Among the various ways of classifying animals is according to body symmetry. Almost all animals have some form of symmetry -- either bilateral or radial. Sponges, meanwhile, are a major exception.
Radial Symmetry - Like Snowflakes or a Pie
An animal with radial symmetry could be divided into equal portions from the center, in the same way that you could cut a pie into wedges. An animal with radial symmetry really has no right and left side or head and rear end. Radial symmetry is ideal for animals that do not move, so they can reach into their environment on all sides. Consider the starfish as an example of radial symmetry.
Bilateral Symmetry - Heads or Tails
Animals with bilateral symmetry can be divided only into mirror halves through a single plane. Higher animals that move are normally bilateral, with matching left and right sides. Bilateral symmetry is associated with having a head or leading end of a body that encounters the environment first, so the sense organs like eyes and mouth are usually there. Bilateral symmetry often gives animals more streamlined shapes for moving through their environments. You, your dog and his fleas are bilateral.
Animals With Radial Symmetry
Animals with radial symmetry tend to have a surface that contains a mouth at the center, and they can reach out in all directions to gather food for that mouth. They can be attached to a substrate, like the sea anemone, or floating, like jellyfish. Radial symmetry occurs in simple animals including the aquatic cnidarians, which include corals, jellyfish and sea anemones, and Ctenophora, which are comb jellies. A group called dchinoderms consists of starfish, sand dollars and sea urchins; these have unique five-point radial symmetry.
Animals With Bilateral Symmetry
Most creatures we see around us have bilateral symmetry. Examples are worms, insects, spiders, fish, birds and mammals, including humans. In evolution, bilateral symmetry was an important step toward the development of a head and the concentration of sensory organs. Animals with bilateral symmetry do not have both halves as perfect mirror images – often one foot or ear can be bigger than another and internal organs are not symmetric in their shape or positioning.
Sponges Are Different
Sponges are multicellular animals, unique in many ways. They are the only group of animals that can be classified as asymmetric, which means they possess no symmetry at all. There is no plane through their bodies where you could cut them in half to produce two mirror images.
- Clinton Community College, Faculty: Introduction to Animals Sponges, Cnidarians
- University of California: Museum of Paleontology: Introduction to the Echinodermata
- University of California: Museum of Paleontology: Introduction to Cnidaria
- Decoded Science: Animal Body Plans and Movement – Symmetry in Action