27.2A: Animal Characterization Based on Body Symmetry (2024)

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Animals can be classified by three types of body plan symmetry: radial symmetry, bilateral symmetry, and asymmetry.

Learning Objectives

Differentiate among the ways in which animals can be characterized by body symmetry

Key Points

Animals with radial symmetry have no right or left sides, only a top or bottom; these species are usually marine organisms like jellyfish and corals.
Most animals are bilaterally symmetrical with a line of symmetry dividing their body into left and right sides along with a “head” and “tail” in addition to a top and bottom.
Only sponges (phylum Porifera) have asymmetrical body plans.
Some animals start life with one type of body symmetry, but develop a different type as adults; for example, sea stars are classified as bilaterally symmetrical even though their adult forms are radially symmetrical.

Key Terms

sagittal plane: divides the body into right and left halves
radial symmetry: a form of symmetry wherein identical parts are arranged in a circular fashion around a central axis
bilateral symmetry: having equal arrangement of parts (symmetry) about a vertical plane running from head to tail

Animal Characterization Based on Body Symmetry

At a very basic level of classification, true animals can be largely divided into three groups based on the type of symmetry of their body plan: radially symmetrical, bilaterally symmetrical, and asymmetrical. Only a few animal groups display radial symmetry, while asymmetry is a unique feature of phyla Porifera (sponges). All types of symmetry are well suited to meet the unique demands of a particular animal’s lifestyle.

Radial Symmetry

Radial symmetry is the arrangement of body parts around a central axis, like rays on a sun or pieces in a pie. Radially symmetrical animals have top and bottom surfaces, but no left and right sides, or front and back. The two halves of a radially symmetrical animal may be described as the side with a mouth (“oral side”) and the side without a mouth (“aboral side”). This form of symmetry marks the body plans of animals in the phyla Ctenophora (comb jellies) and Cnidaria (corals, sea anemones, and other jellies). Radial symmetry enables these sea creatures, which may be sedentary or only capable of slow movement or floating, to experience the environment equally from all directions.

27.2A: Animal Characterization Based on Body Symmetry (2)

Bilateral Symmetry

Bilateral symmetry involves the division of the animal through a sagittal plane, resulting in two mirror-image, right and left halves, such as those of a butterfly, crab, or human body. Animals with bilateral symmetry have a “head” and “tail” (anterior vs. posterior), front and back (dorsal vs. ventral), and right and left sides. All true animals, except those with radial symmetry, are bilaterally symmetrical. The evolution of bilateral symmetry and, therefore, the formation of anterior and posterior (head and tail) ends promoted a phenomenon called cephalization, which refers to the collection of an organized nervous system at the animal’s anterior end. In contrast to radial symmetry, which is best suited for stationary or limited-motion lifestyles, bilateral symmetry allows for streamlined and directional motion. In evolutionary terms, this simple form of symmetry promoted active mobility and increased sophistication of resource-seeking and predator-prey relationships.

27.2A: Animal Characterization Based on Body Symmetry (3)

Animals in the phylum Echinodermata (such as sea stars, sand dollars, and sea urchins) display radial symmetry as adults, but their larval stages exhibit bilateral symmetry. This is termed secondary radial symmetry. They are believed to have evolved from bilaterally symmetrical animals; thus, they are classified as bilaterally symmetrical.

27.2A: Animal Characterization Based on Body Symmetry (4)

Asymmetry

Only members of the phylum Porifera (sponges) have no body plan symmetry. There are some fish species, such as flounder, that lack symmetry as adults. However, the larval fish are bilaterally symmetrical.