Cnidarians are a diverse group of aquatic animals that includes familiar organisms like jellyfish, sea anemones, and corals. A primary way biologists categorize animals is by the symmetry of their body plan, which describes how their parts are arranged. This has led to the question of where cnidarians fit within these classifications, and specifically, if they have bilateral symmetry.
Defining Radial and Bilateral Symmetry
Bilateral symmetry is a body plan where an organism can be divided into two mirror-image halves, a left and right side, but only along one specific plane. This single plane, called the sagittal plane, runs from the head to the tail. Humans, insects, and crustaceans are examples of bilaterally symmetrical animals. This body plan is associated with active movement and the development of a head region, a phenomenon known as cephalization.
In contrast, radial symmetry describes a body plan arranged around a central point. Organisms with this type of symmetry can be divided into similar halves by any plane that passes through the central axis, much like slicing a pie. Instead, they have a top surface where the mouth is located (oral side) and a bottom surface (aboral side).
The Radial Body Plan of Cnidarians
The vast majority of cnidarians exhibit radial symmetry, not bilateral symmetry. Their bodies are organized around a central axis that extends from the mouth to the base. This structural plan is advantageous for their lifestyles.
Many cnidarians are either sessile, meaning they are fixed in one place like sea anemones and corals, or they are planktonic, drifting with water currents like jellyfish. This radial arrangement allows them to encounter their environment—including food, predators, and mates—from all directions equally. This body plan is evident in both of the main cnidarian forms: the polyp, which is attached to a substrate with its mouth and tentacles pointing upward, and the medusa, which is free-swimming with its mouth and tentacles pointing downward.
Biradial Symmetry: An Exception to the Rule
While radial symmetry is the standard for the phylum, some cnidarians display a modified form known as biradial symmetry. This is particularly true for animals in the class Anthozoa, which includes sea anemones and corals. Biradial symmetry is a hybrid condition where parts are arranged radially, but the body can only be divided into two perfect mirror images along two specific planes, not multiple planes as in true radial symmetry.
This condition arises from the presence of certain structures that occur in pairs on opposite sides of the oral-aboral axis. For example, in sea anemones, a groove-like structure called a siphonoglyph runs down the pharynx, and its presence on only one side breaks the complete radial pattern. Scientists view biradial symmetry not as a completely separate category but as a modification of the underlying radial plan, perhaps indicating an evolutionary pathway toward the bilateral symmetry seen in more complex animals.