Animal Body Plans and Movement: Symmetry in Action

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Pentaradial Symmetry: The Echinoderms

The Phylum Echinodermata is an oddity where body plans are concerned. Comprising starfish, brittle stars, sea urchins, sea cucumbers and crinoids, the echinoderms are unique in having pentaradial – or five-sided – symmetry.

Brittle stars can use any one of their five arms as a “head.” Photograph by Brocken Inaglory.

Like radially symmetric animals, most echinoderms do not have anything resembling a brain. (The sea cucumber is the exception that proves the rule; it is worm-like in shape, and has a ring of nerve tissue and, in some species, rudimentary eyes at its front end.) However, unlike sessile sea anemones and drifting jellyfish, starfish, brittle stars and sea urchins are all highly (if slowly) mobile, covering large areas of the seabed as they forage for food.

Brittle Star: How It Moves

How an animal with no head decides in which direction to move has puzzled zoologists, though research is beginning to shed light on the mystery. A recent study showed that a moving brittle star has a single “central arm” that points in the direction of motion, while the two arms on either side of the center propel the animal along with a rowing motion. To change direction, the brittle star simply chooses a different central arm and resumes its journey. Echinoderm motion thus mimics the central control of bilaterally symmetric animals, but it is inefficient by comparison. Changing “heads” takes a brittle star about two seconds to accomplish, which leaves little time to escape a hungry predator.

The Origin of Symmetry

It is unclear when radial and bilateral symmetry first arose in animals. Most zoologists agree that bilateral organisms evolved from a simpler radial body plan, though they debate over when exactly this occurred. The oldest known fossils of multicellular animals include both radially and bilaterally symmetric species, as well as asymmetric forms, suggesting that the division of the Animal Kingdom into “Radiata” and “Bilateria” occurred very early in the history of life. There is no doubt, however, that having distinct front and back ends has been crucial to the evolutionary success of the Bilateria.

Sources

Astley, H.C. Getting around when you’re round: quantitative analysis of the locomotion of the blunt-spined brittle star, Ophiocoma echinata. (2012). Journal of Experimental Biology 215, 1923–1929. doi: 10.1242/jeb.068460. Accessed May 14, 2012.

Farabee, M.J., Ph.D. Biological diversity: animals I. (2007). Estrella Mountain Community College. Accessed May 14, 2012.

Gregory, M., Ph.D. Introduction to Animals, Sponges, Cnidarians. (2012). Clinton Community College. Accessed May 14, 2012.

Hejnol, A., Martindale, M.Q. Acoel development supports a simple planula-like urbilaterian. (2008). Philosophical Transactions of the Royal Society B Biological Sciences 363, 1493–1501. doi: 10.1098/rstb.2007.2239. Accessed May 14, 2012.

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