The rigid or semirigid framework that
supports an animal's body, protects its
internal organs, and provides anchorage
points for its muscles. The skeleton may be
composed of bone and cartilage (vertebrates),
chitin (arthropods), calcium carbonate
(molluscs and other invertebrates, or silica
(many protists). It may be internal, forming
an endoskeleton, or external, forming an
exoskeleton. Another type of skeleton, found
in invertebrates such as earthworms, is the
hydrostatic skeleton. This gains partial
rigidity from fluid enclosed within a body
cavity. Because the fluid cannot be
compressed, contraction of one part of the
body results in extension of another part,
giving peristaltic motion.