What Type of Symmetry Do Mollusks Exhibit?

Symmetry

• Symmetry is the concept that the parts of certain objects exhibit a sense of balance. For example, a sphere is considered symmetrical because it looks the same from every angle as it spins around its axis. Biologists believe that most of Earth's life forms also exhibit symmetry. If most plants or animals are cut in two along a line running through the middle, the two halves will be mirror images of each other.

Mollusks

• The more than 100,000 species of mollusks are the second largest group of invertebrates after arthropods. Some familiar examples of mollusks include snails, clams, mussels, whelks, oysters, octopuses and squids. Mollusks have several features in common. These include a muscular foot, a hump containing the digestive tract and reproductive organs, a head containing sensory organs and a mouth and a structure called the "mantle," which usually creates a protective shell. They also exhibit bilateral symmetry.

Bilateral Symmetry

• In general, bilateral symmetry occurs in animals where one part of the body such as the head always leads when the animal moves. If an animal exhibiting bilateral symmetry is cut in half, the two halves are only mirror images along one plane. For example, if a mollusk is cut in half from front to back, the left and right halves are mirror images. If a mollusk is cut in half from side to side the two halves are not mirror images.

Other Types of Symmetry

• Scientists have observed two other forms of symmetry in the natural world. Radial, or line, symmetry occurs in animals with cone-shaped or disc-shaped bodies such as sea anemones and starfish. When an organism exhibiting radial symmetry is cut in half, both halves will be mirror images regardless of the direction of the cut.

  Point symmetry, also called "spherical symmetry" or "universal symmetry," occurs in certain protozoa with spherical bodies. A straight cut through the center of an organism with spherical symmetry will always produce halves that mirror each other. Some microscopic animals such as amoebas are asymmetrical, or lacking symmetry. Two halves of an amoeba never mirror each other.