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Symmetry in the Animal Kingdom

Denise DeCooman, Megan Wahl
  • Author
    Denise DeCooman

    Denise DeCooman was a teaching assistant for the General Zoology course at California University of Pennsylvania while she earned her Master's of Science in Clinical Mental Health Counseling from fall semester of 2015 and spring of 2017. She also has a Bachelor's of Science in Biological Sciences from California University. She has been writing instructional content for an educational consultant based out of the greater Pittsburgh area since January 2020.

  • Instructor
    Megan Wahl

    Megan has taught middle school science and developed curriculum for k-higher ed. She has a master's degree in Educational Technology.

Explore types of symmetry in animals. Learn about the different types of symmetry, how many types of symmetry there are, and what symmetry means in animals. Updated: 12/10/2021

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What is Symmetry?

When referenced in the study of biology, symmetry refers to the balance, or agreement, in dimensions of an organism. Symmetry can be found via the use of mathematics by dividing the organism into as many equal parts as possible (which is typically two or more equal parts) along an axis. For example, a human being could be divided into left and right halves, and a marine jelly could be divided into almost infinitely identical parts. Almost all animals are observed to have some type of symmetry, with few exceptions to this rule.

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  • 0:04 What Is Symmetry?
  • 0:51 Types of Symmetry
  • 1:11 Symmetries and Asymmetry
  • 4:11 Why is Body Symmetry…
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Types of Symmetry

There are multiple types of symmetry observed in the animal kingdom. Symmetry is a characteristic in the animal kingdom used to classify organisms as well as determine common ancestors for these groups; since most living organisms share the type of symmetry they present with their most recent common ancestor. The types of symmetry this lesson will elaborate on include: asymmetry, bilateral, biradial, radial, and spherical symmetry.

Asymmetry

An organism which is said to exhibit asymmetry has no symmetry whatsoever, regardless of how it would be cut on any axis or plane. Asymmetrical animals include some species of sponges, which are considered one of the most primitive members of the animal kingdom.

Drawing of a sponge that is asymmetrical

Drawing of a Sponge to Show Asymmetry

Bilateral Symmetry

Organisms which display bilateral symmetry have nearly (not totally) identical left and right sides. Bilateral symmetry is an evolutionary adaptation which evolved along with cephalization. Cephalization refers to the concentration of sensory and nervous tissues within a head-like structure of an organism. Bilateral symmetry is observed typically in animals which have well-formed and complex heads, or head-like structures. Bilaterally symmetrical animals include:

  • Insects
  • Fish
  • Humans
  • Dogs
  • Lampreys
  • Worms
  • Amphibians

Animals that are bilaterally symmetrical and cephalized are able to typically move about quickly in order to escape possible predation as well as attack prey. The example of a fish fits well because fish are bilaterally symmetrical and cephalized. They are able to respond quickly to external stimuli due to these traits and move quickly toward, or away from, the stimulus. All vertebrates display bilateral symmetry.

Fish, and other bilaterally symmetrical animals, have different bodily regions such as anterior, posterior, dorsal, and ventral portions. The anterior refers to the top of the body, typically where the head is observed. The posterior of the body refers to the region where the tail, or tailbone, is located. Dorsal is the anatomical term for the back side of an animal (for example, fish have dorsal fins). Ventral is used to refer to the front, or belly, of a bilaterally symmetrical animal.

Humans display bilateral symmetry

Human Beings Display Bilateral Symmetry

All fish display bilateral symmetry

All Fish Display Bilateral Symmetry

Insects display bilateral symmetry

Insects Display Bilateral Symmetry

Biradial Symmetry

Biradial symmetry is a type of symmetry exhibited in organisms that display both bilateral and radial types of symmetry and is very rare in the animal kingdom. These animals typically have an oval-shaped body and are either weak swimmers, floaters, or sessile (do not move). Examples of animals that have biradial symmetry include ctenophores (comb jellies) and sea walnuts.

Ctenophores (Comb Jellies)

Ctenophores (Comb Jellies)

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Frequently Asked Questions

What is symmetrical nature?

Symmetrical nature refers to the the balance in proportions of an organism where the entity can be divided into one or more equal portions. All animals, aside from certain sponges, display some sort of symmetry.

Why are animals symmetrical?

Animals display symmetry because some of their bodily proportions are balanced. Different types of symmetry are witnessed in most animals that move freely throughout their environments.

What are the types of symmetry?

The different types of symmetry are: bilateral symmetry, radial symmetry, biradial symmetry, spherical symmetry, and asymmetry. Bilaterally symmetrical organisms have left and right sides. Radially symmetrical animals bodies radiate outwards from the center. Biradially symmetrical entities have both radial and bilateral symmetry. Those that exhibit spherical symmetry are sphere-shaped and animals considered asymmetical completely lack symmetry.

Do all animals have symmetrical bodies?

Most animals, aside from some species of sponges that are considered asymmetrical, exhibit some form of symmetry. Most animals are bilaterally symmetrical, while others may be radially or biradially symmetrical.

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