What Are Radial Symmetry Animals? Incredible Examples For Kids

Rajnandini Roychoudhury
Jan 17, 2023 By Rajnandini Roychoudhury
Originally Published on Oct 22, 2021
Edited by Luca Demetriou
Fact-checked by Niyati Parab
A pink jellyfish swimming in an aquarium

One of the major ways of classifying animals is in accordance with their body symmetry.

They can be majorly classified into two types of body symmetry: bilateral symmetry and radial symmetry. However, sponges as the members of the phylum Porifera lack any sort of body symmetry and exhibit asymmetrical properties.

An animal exhibiting radial symmetry means that it does not have any vertical or horizontal sides. The central axis of the body has an equal distance from all sides and there is a lack of proper head or mouth.

This characteristic is highly uncommon in the animal kingdom, with very few creatures possessing this form of symmetry. Examples of animals with radial symmetry are jellyfish, sea urchins, and sea stars.

The bodies of most animals can be divided into equal left and right halves along the central axis and this is known as bilateral or mirror symmetry. Such creatures usually have a head and a tail, and the body shape becomes more streamlined to aid them in moving through the environment.

Almost all the animals we encounter in our day-to-day life show bilateral symmetry, such as cats, dogs and fellow human beings.

The third group consists of a very limited number of animals, namely the members of the phylum Porifera, such as the sponges. These animals possess no symmetry or body shape at all and are hence classified as asymmetrical creatures.

Read on this article to know more about animals that have radial symmetry. For more related educational articles, please check out our articles on midnight zone animals and hairless animals.

What is radial symmetry?

The Greek word ‘symmetry’ was coined to describe the harmonic arrangement of parts that have the same measures and proportions.

Symmetry is an important aspect in biology for it provides certain advantages in moving around for animals and organisms, apart from being the primary reason behind harmony and aesthetic aspects.

For example, humans need symmetrical right and left legs to walk around without any impedance, or birds need both the left and right wings to have symmetrical wingspan which is a prerequisite for them to fly.

Even on microscopic levels, symmetry is observed in cells and organs for them to function accurately and to ascertain developmental advantages.

The number of duplicate body parts or shapes that make an animal symmetrical determines the type of symmetry the organism has got, such as, radial symmetry or bilateral symmetry. In radial symmetry, multiple planes divide the animal body along a central point to produce identical parts.

Radially symmetric bodies can occur from three-fold up to infinity-fold radial symmetry, as in the case of a completely spherical organism.

Radial symmetry is a very rare characteristic seen in animals and organisms, as opposed to bilateral symmetry. Radial symmetry is found in sessile organisms such as floating organisms and echinoderms, such as sea stars and sea urchins.

The equal distribution of body parts and sense organs makes them better able to react to environmental stimuli coming from all around their bodies. As shapes of organs and cells are strictly connected to their activities and functions, symmetry is an important matter also at those scales.

Cnidarians With Radial Symmetry

Those organisms that belong to the phylum Cnidaria are known as cnidarians. These types of organisms generally inhabit saltwater bodies and possess uncomplicated body structures which lack any distinct organ or limb.

They have specialized cells in their bodies, known as nematocysts, that sting prey and help them catch their food. Each organism possesses a mouth-like opening that leads to a pouch-like gastrointestinal cavity. Some of the major cnidarians are jellyfish and coral.

These cnidarians have unique body structures that exhibit radial symmetry, making these organisms vastly different from those prior to them. Radial symmetry means that they have a circular or symmetrical body plan, and dividing them from any side through the central point or axis of the organism leaves two equal halves.

Cnidarians have two basic body forms: polyp and medusa. While animals having the medusa body form have mouth and tentacles bearing downward from a bell-shaped body, an animal possessing the polyp body form has the mouth facing upward with a body that is shaped like a cup.

Organisms like the sea anemone and coral exhibit polyp body form, while jellyfish show the medusa body form.

Unlike sponges which evolved way before the cnidarians, the latter is composed of true tissues. The inside of a cnidarian is called the gastrovascular cavity which is basically a huge space that aids the organism's digestion and movement of nutrients throughout the body.

Cnidarians also possess nerve tissue that is organized in a net-like structure, known as a nerve net. This acts as a nervous system with connected nerve cells dispersed all around the body. However, as already mentioned, these organisms lack the presence of true organs.

Echinoderms With Radial Symmetry

Echinoderms have bodies akin to that of skeletons, with the bones being substituted by a water vascular system and minuscule plates. They lack heads with a body structure that is based on five-part symmetry.

Aided by their little, water-filled tube feet that are controlled by a central hydraulic system, these slow creatures are very skilled when it comes to catching prey with their tube feet.

The number of duplicate body parts or shapes that make an organism (or an organ) symmetric defines the type of symmetry the body has got, such as radial or bilateral symmetry.

However, the type of body symmetry is not always fixed for life and can be either broken or changed from one type to another through the lifespan of an organism.

Transitions, where a whole radially symmetric animal is converted into a bilaterally symmetric one, are relatively common, whereas transitions from bilateral to radial symmetry are more rarely observed.

Echinoderms are animals with bodies that are shaped like discs and elongated arms known as tube feet. In such animals, we can observe bilateral symmetry during the larval stages.

The larvae are the elongated worm-like structures in the inception stages of the life cycle of animals. When the larvae grow to form a full-fledged adult, the radial form of body symmetry is attained by the animal. The presence of the extending arms that are protruding outwards lends the characteristic of radial symmetry to the animals.

Radial symmetry is considered the secondarily adapted symmetry in echinoderms. Sea stars, sea urchins, sea cucumber, crinoids, and brittle stars are some examples of echinoderms.

Are humans radial or bilateral?

Humans, like almost the majority of the animal kingdom, exhibit bilateral symmetry, which is also known as mirror symmetry. Bilateral symmetry is a type of body symmetry in which both the right and left sides if cut along a central axis, mirror each other.

The outer surface is the exact same on either left and right sides along a sagittal plane, which is the structural plan of humans and most members of the animal kingdom. Humans exhibit structural symmetry about a plane running from head to toe.

An intriguing scientific notion in biology has gained prominence in recent times. Scientists have started believing that the extensive prevalence of bilateral symmetry in the animal kingdom is not a coincidence at all.

Going by mathematical probabilities, nature had infinitely more ways to build an asymmetrical body than a symmetrical one. Yet, a study of fossils gives ample proof that bilateral symmetry had already taken over animal bodies as early as 500 million years ago.

Thus, scientists and researchers of biology are certain that bilateral symmetry must have evolved for some specific reason or purpose. The most prominent and well-received hypothesis is that bilateral symmetry makes visual perception more effortless as such mirror symmetry is easier for the brain to recognize while in different orientations and positions.

The other most popular theory is that mirror symmetry factored into evolution as it aids with the selection of mates. Scientific experiments involving many animal groups have proven that the female partner prefers males exhibiting the most symmetrical reproductive organs.

Examples include female barn swallows choosing mates that have long, symmetrical tail feathers, or peacocks opting for mates having more symmetrical and exorbitant tails.

Human beings tend to exhibit the same pattern. Experiment groups have concurred that females are more attracted to men having more symmetrical features than those possessing slightly less symmetrical features.

With the discovery of connections between symmetrical bodies and health, the correlation between mate selection and body symmetry began to emerge also.

Scientific studies began to show appalling results between mental and physical health with structural symmetry. It was seen that women having faces exhibiting asymmetry tend to suffer more from depression and emotional vulnerability, while men having faces with asymmetry are more prone to anxiety, stomach disorders, headaches, and depression.

Other studies came up with discoveries such as individuals were more probable to aggressive behavior when provoked if their bodies show more asymmetry than others.

Symmetry is not only seen in biology but is also prevalent in the physical sciences and is even interwoven with the laws of Nature and the universe.

Here at Kidadl, we have carefully created lots of interesting family-friendly factsfor everyone to enjoy! If you liked our suggestions for radial symmetry animals then why not take a look at animals with opposable thumbs, or animals in the Arctic.

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Written by Rajnandini Roychoudhury

Bachelor of Arts specializing in English, Master of Arts specializing in English

Rajnandini Roychoudhury picture

Rajnandini RoychoudhuryBachelor of Arts specializing in English, Master of Arts specializing in English

With a Master of Arts in English, Rajnandini has pursued her passion for the arts and has become an experienced content writer. She has worked with companies such as Writer's Zone and has had her writing skills recognized by publications such as The Telegraph. Rajnandini is also trilingual and enjoys various hobbies such as music, movies, travel, philanthropy, writing her blog, and reading classic British literature. 

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Fact-checked by Niyati Parab

Bachelor of Commerce

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Niyati ParabBachelor of Commerce

With a background in digital marketing, Niyati brings her expertise to ensure accuracy and authenticity in every piece of content. She has previously written articles for MuseumFacts, a history web magazine, while also handling its digital marketing. In addition to her marketing skills, Niyati is fluent in six languages and has a Commerce degree from Savitribai Phule Pune University. She has also been recognized for her public speaking abilities, holding the position of Vice President of Education at the Toastmasters Club of Pune, where she won several awards and represented the club in writing and speech contests at the area level.

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