An important characteristic of the body plan of an animal is its symmetry. Symmetry refers to the equivalence in size and shape of sections of an animal’s body. Most animals exhibit body symmetry, but a few species of sponges are asymmetrical. If a plane were passed through the body of an asymmetrical sponge, slicing it in two, the parts would not be the same.
Some animals are radially symmetrical. Shaped like either short or long cylinders, these stationary or slow-moving organisms have distinct top and bottom surfaces but lack fronts and backs, heads or tails. A plane could pass through a radially symmetrical animal in several places to create two identical halves. Starfish, jellyfish, sea cucumbers, sea lilies, and sand dollars are a few examples of radially symmetrical animals.
The bodies of most animals are bilaterally symmetrical, a form in which a plane could pass through the animal only in one place to divide it into two equal parts. The two halves of a bilaterally symmetrical animal are mirror images of each other. Bilateral symmetry is associated with animals that move around. The leading part of a bilaterally symmetrical animal’s body contains sense organs such as eyes and nose. Fish, whales, birds, snakes, and humans are all bilaterally symmetrical.
Scientists have special terms to describe the body of a bilaterally symmetrical animal, depicted in Figure below. The head or front region is called the anterior portion and the opposite end, the hind region, is the posterior. The stomach or underside is the ventral side, and opposite that is the back, or dorsal, side. Structures located on the side of an animal are described as lateral.
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| A sponge (a) is an asymmetrical animal. Starfish and jellyfish (b) are radially symmetrical; snails, turtles, and fish (c) are bilaterally symmetrical. In a bilaterally symmetrical animal, the head or front end is described as anterior and the tail end as posterior. The front or stomach side is ventral and the back or top side is dorsal. The sides of the animal are described as lateral. |