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© Copyright 1997, Jim Loy
Someone once asked me, "Is a spider an animal or an insect?" That's easy to answer, but it's a strange question. A spider is an animal, and it is not an insect. But insects are animals, too. So, the question shows a basic lack of knowledge.
Some people think that "animal" is the same as "mammal". That is incorrect. Animals are all those creatures which move around under their own power, from worms to insects to snakes to humans. Animals are the members of the animal kingdom. Mammals are those animals which have hair, give birth live (most of them), and have mammary (milk) glands for feeding their babies.
Until recently, most one-celled organisms were considered animals, too. But, in some ways, these critters are more different from the rest of the animal kingdom, than plants are. So, one-celled organisms have been put into two separate kingdoms, with bacteria being one of those two kingdoms. And fungi (plural of fungus) have been put into a fifth kingdom, as they are much different from plants.
And viruses are not considered to be alive at all, but are merely bits of DNA or RNA. So, they are not part of any of the five kingdoms.
Comments:
Some animals don't get around much. Sea anemones, sea cucumbers, hydras, sponges, barnacles, and corals, which are stuck to rocks and mud and other organisms, are all animals.
Some of the things that look like animals or plants are actually colonies of organisms. A portuguese man-of-war is not a jellyfish, but a colony of many thousands or smaller organisms. A lichen is a colony of plants and fungi.
Some mammals (platypuses and echidnas) lay eggs. Other mammals have very little hair (dolphins and other whales).