Placentals
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The third subclass of mammals, called placentals, includes about 4300 species, making it by far the largest of all three mammal groups. Unlike young marsupials, young placental mammals spend a relatively long time developing inside their mother’s body before birth. Warm and protected within the mother’s womb, the unborn young are nourished by a spongy organ called the placenta, which absorbs nutrients from the mother's blood and transfers them to the developing animal. By the time a young placental mammal is born it is usually fully formed, although it may not yet have fur or functioning eyes or teeth.
Biologists classify placental mammals into about 19 groups called orders (the exact number varies in different classification systems). The largest group, with about 1500 species, contains the rodents, such as rats, mice, squirrels, and porcupines. Animals with sharp, chisel-like front teeth that grow throughout life, rodents use these teeth to gnaw into their food, and also to cut through any obstacles in their path. Another major group of mammals, with about 1000 species, contains the bats. Insect-eating bats are generally small animals, but some fruit-eating species have a wingspan of over 1.5 m (5 ft).
See Pictures of Mammals.
The third subclass of mammals, called placentals, includes about 4300 species, making it by far the largest of all three mammal groups. Unlike young marsupials, young placental mammals spend a relatively long time developing inside their mother’s body before birth. Warm and protected within the mother’s womb, the unborn young are nourished by a spongy organ called the placenta, which absorbs nutrients from the mother's blood and transfers them to the developing animal. By the time a young placental mammal is born it is usually fully formed, although it may not yet have fur or functioning eyes or teeth.
Biologists classify placental mammals into about 19 groups called orders (the exact number varies in different classification systems). The largest group, with about 1500 species, contains the rodents, such as rats, mice, squirrels, and porcupines. Animals with sharp, chisel-like front teeth that grow throughout life, rodents use these teeth to gnaw into their food, and also to cut through any obstacles in their path. Another major group of mammals, with about 1000 species, contains the bats. Insect-eating bats are generally small animals, but some fruit-eating species have a wingspan of over 1.5 m (5 ft).
See Pictures of Mammals.
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