EGGS, POUCHES AND PLACENTAS
The platypus: A primitive mammal
The duck-billed platypusOrnithorhynchus anatinus (bird-billed) from Australia is animal belonging to the most primitive order of mammals (Monotremata). This animal is the size of a rabbit, possesses thick fur, webbed and clawed feet, a cloaca combining both excretory and reproductive functions and a large pliable flat beak like a duck's. It lives in the rivers of eastern Australia, swimming using its webbed flat feet and steering with its hind-limbs. When it dives, it closes its ears and tiny eyes with little muscular flaps of skin and hunts for aquaticinvertebrates using its bill, which is rich in nerve endings and very sensitive. It is also a powerful burrower, excavating tunnels up to 18 metres in length through the river banks. These animals roll back the webbing of their fore feet into their palms and this frees the claws for burrowing. Within these tunnels the female constructs an underground nest of grass and reeds and lays two eggs that are nearly spherical, the size of marbles, and soft shelled and therefore similar to a reptile's egg. Since platypuses have fur, they are warm blooded and possess rudimentary mammary glands; they definitely belong to the class Mammalia and is one of only two primitive living mammal families which lays eggs. The female platypus develops on her belly special glands, that are similar in structure to sweat glands but are enlarged and produce a thick rich milk which oozes into the fur. The young platypus suck the fur. This is the beginning of the true mammary gland found in all higher mammals. The other important mammalian feature of endothermy is also incompletely developed and the platypus allows its body temperatures to fluctuate more greatly than other mammals (viz can drop to 30ºC).