Animal Body Plans and Movement: Symmetry in Action

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Radial symmetry is best suited to sessile animals such as these coral polyps. Photograph by the US National Oceanic and Atmospheric Administration.

Almost all animals exhibit some form of symmetry; the main exceptions are sponges, which are asymmetric in shape. The most common body plan is the bilateral symmetry seen in groups as diverse as worms, insects and vertebrates, while many simpler animals are radially symmetric, and a few (the echinoderms) are pentaradially symmetric. Some organisms move more efficiently than others, and many not at all, but how the pentaradial echinoderms control their locomotion has been a puzzle to zoologists. Now, however, research has uncovered the method behind the movement of one such animal, the brittle star.

What is Radial Symmetry?

An animal with radial symmetry is essentially a disk with a top and a bottom, but no left or right side. If a radially symmetric animal were to be divided in half, a cut in any direction would produce two approximately similar parts. This pattern is seen in simple animals such as jellyfish, corals, and sea anemones (members of the Phylum Cnidaria) and comb jellies (Phylum Ctenophora).

Bilateral Symmetry: Animals with Heads and Tails

Bilaterally symmetric organisms have left and right sides that are superficially mirror images of each other. The reflection is not perfect, as you can see by observing your own body – it is common to have one foot larger than the other, for example, and our internal organs are not all symmetrically shaped or positioned. However, the evolution of bilateral symmetry was an important advance – it opened the way for the development of directed motion, improved organs of sense and, eventually, the enlarged and highly complex mammalian brain.

Bilaterally symmetric animals have a distinct head and tail. Photograph by Patrick Giraud.

While being radially symmetrical is ideal for creatures that either do not move (e.g. corals, sea anemones) or rely largely on water currents for transport (e.g. jellyfish, comb jellies), this body plan does not facilitate purposeful movement towards new habitats, sources of food or mates, or away from danger. The juvenile forms of sea anemones and corals are bilaterally symmetric for this very reason. After spawning, these tiny larvae called “planulae” swim away from their parents until they find a suitable new location, where they attach themselves to the substrate and transform into the sessile adult form.

The first bilaterally symmetric animals may have evolved from organisms similar to planulae that remained free-swimming as adults instead of settling down. For an animal that moves forwards through its environment, it is an advantage to have its organs of sense at the end of its body that encounters new things first. This aggregation of sensory cells and nerves in what would become the head of bilateral organisms is likely to have been the first step in the evolution of the brain. In contrast, the sensory systems of radially symmetric animals are simple networks spread uniformly throughout the body, with no single concentration of nerve cells.

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