Definition of "bilateral symmetry" []

• Symmetrical arrangement, as of an organism or a body part, along a central axis, so that the body is divided into equivalent right and left halves by only one plane. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "For instance, there was a tendency toward bilateral symmetry in the animal kingdom at a fairly early period, and individual echinoderms attained it; but it was not general until the era of the worms."