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Life Science: Session 2

Classifying Living Things: The Animal Kingdom

What features distinguish animals from other life forms?

frog and frog cells
Frog (inset) and frog cells. Roll your mouse over the
word "nuclei" to highlight this structure.

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.

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.

earthworm
fish
Invertebrate (earthworm)
Lacks a backbone
Vertebrate (fish)
Has a backbone
prev: the plant kingdom next: the fungus kingdom



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