While the term "animal" is commonly used in speaking of our four-footed
wildlife, it is best to record with more complete accuracy that
"animals" include any living thing having sensation and the power of
voluntary movement. This would therefore admit a great variety of
creatures such as one-celled protozoa, worms, fish, frogs, snakes,
birds, and finally the four-footed animals mentioned--mammals. Mammals
are set apart as a special group of animals for two reasons: they have
some sort of hair covering on their bodies and the females are equipped
with mammary (milk) glands for nursing their young, features which none
of the other "animals" possess.