Body Symmetry

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Animals may be asymmetrical, radial, or bilateral in form (Figure 15.4). Asymmetrical animals are animals with no pattern or symmetry; an example of an asymmetrical animal is a sponge (Figure 15.4 a). An organism with radial symmetry (Figure 15.4 b) has a longitudinal (up-and-down) orientation: Any plane cut along this up–down axis produces roughly mirror-image halves. An example of an organism with radial symmetry is a sea anemone.

Figure 15.4 Animals exhibit different types of body symmetry. 

The (a) sponge is asymmetrical and has no planes of symmetry, the (b) sea anemone has radial symmetry with multiple planes of symmetry, and the (c) goat has bilateral symmetry with one plane of symmetry.

 

Bilateral symmetry is illustrated in Figure 15.4 c using a goat. The goat also has upper and lower sides to it, but they are not symmetrical. A vertical plane cut from front to back separates the animal into roughly mirror-image right and left sides. Animals with bilateral symmetry also have a “head” and “tail” (anterior versus posterior) and a back and underside (dorsal versus ventral).

Watch this video (http://openstaxcollege.org/l/symmetry2) to see a quick sketch of the different types of body symmetry.