The single-celled protozoan ancestors of animals had their weight supported by water and were able to move by cilia or other simple organelles. Animals use their muscular and skeletal systems for support, locomotion, and maintaining their shape. This movement is a result of contraction of muscles. The skeleton helps transmit that movement. Skeletons are either a fluid-filled body cavity, exoskeletons , or internal skeletons.
Hydrostatic skeletons consist of fluid-filled closed chambers.
Internal pressures generated by muscle contractions cause movement as well as maintain the shape of the animals, such as the sea anemone and worms. The sea anemone has one set of longitudinal muscles in the outer layer of the body, and a layer of circular muscles in the inner layer of the body. The anemone can elongate or contract its body by contracting one or the other set of muscles. Images from W.H. Freeman and Sinauer Associates, used by permission. Exoskeletons are characteristic of the Phylum Arthropoda.