Storer, Tracy I. Department of Zoology, University of California, Davis, California.
Last reviewed:October 2019
- Related Primary Literature
- Additional Reading
The symmetrical disposition of organs and other constituent parts of the bodies of living animals with respect to imaginary axes. Animal symmetry relates the organization of parts in animal bodies to the geometrical design that each type suggests. The term asymmetrical applies to most sponges and some protozoa because the body lacks a definite form or geometry, and it cannot be subdivided into like portions by one or more planes. Spherical symmetry is exhibited by some protozoa, including the Heliozoia and Radiolaria. The body is spherical, with its parts concentrically around, or radiating from, a central point. Radial symmetry is exemplified by the echinoderms and most cnidarians. The body is structurally a cylinder, tall or short, having a central axis named the longitudinal, anteroposterior, or oral-aboral axis (see illustration). Any plane through this axis divides the animal into like halves. Often several planes, from the axis outward, can divide the body into a number of like portions, or antimeres, with five being most common in echinoderms. Ctenophores and many sea anemones and corals possess biradial symmetry, which is basically radial but with some parts arranged on one plane through the central axis. Most animals have bilateral, or two-sided, symmetry, in which a median or sagittal plane divides the body into equivalent right and left halves, with each being a mirror image of the other. See also: Animal; Bilateria
The content above is only an excerpt.
for your institution. Subscribe
To learn more about subscribing to AccessScience, or to request a no-risk trial of this award-winning scientific reference for your institution, fill in your information and a member of our Sales Team will contact you as soon as possible.
to your librarian. Recommend
Let your librarian know about the award-winning gateway to the most trustworthy and accurate scientific information.
AccessScience provides the most accurate and trustworthy scientific information available.
Recognized as an award-winning gateway to scientific knowledge, AccessScience is an amazing online resource that contains high-quality reference material written specifically for students. Contributors include more than 9000 highly qualified scientists and 43 Nobel Prize winners.
MORE THAN 8700 articles covering all major scientific disciplines and encompassing the McGraw-Hill Encyclopedia of Science & Technology and McGraw-Hill Yearbook of Science & Technology
115,000-PLUS definitions from the McGraw-Hill Dictionary of Scientific and Technical Terms
3000 biographies of notable scientific figures
MORE THAN 19,000 downloadable images and animations illustrating key topics
ENGAGING VIDEOS highlighting the life and work of award-winning scientists
SUGGESTIONS FOR FURTHER STUDY and additional readings to guide students to deeper understanding and research
LINKS TO CITABLE LITERATURE help students expand their knowledge using primary sources of information