Hydrostatic Skeletons, Exoskeletons & Endoskeletons

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  • 0:02 Bones and Skeletons
  • 0:33 The Endoskeleton
  • 1:18 The Exoskeleton
  • 2:30 The Hydroskeleton
  • 4:36 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

You have a skeleton, right? Do you know what kind of skeleton? Do you know the other types of skeletons and what animals have them? This lesson will help answer these questions for you!

Bones and Skeletons

The bones of your body make up your skeleton, the hard structural framework of your body on which the rest of your tissues and organs sit on and within, or are attached to. The skeleton is like the framework and foundation of your home. Without it, you'd be a kind of blob on the floor.

Do you know what kind of skeleton you have? There's an actual name for it. And did you know that there is more than one kind of skeleton, two kinds of which you do not have? Well, if you don't know, then don't worry; you're about to find out!

The Endoskeleton

Whether you're a person or a sponge, you have what's known as an endoskeleton, a hardened internal skeleton. The human skeleton is composed of more than 200 bones. Some of these bones, like the bones that house the brain, are fused together into a rigid structure. Other bones, like the bones found at the knee joint, are very mobile with respect to one another to allow for movement. Other mammals, like dogs, elephants, whales, and beyond, have very similar, albeit differently arranged, bones comprising their endoskeleton.

Again, the endoskeleton is an internal framework. It is not visible directly because it is covered by tissues and organs of various kinds.

The Exoskeleton

This concept is completely the opposite of an exoskeleton, a hard external framework found on the bodies of some kinds of animals.

Be they clamshells or insects like a cockroach, the crunch you hear when opening them up or stepping on them comes from this exoskeleton. Mollusks and insects are by no means the only animals with exoskeletons. Let's not forget crustaceans, like crabs and lobsters, too!

The exoskeleton found on animals like clams is made of calcium carbonate, while the exoskeleton found on insects is partially made of chitin, a kind of polysaccharide. A polysaccharide is a chain of many sugar units.

Unlike the endoskeleton, an exoskeleton must be shed in order to produce a larger one as the animal grows. This is called molting. However, just like the exoskeleton, the parts of the endoskeleton that do not require movement can be hardened for added protection against predators and impacts.

If an endoskeleton is like the framework and foundation of a home for the body that utilizes one, then an exoskeleton is like a helmet for the animal that uses one.

The Hydroskeleton

But I saved the coolest, most confusing, and least known type of skeleton for last. It's the hydrostatic skeleton, aka the hydroskeleton, which is a kind of skeleton that is composed of soft tissue filled with an incompressible fluid or a gel-like substance.

Confused? I thought you might be. Let me give you a very good metaphor for this using something you're very well aware of. Have you ever ridden in a car or on a bicycle, or have you ever inflated a tire? What is a tire but walls of rubber filled with essentially incompressible - that is to say, pressurized - air? This air, a kind of pressurized fluid, holds the tire up and prevents it from collapsing.

This is what happens in a hydroskeleton. Except the walls of the skeleton aren't made of rubber or bone but of soft tissue, specifically muscle. And it's not air or bone that holds up the soft-bodied cavities of these animals, but pressurized liquid.

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