The bat Eptesicus fuscus. Dr. Lloyd Glenn Ingles, © 1999 California Academy of Sciences
Black Rat. Photo © 2004 Larry Jon Friesen.
are a rather diverse group, with nearly 4000 described species, mostly
rodents and bats (photos at left). The placental mammals include such diverse forms as
whales, elephants, shrews, and armadillos. They are also some of the
most familiar organisms to us, including pets such as dogs and cats, as well
as many farm and work animals, such as sheep, cattle, and horses. And
humans, of course, are also placental mammals.
Placental mammals all bear live young, which are nourished before birth
in the mother's uterus through a specialized embryonic organ attached
to the uterus wall, the placenta. The placenta is derived from
the same membranes that surround the embryos in the
eggs of reptiles, birds, and monotreme.
mammals. The term "placental mammals" is somewhat of a misnomer because
also have placentae. The difference is that the placenta of marsupials
is very short-lived and does not make as much of a contribution to fetal
nourishment as it does in eutherians, as "placental mammals" are