Biological Symmetry

Animals exhibit one of two types of symmetry: bilateral, wherein an animal's right half is essentially a mirror image of its left half; or radial, wherein multiple lines of symmetry passing through a central point exist. (See figure below).

Figure 1. A comparison of radial and bilateral symmetry in animals.

A snow board is a good example of a human-made object that exhibits bilateral symmetry (draw a line along the length of the ski to divide it in half and each side will be a mirror image of the other). A snow tube is a good example of a human-made object that exhibits radial symmetry (an infinite number of lines of symmetry can be drawn through the center of the tube as it lays flat on the snow).

Figure 2. A snowboard (an example of a human-made bilaterally symmetrical object).		Figure 3. A snowtube (an example of a human-made radially symmetrical object).

To appreciate the functional difference between the two forms of symmetry, imagine yourself sliding down a hill on a snowboard and a tree appears in front of you. Now imagine the same scenario with a snowtube. Which object will be easier to steer away from the tree? Did you know that animals which move considerable distances under their own power are bilaterally symmetrical? Can you generate a hypothesis to explain this observation?
Additional examples of radial and bilateral symmetry in organisms are presented below. Can you identify which organisms exhibit which form of symmetry? How do each of these species move long distances in their environments?

Shark

Butterfly

Jelly Fish