Muscles can contract and exert force, or they can relax. To create significant movement, they must have something to pull on and something that stretches the muscle back to a longer position. In some cases muscles pull on each other, as in the trunk of an elephant or the arms of an octopus. In most cases, however, skeletal systems are the rigid supports against which muscles pull to create directed movement. In this section we will examine the three types of skeletal systems: hydrostatic skeletons, exoskeletons, and endoskeletons.
focus your learning
Three types of skeletons have evolved in animals, and each provides resistance against forces exerted by muscles to enable locomotion.
Three cell types, osteoblasts, osteoclasts, and osteocytes are responsible for the development and continuous remodeling of bones of vertebrates.
Bones and muscles act together across joints to form levers that produce specific types of motion.