Note: Mammalia are divided into three subclasses; -- I.
{Placentalia}. This subclass embraces all the higher
orders, including man. In these the fetus is attached
to the uterus by a placenta. II. {Marsupialia}. In
these no placenta is formed, and the young, which are
born at an early state of development, are carried for
a time attached to the teats, and usually protected by
a marsupial pouch. The opossum, kangaroo, wombat, and
koala are examples. III. {Monotremata}. In this group,
which includes the genera {Echidna} and
{Ornithorhynchus}, the female lays large eggs
resembling those of a bird or lizard, and the young,
which are hatched like those of birds, are nourished by
a watery secretion from the imperfectly developed
mamm[ae].