Monotremes
These primitive mammals produce a shelled egg like their reptilian ancestors. Only three species exist today: two species of spiny anteater (echidna) and the duckbill platypus. [View (48K)]
Marsupials ...
monotreme -- n. A mammal that lays eggs rather than giving live birth. Though laying eggs is a primitive reptilian trait, monotremes share many morphological, physiological, and reproductive characteristics with other mammals.
monotreme
[Gk. monos, single + trema, hole]
An egg-laying mammal, represented by the platypus and echidna.
morphogen ...
monotremes Egg-laying mammals; e.g., the spiny anteater and the duck-billed platypus.
morph A distinct phenotypic variant within a population.
[edit] Monotremes
Monotremes, only five species of which exist, all from Australia and New Guinea, lay eggs. They have one opening for excretion and reproduction called the cloaca.
Subclass Prototheria: Order Monotremata: Monotremes (typified by the platypus and echinda) lay eggs that have similar membranes and structure to reptilian eggs. Females burrow in ground and incubates their eggs.
There are many more marsupials than monotremes. Kangaroos, koalas, bandicoots, and possums. You'll find a lot of them in Australia. Australia is an island continent. Because of its isolation, placental mammals didn't take hold in their ecosystems.
A subclass of the Class Mammalia (others are monotremes and marsupials). Embryo and fetus are nourished by a placenta.
For example, the monotremes are mammals that lay eggs and the marsupials are mammals that give birth to immature young which remain in a special pouch which has mammary glands within it.
 See also: Mammals, Monotremes, Animals, Animal, Organ
  
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