|Themes > Science > Zoological Sciences > Animal classification > Mammals|
Mammals alone feed their young with milk from mammary glands. They have distinctive dentition and, typically, hair as well as an internal skeleton, a nerve cord, a three-part brain, and sensory organs inside the skull. Their young typically require an extended period of dependency and learning.
The three classes of mammals are:
Monotremes are mammals of the order Monotremata and include the duck-billed platypus and the spiny anteaters. The name of the order means "single opening" and refers to the fact that in these mammals, as in birds and reptiles, the intestinal tract, the urinary ducts, and the genital ducts all open into one chamber, the cloaca, which has a single opening to the outside.
Monotremes are classified as mammals because they possess certain strictly mammalian characteristics, including milk glands (but not teats or nipples) to nourish their young, warm-bloodedness, a muscular diaphragm separating the lungs from the abdominal cavity, a single bone making up each side of the lower jaw, three middle ear bones (auditory ossicles), and hair. They are unique among mammals, however, because they possess so many reptilian characteristics, such as they lay eggs rather than bear the young alive.
Marsupials are distinguished from other mammals by anatomical, physiological, and developmental differences. One characteristic is the presence of a pouch, or marsupium, which provides continued maternal protection to the undeveloped newborn young. Not all species of marsupials, however, have pouches, and in some it develops only during the nursing period. Further, a non-marsupial, the spiny anteater, also develops a pouch for its nursing young. Pouches range from simple folds of skin to well-developed sacs that can be closed by muscular action. Pouches may open to the front or rear, depending on the species.
Another marsupial characteristic is the female has two vaginas, which open to the outside through a single opening. The two vaginas have not fused to form a single vagina because the tubes (ureters) from the kidneys to the bladder pass between them. Although the vaginas serve as passageways for the sperm to the egg, the young are not born through the vaginal passages. Instead, a new opening develops in the connective tissue between the vaginas and functions as the birth canal. In kangaroos this passageway becomes permanent after the birth of the first young. Male marsupials have forked penises, and the testes are located in a scrotal sac in front of the penis, rather than behind it (except in the marsupial mole, Notoryctes, in which the testes are internal)
Placental mammals are separated on the general basis of the presence or absence of a placenta, a special organ that develops between the mother and the fetus during pregnancy. This spongy tissue in the means by which the baby receives nutrients and oxygen and gets rid of metabolic wastes. Placental mammals grow faster in the uterus than marsupials do in a pouch, and many are fully developed at birth. All living mammals, except the marsupials and egg-laying monotremes, are placental mammals.