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Within the great phylum of vertebrate animals, mammals are distinguished by their young being nourished for some time after birth on milk from the mothers mammary glands. The possession of mammary glands gives rise to the name mammal.
Other characteristics are; Hair covering the body, jawbone consisting of only one bone on each side, other animals have more, middle ear bones are three in number where other animals have less, Occipital condyles, the joint between the head and vertebra consists of two rounded knobs as opposed to one in other animals except amphibians, a diaphragm separating the heart and lungs from the abdominal cavity, the largest part of the brain known as the fore brain being greatly enlarged and with this enlargement comes the development of intelligence. There is also an increased complexity in the cross connections or commissures between the left and right halves with the diagnostic feature being the presence of the corpora quadrigemina, in the mid brain, connected with the functions of sight and hearing, finally the main artery in mammals leaving the heart curves round on the left side to form the aortic arch, in birds it curves to the right and in other vertebrates one or more curves are present on both sides.
The class Mammalia is divided into 3 subclasses, one of which is extinct the other two are Prototheria (containing egg laying mammals or monotremata) and Theria.
The subclass Theria is then split again into two infraclass‘s consisting of Marsupialia (marsupials) and Placentalia (mammals that gestate with a placenta) which contains the vast majority of living mammals divided into 16 orders, of which only 8 are present in Britain and Ireland. The eight orders are; Insectivora, Chiroptera, Carnivora, Lagomorpha, Rodentia, Ungulata, Cetacea(whales) and primates represented by man.