The
duck-billed platypus (Ornithorhynchus
anatinus) and the echidna or spiny anteater (family Tachyglossidae), indigenous
to Australia, Tasmania, and New Guinea, are the only two species of mammals
that lay eggs (a non-mammalian feature) but suckle their
young (a mammalian feature). These mammals (order Monotremata) resemble
reptiles in that they lay rubbery shell-covered eggs that are incubated and hatched
outside the mother's body. In addition they
resemble reptiles in their digestive, reproductive, and excretory systems, and
in a number of anatomical details (eye structure, presence of certain skull
bones, pectoral [shoulder] girdle, and rib and vertebral structures). However
they are classed as mammals because they have fur and a four-chambered heart,
nurse their young from gland milk, are warm-blooded, and have some mammalian skeletal
features. (The
Handy Science Answer Book, compiled by the Science and Technology department of
the Carnegie Library of Pittsburgh)
