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Classifying Living Things: The Animal Kingdom
What features distinguish animals from
other life forms?
Like plants, animals are eukaryotic
organisms. They are multicellular with cells that contain nuclei and
many different organelles that carry out various cell functions. Unlike
plants, animals cannot produce their own food. They obtain food by
ingestion — they take it in whole or in small chunks. Animals
may eat plants, or other animals. Some animals, such as humans and
bears, have a mixed diet of plant and animal food sources.
Animal cells also lack cell walls. Their cells are simply enclosed
by a cell membrane. Thus, they lack the structural rigidity seen in
plant cells. However, animals have a startling diversity of shapes
and sizes that are supported by different types of skeletons.
Frog (inset) and frog cells.
Roll your mouse over the word "nuclei" to
highlight this structure.
How are animals important
to people?
The immense diversity of animals and their importance
on this planet is truly remarkable. Of course, animals are a food source.
But animals play many roles in their environments. Animals with a backbone,
known as vertebrates, represent those species best known by children.
The majestic creatures of the African bush and the brightly colored
fish of a coral reef are well-recognized examples of vertebrates. However,
the diversity of invertebrates, which are animals that lack a backbone
and are typically structurally less complex, is equally, if not more,
amazing. From the insects of the terrestrial world to the untold wealth
of life in the sea, millions of species exist that possess bizarre
and novel adaptations for success in their respective environments.
These adaptations are based on avoiding predators, attracting prey,
and mating.
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Invertebrate (earthworm)
Lacks a backbone |
Vertebrate (fish)
Has a backbone
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