Echinoderm Skeleton Structure
The interlocking plates of a sea urchin test.
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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.
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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.
Even though adult echinoderms exhibit pentaradial symmetry, it's important to remember that the larvae, or young, have bilateral symmetry - symmetry that is nearly equal on either side of a midline. Humans have bilateral symmetry; imagine looking in the mirror and drawing an imaginary line down the center of your face. With the exception of a few freckles or other marks, you'd have the same features on both sides of your face. That same imaginary line can be drawn down your entire body, with the same results.
The free-floating larvae of echinoderms begin to develop pentaradial symmetry as they mature, but this initial bilateral nature unites us, as humans, with the echinoderms under the superphylum Deuterostomia.
Lesson Summary
Strange, living defense structures. Spiny, radial creatures. They aren't science fiction, they're echinoderms - and they're even our relatives!
The animals within Phylum Echinodermata are members of the superphylum Deuterostomia, along with the Phylum Chordata - the chordates. We humans are chordates, and we all share a common embryologic development within the deuterostomes. The anus develops first, rather than the mouth. We also share bilateral symmetry during our development.
The echinoderm skeleton, however, is quite different from our own skeleton. Although we share an internal endoskeleton, underneath our outer skin or epidermis, the endoskeleton of echinoderms is made up of interlocking or loosely-held calcium carbonate plates. There are pores within these plates, as well as openings for the mouth and anus. Some echinoderms, like sea urchins and sand dollars, have a very tightly interlocking series of plates known as a test.
Others have more fluid plate arrangements, such as sea stars and sea cucumbers. Even sea lilies, or crinoids, have this calcium carbonate skeleton. In microscopic view these plates make up a network of extended calcium carbonate crystals that form a structure known as a stereom.
All echinoderms also exhibit radial symmetry in their body plans - specifically pentaradial symmetry, or fivefold symmetry, which is unique among animals. Only cnidarians, like jellyfish and hydra, have anything close to this kind of arrangement.
