotheria appeared in the 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 the first dinosaurs — archosaurus, in due time, was marked with the mass (but not completely) extinction of terapcyds, the highest form according to its organization were very similar to single-pass mammals and according to some assumptions, probably had mammary glands and wool.
Inhabitation:
Mammals are prevalent worldwide and are met on all continents, in all oceans and on the majority of existing islands. The natural habitat of the koala is limited to Australia and New Guinea, marsupials — space of Australia, Oceania and both American continents. The wide prevalence of placentary mammals, which as of today are present on all the continents, notably bats and mice, inhabited Australia before the arrival of Europeans. Only limited fauna was found on the remote islands before the inhabitation of human beings 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. The Pacific Ocean and interiors of Antarctica belong to a few regions and biospheres in which mammals are not found (except for the 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. Tag funny pix 
