Echinoderm Structure: Skeleton & Symmetry

Instructor: Taormina Lepore

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

The weird and wonderful animals of the Phylum Echinodermata have a calcium carbonate skeleton and a kind of symmetry that is fivefold. They are unique in the animal kingdom in having this symmetry. In this lesson we'll delve into the skeleton and symmetry of the Echinodermata.

What Is An Echinoderm?

First of all - how do you say it? The Phylum Echinodermata (ekkin-o-der-MAH-tah) might be a tongue twister, but you should be familiar with this group of animals. They're our cousins!

Echinoderms are members of the superphylum Deuterostomia; we humans, and all other chordate animals, are also a part of this superphylum.

In deuterostome embryological development, the anus forms first and the mouth forms second. That's where the name comes from: the Greek deutero-, or second, and the Greek -stomia, or mouth.

Echinoderms include sea stars or starfish, sea lilies, sea cucumbers, sea urchins, and sand dollars. The name echinoderm derives from the Greek words for 'hedgehog' - yes, the spiny animal! - echinos, and the word for 'skin', dermos. So, echinoderms have spiny skin.

All echinoderms have some form of radial symmetry as adults, meaning they have body parts that branch out from a central point - think of the five arms of a starfish. That's fivefold radial symmetry.

Echinoderms are also united by a water vascular system that pushes water throughout their bodies, and by tube feet with tiny suckers on the ends. The Phylum Echinodermata doesn't always have spiny skin, or a spiny surface, but these animals do all share a calcium carbonate internal skeleton, or endoskeleton.

Let's talk a bit more about how the skeletal structure and symmetry of echinoderms works.

Echinoderm Skeleton Structure

The interlocking plates of a sea urchin test.
sea urchin test

Think of an ornate building with windows and bricks. It has interlocking blocks that form beautiful spires and towers, with windows of different shapes and sizes.

The endoskeleton of echinoderms is built just like an ornate and beautiful building. Just like a building has windows within its connected blocks, the internal calcium carbonate skeleton of echinoderms is made up of connected plates.

There are tiny pores, and openings for the mouth and anus in these plates, much like the windows in a building. And, because the skeleton is covered in an epidermis, or outer skin covering, it is internal - that's where the endo- in endoskeleton comes in, since it means inside in Greek.

Some echinoderms have very tightly interlocking plates, like sea urchins. Think of them as tight defense systems around an armed building. In sea urchins and sand dollars, these interlocking plates form an enclosure, almost like a shell, called a test. Sea urchins also have spines protruding from their tests, in life.

Others have more movable plates, like sea stars or starfish. Sea cucumbers, the worm-like echinoderms, have an endoskeleton that is nearly microscopic. They may not have as strong a defense system, but they are much more flexible and movable.

If you zoom in on an echinoderm's calcium carbonate skeletal plates under an electron microscope, you'll see that each plate is made up of a very fine webbing. Each piece of webbing is actually a crystal of calcium carbonate that has branched and spread to form an overall skeletal network, called a stereom.

Echinoderm Symmetry

Since we've been discussing ornate buildings, let's think about how a building might be organized. Imagine a structure like the Pentagon building in Washington, D.C., with corners set at five points, like a star.

This is exactly how echinoderm symmetry is arranged. It's a type of radial symmetry, which is when an animal's body plan is arranged with one central point and a branching series of body parts. The cnidarians (nid-AIR-ee-ans), or jellyfish and hydras, have radial symmetry, along with our echinoderms.

Pentaradial symmetry in a starfish.
starfish

However, echinoderms have an even more specific type of radial symmetry called pentaradial symmetry. Just like the Greek word for 'five', penta-, echinoderms have a fivefold symmetry. Your five-pointed starfish is a good example of this kind of symmetry.

You can also see pentaradial symmetry in the tentacled mouths and internal calcium carbonate skeleton of sea cucumbers, and in the tests of sea urchins and sand dollars. Even sea lilies, or crinoids, have a pentaradial arrangement in their feathery tentacles.

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