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An Introduction to Zoology:  Chapter 4
An Introduction to Zoology: Chapter 4
Published by The Lone Ranger
Default Radial Symmetry

Radial Symmetry:
An animal shows radial symmetry if its body can be divided into mirror-image halves in more than one way. Such an animal has a top side and a bottom side, but no front and no back. Neither do radially-symmetrical animals have left and right sides.

Animals that have radial symmetry include members of the phylum Cnidaria, such as sea anemones and “jellyfishes” (properly known as jellies). Some members of the phylum Echinodermata, such as the sea stars (“starfishes”), have a modified form of radial symmetry. Most radially-symmetrical animals move very slowly or not at all. An animal that is more or less permanently attached to some surface and cannot move on its own is said to be sessile, and most animals that are sessile are radially symmetrical.

Radial symmetry may provide some advantages, but it seems to come with a number of disadvantages, too. A radially-symmetrical animal has no front and no back, no right and no left; such animals typically have their sense organs scattered around the body surface instead of concentrated in one region. This gives a radially-symmetrical animal the ability to sense danger (or prey) from any direction. On the other end, since radially-symmetrical animals are almost always either sessile or very slow-moving, it seems that radial symmetry is not compatible with quick and coordinated movement.

A hydra (phylum Cnidaria) has a radially-symmetrical
body. Note that there are many different ways that
its body could be divided into mirror images.


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Corona688 (08-10-2008), curses (08-04-2008), Ensign Steve (08-05-2008), monruw (03-30-2011), Stormlight (08-05-2008)

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