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.
Exoskeletons are hard segments that cover the muscles and visceral organs.
Muscles for movement attach to the inner surface of the exoskeleton. The bulk and weight of the exoskeleton and associated mechanical problems limits the size animals can attain. Spiders use a combination of an exoskeleton for protection and fluid pressure for movement. Image from W.H. Freeman and Sinauer Associates, used by permission. Muscles are on the outside of the endoskeleton.