William Neuguth, age 12, of Coventry, R.I., for his question:

IS THE PLATYPUS A MAMMAL OR A MARSUPIAL?

The furry mammals are warmblooded, air breathing animals that give birth to live babies and feed them on mother's milk. A few variations are permitted, but every female who feeds her offspring on mother's milk qualifies as a genuine mammal. The kangaroo, the platypus and all the other pouched marsupials who do this are classed as mammals.

Scientists base their animal classifications on important features such as skin covering, bones and teeth, if any. The list includes the life cycle and the all important happening of birth. For example, all the feathery, toothless birds lay eggs. So do the clammy amphibians, the scaly fishes and most of the scaly reptiles.

In these animals, the fertilized egg cell develops inside a shell  and hatches when ready. Among the more advanced mammals, the fertilized egg develops through its embryo stages during a so called gestation period inside the mother's body and comes forth as a live baby. If the infant is nursed on mother's milk, he or she is a qualified mammal.

The marsupials are pouched mammals who give birth to premature, undeveloped infants. The helpless little creatures transfer to a pouch on the female's tummy. There they feed on mother's milk, while they progress through further stages of development.

This was known ages ago, when experts were sure that no qualified mammal lays eggs. Then the impossible platypus was discovered, living around the streams of Australia. He has stubby webbed feet, a thick, furry coat and a rubbery snout like a duck's bill. Thus far, his features qualify him as a mammal. Then it was discovered that the female duckbill platypus lays eggs.

Surely this should disqualify her as a genuine mammal. But not at all  because the helpless hatchlings are nursed on mother's milk. About two weeks after mating, the female lays a couple of small, soft shelled eggs in a deep burrow. She incubates them for seven to 10 days. Then the blind, helpless infants transfer to her marsupial pouch. There they nurse on mother's milk, which oozes through special pores in her skin.

Hence the fantastic platypus must be classed among the sophisticated mammals. And since the female has a built in baby carrying pouch, she also must be classified as a marsupial. We now know that she is not the only such oddity in the animal world. The echidna, alias the spiny anteater, also is an egg¬ laying mammal marsupial.