Bones and Muscles

In the last lesson, we introduced you to the skeletal system (bones) and the muscle system (muscles). These systems work together to allow animals (including us) to move. That's why scientists sometimes consider them to be a single organ system -- the musculoskeletal system.

As you learned in our last lesson, there are three different kinds of skeletons:

1. Hydrostatic skeletons -- closed fluid-filled chambers (such as you'd find in a sea anemone or worm)
2. Exoskeletons -- hard segments that cover the muscles and visceral organs of certain organisms like insects, shrimp and lobsters. These organisms actually "wear" their skeletons on the outside of their bodies.
3. Endoskeletons -- internal mineralized skeletons made up of bone and/or cartilage

Vertebrates, like us, have endoskeletons -- so that's what most of this lesson is about.

But first we'd like to tell you a little about organisms that have hydrostatic skeletons or exoskeletons.