Class Mammalia

Class Mammalia
Class Mammalia is a relatively small group of animals that are very well known. There are approx. 4500 species of mammals, with very few left to be discovered. Some discoveries are still possible, as is the case of the cloudrunner, a squirrel-sized mammal discovered in the late 1990's. All mammals have similar characteristics. First off, they all, just like other vertebrates, have a backbone and spinal column. They also have a notochord at some stage of development. All mammals are warm-blooded, meaning they can control their body temperature. They can keep it at a steady 36-38°C. Mammals lack feathers but rather are covered with fur or hairy skin. They all have specialized teeth, with the exception of the Edentates (armadillos), who lack teeth entirely. Mammal bones are solid and filled with either marrow (as in humans) or spongy bone (as in elephants). No mammals are completely cartilaginous. Mammals lack wings, with the exception of Chiroptera (bats), which numbers to 900 species. All female mammals have mammary glands which produce milk for the young. Mammary glands are the best way of defining mammals and is where the term Mammalia came from.
Mammals are divided into 3 sub-classes based on their mode of reproduction:
Subclass Prototheria
Prototheria is composed of egg-laying mammals. There are only 6 species of these and they belong to one order:
Monotremata (platypus and echidna)
Subclass Metatheria
Metatheria numbers to 250 species and is composed of seven orders (previously 1 - Marsupialia). Their young are born tiny and immature and must climb into the mother's pouch, where they grab hold of a teat and do not leave until they are mature.
Didelphimorphia (New World opossums) Paucituberculata (South American rat opossums) Microbiotheria (colocolo) Dasyuromorphia (dasyurids, thylacines) Peramelemorphia (bandicoots) Notoryctemorphia (marsupial moles) Diprotodontia (kangaroos, koalas, wombats, possums)
Subclass Eutheria
The sub-class Eutheria is composed of all the placental mammals, whose young form as embryos in the mother's stomach. Humans are an example of the sub-class Eutheria. Below is a list of all 19 orders in this sub-class:
Insectivora (moles, shrews) Dermoptera (flying lemurs) Chiroptera (bats) Cetacea (whales) Carnivora (cats, bears, dogs, otters, seals, sea lions) Tubulidentata (aardvarks) Proboscidea (elephants) Hyracoidea (hyraxes) Primates (monkeys, lemurs, bushbabies, aye-ayes) Xenarthra or Edentata (armadillos, anteaters, sloths) Pholidota (pangolins) Lagomorpha (rabbits, hares, pikas) Rodentia (mice, rats, squirrels, porcupines, beavers, voles, hamsters) Sirenia (manatees, dugongs) Perissodactyla (horses, donkeys, zebras, rhinoceroses,tapirs) Artiodactyla (pronghorns, deer, camels, gnus, goats, giraffes, hippopotami, pigs, peccaries, chevrotains, musk-deer, cows) Scandentia (tree shrews) Macroscelidea (Elephant Shrews