Key Vocab - 12. Characteristics of Animals

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Symmetry

Back

The balanced arrangement of duplicate body parts or shapes on opposite sides of an organism, measured around a central point, line, or axis.

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Asymmetrical

Back

Describing an organism whose body has NO line that can divide it into matching or mirror-image halves. (e.g., sponges)

Front

Radial Symmetry

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A body plan in which similar parts are arranged equally around a central point, so the body can be divided into matching halves by more than one plane — like the spokes of a wheel. (e.g., starfish, jellyfish)

Front

Bilateral Symmetry

Back

BA body plan in which only one plane divides the organism into a mirror-image LEFT

side and RIGHT side. (e.g., humans, worms)

Front

Cephalization

Back

The evolutionary trend in which sensory organs (like eyes and ears) and the nervous system become concentrated at the front end of the body, forming a distinct head region. This is closely linked to bilateral symmetry. The organisms have cephalazations can show complex behaviour.

Front

Coelomate

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An animal that has a true body cavity (body space) that is completely surrounded and lined by a tissue layer called mesoderm. This cavity cushions and protects internal organs. (e.g., earthworms, humans)

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Pseudocoelomate

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An animal that has a body cavity that is only partially lined by mesoderm tissue.

The prefix pseudo- means FALSE, indicating this is NOT TRUE coelom. (e.g., roundworms)

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Acoelomate

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An animal that has NO body cavity at all. The space between the digestive tract and the outer body wall is filled entirely with solid tissue. The prefix a- means

without. (e.g., flatworms)

Front

Segmentation

Back

The organization of an animal's body into a series of repeating sections called segments, where each segment can develop specialized structures to carry out

diMerent functions, giving the animal greater flexibility and mobility. (e.g., earthworms, insects)