Skin covering is feathers, but the feet are covered by scales
Breathe with lungs
The nervous, digestive (toothless), circulatory (with 4 chambered heart), skeletal (light strong bones), muscular and excretory (no sweat glands) systems are similar to higher chordates.
Constant body temperature (homoiothermic, endothermic)
Skin covering is hair or fur
The young are nourished with milk from mammary glands of the mother.
Complex nervous, digestive, circulatory, respiratory (lungs and muscular diaphragm) and excretory systems
Aquatic mammals have no sweat glands.
Most have 2 pairs of appendages, usually legs, although some have modified appendages for swimming (e.g. seal) or flying (e.g. bat).
Sexual reproduction, internal fertilisation
Monotremes - produce leathery-shelled eggs
Marsupials - have a gestation period of about 10 to 15 days; the young crawls into the mother's pouch after birth, where most of its early development continues
Placentals - have a long gestation period; the young receives its nourishment through blood vessels inside an umbilical cord between the mother's placenta on the wall of the womb and the baby
Did You Know That...?
Bulls cannot see red. They are colour-blind. In bullfights, they chase the matadors' capes because they are moving and not because of the colour.
A platypus' bill can detect the weak electrical fields given out from the muscles and nerves of a shrimp which is its prey.