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Spherical Symmetry: Definition, Biology & Animals

Instructor: Danielle Haak

Danielle has a PhD in Natural Resource Sciences and a MSc in Biological Sciences

Only two types of organisms exhibit spherical symmetry. Read this lesson to learn about biological symmetry and what makes spherical symmetry rare and unique.

Biological Symmetry

What is biological symmetry? In biology, the term symmetry refers to the arrangement of body parts in relation to a line or point. Some organisms are asymmetrical, meaning they have completely irregular body shapes. However, many organisms exhibit some type of symmetry, and there are four types of symmetry identified in biology: spherical, radial, biradial, and bilateral.

These terms refer to the arrangement of an organism's body parts on opposite sides of a dividing line or around a point. Bilateral symmetry is the most commonly found symmetry in living organisms, and this just means that the body of the organism is divided into two equal halves by a line called the sagittal plane. Typically, the sagittal plane divides the body into a left half and a right half that mirror each other.

Spherical Symmetry

Though there are types of biological symmetry that are more commonly found in nature, in this lesson we're interested in learning about spherical symmetry, which describes an organism that can be divided into two identical halves by any cut or line that passes through its center.

For an organism to exhibit spherical symmetry, its body must be shaped like a sphere, and all parts must be arranged or radiate equally around a center point. Roughly speaking, you can think of the sun as having spherical symmetry. If you cut the sun into two halves through its core, each half would be almost identical. We're just applying that same idea to a living organism.

Thus, any division through the center point of the organism creates two halves that mirror one another. This unique level of symmetry is sometimes referred to as point symmetry or universal symmetry.

Spherical symmetry is only possible in very small organisms because the necessary ratio of internal mass to surface area is relatively large and could not be sustained in bigger organisms.

Examples of Organisms with Spherical Symmetry

Spherical symmetry is so rare that very few groups exhibit this feature.

You can find spherical symmetry in two protozoan groups, Radiolaria and Heliozoa.

Some members of the single-celled protozoan group Radiolaria exhibit spherical symmetry
Radiolaria

Additionally, colonial algae, such as the genus Volvox, exhibit external spherical symmetry.

Colonial algae like Volvox display external spherical symmetry
Volvox

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