Pentaradial Symmetry: Definition & Types

Instructor: Taormina Lepore

Taormina has taught advanced high school biology, is a science museum educator, and has a Master's degree in museum paleontology.

Among invertebrate animals, only the echinoderms have this kind of radial symmetry. In this lesson, we'll define pentaradial symmetry and explore a few different types we see in nature.

Definition of Pentaradial Symmetry

Think about your own body; you have the same number of arms, legs, and other features on each side, with the exception of a few scars or freckles. This is called bilateral symmetry, with the prefix bi- meaning 'two' and lateral meaning 'sides'.

One way to differentiate different groups of animals is by their body symmetry. Radial symmetry is another type of animal symmetry, where parts of an animal branch out from a central point, with each part matching the other. Cnidarians, or jellyfish and hydras, have a type of radial symmetry where their body plan branches out from a central focus. Think of all those tentacles; they are part of that branching pattern.

Some animals, however, have a special kind of radial symmetry that is based on the number five. Pentaradial symmetry ('penta-' means five) follows a pattern where the parts of the animal branch out into five distinct compartments, or arms. Pentaradial symmetry is also known as pentamerism, another term referring to the number five.

Diagram of pentaradial symmetry
pentaradial diagram

Let's take a look at the examples of pentaradial symmetry among invertebrate animals.

Examples of Pentaradial Symmetry

In the animal kingdom, there is only one known phylum that exhibits pentaradial symmetry, and that's the Phylum Echinodermata. Echinoderms include sea stars (or starfish), sea urchins, sea lilies, sand dollars, and sea cucumbers. They're a weird and wonderful group that are members of the superphylum Deuterostomata - a group we also belong to in which the anus forms first during embryological development (as opposed to the mouth).

Let's look at some of the more known pentaradial organisms:

Sea Stars

A sea star, or starfish.
sea star

When you think of a sea star, or starfish, you're probably imagining pentaradial symmetry symmetry based on five parts. Their five tentacle-like arms radiate from a central point.

Sand Dollars

If you've ever seen a dried or living sand dollar, you might have noticed that they, too, have a tiny five-armed structure in the center of the circular disk that makes up much of their body. This circular disk, called a test, is made up of five calcium carbonate plates in a radial pattern. The tiny five-armed structure in the center is made up of pores that facilitate gas exchange in a living sand dollar.

Examples of sand dollars.
sand dollar

Sea urchins

Sea urchins, which are modified sand dollars, also have this five-fold radiation in their spiny calcium carbonate plates. In life, spines stick out from the living sea urchin's surface.

A sea urchin with its spines removed, showing the calcium carbonate plates.
sea urchin

Sea lilies

Sea lilies are an ancient group of echinoderms, once very diverse, but is now limited to around 600 species. These animals, also known as crinoids, have a feathery appearance atop a long stem, and are common in tropical marine environments. They are abundant in the fossil record.

Examples of sea lilies.

Sea cucumbers

Sea cucumbers may not appear to have pentaradial symmetry at first glance. They look like worms! But these echinoderms have the same five-fold symmetry at their tentacles and along their internal calcium carbonate skeleton.

A worm-like sea cucumber.
sea cucumber

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