Features Used to Classify Animals

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. Asymmetry is a unique feature of Parazoa (Figurea). Only a few animal groups display radial symmetry. All types of symmetry are well suited to meet the unique demands of a particular animal’s lifestyle.

Radial symmetry is the arrangement of body parts around a central axis, as is seen in a drinking glass or pie. It results in animals having top and bottom surfaces but no left and right sides, or front or back. The two halves of a radially symmetrical animal may be described as the side with a mouth or “oral side,” and the side without a mouth (the “aboral side”). This form of symmetry marks the body plans of animals in the phyla Ctenophora and Cnidaria, including jellyfish and adult sea anemones (Figurebc). Radial symmetry equips these sea creatures (which may be sedentary or only capable of slow movement or floating) to experience the environment equally from all directions.

Part a shows several sponges, which form irregular, bumpy blobs on the sea floor. Part b shows a jellyfish with long, slender tentacles, radiating from a flexible, disc-shaped body. Part c shows an anemone sitting on the sea floor with thick tentacles, radiating up from a cup-shaped body. Part d shows a black butterfly with two symmetrical wings.
The (a) sponge is asymmetrical. The (b) jellyfish and (c) anemone are radially symmetrical, and the (d) butterfly is bilaterally symmetrical. (credit a: modification of work by Andrew Turner; credit b: modification of work by Robert Freiburger; credit c: modification of work by Samuel Chow; credit d: modification of work by Cory Zanker)

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 (Figured), 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 (Figure). All true animals except those with radial symmetry are bilaterally symmetrical. The evolution of bilateral symmetry that allowed for 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.

The illustration shows a woman’s body dissected into planes. The coronal plane separates the front from the back. The front of the body is the ventral side, and the back of the body is the dorsal side. The upper body is defined as cranial, and the lower body is defined as caudal.  The sagittal plane dissects the body from side to side. The medial line goes through the center of the body. The areas to the left and right of the medial line are defined as lateral. Parts of the body close to the medial line are proximal, and those further away are distal.
The bilaterally symmetrical human body can be divided into planes.

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.

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Watch this video to see a quick sketch of the different types of body symmetry.

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