Snakes and birds lay eggs and hatch baby snakes and birds out of their shells; they are oviparous. Rabbits and puppies, on the other hand, are born directly from their mothers and grow up on their mothers' milk; they are medium mammals. However, the platypus, which lives along the creeks, rivers and lakes of eastern Australia, is different from both ovoviviparous and mammalian animals and is very strange. It is both oviparous and yet feeds its young by nursing. Platypus body length of 50 centimeters or so, covered with fine and dense soft short fluffy hair, shiny, not afraid of water, flat mouth, flat and hard and duck's the same as its feeling than the cat's whiskers are also sensitive, can accurately catch the silt in the underwater small animals. Platypus has no ears, swimming and diving on the ear holes and eyes on the muscle folds on the ear holes and eyes tightly covered, so that the water can not enter. In addition it has a flat tail about 15 centimeters long, can play the role of the rudder; four feet of the toes in the middle of the webbing, can be extended and retracted, swimming can be stretched out of the claws; claws sharp, male beasts hind feet on the big toe born can secrete venom structure. These characteristics indicate that the platypus is very suitable for life in the water.

The platypus spends most of its time in the water, but it also digs holes for nesting. One hole is dug under a tree near the water on the shore, and the eggs are incubated in the hole with crushed platypus while the female echidna incubates the eggs in a brood pouch of crushed grass to cover the camouflage; the other hole is dug underwater. The nest is lined with grass and leaves.

Platypus mating in the water, half a month later, the output of two white, only 1.7 centimeters in diameter of the soft eggs, and by this time the embryo has been in the mother's body for about 15 days of development, which is different from both reptiles and birds. After the eggs are laid, the mother begins to stay in the nest to incubate the eggs, and seldom moves. 10 days or so, the young break out of their shells, and the newborn fetus is very small, with no hair on its body, its eyes are not open, and its small beak is not a "duck's beak". Ten days later, the young beast began to eat milk, the mother lying on her back, the abdomen of the low concave oozing milk, the young beast crawling in the mother's abdomen to lick the milk, because there is no nipple, it is eaten while the mother's abdomen with a small mouth pressure on the breast area, to get enough milk. After more than 120 days, the young beast is fully developed, gradually do not eat milk, the night out of the nest swimming, foraging, began to live independently.