Muscles only do biological work when they contract; relaxation is a passive process. Therefore, muscles are usually found in antagonistic pairs, so that when one muscle group contracts and performs work the other relaxes. However, in order to perform useful work, such as movement, muscles need something to pull against. In vertebrates, it is the skeleton against which muscles pull, whilst in invertebrates, e.g. annelid worms, it is the substrate across or through which they are moving that muscles pull against.
Hydrostatic skeletons are found in soft bodied invertebrates, such as annelid worms. In some respects, they function in a similar manner to the amoeboid movement found in unicellular animals and other motile cells. Essentially, they consist of fluid enclosed in a body cavity which is surrounded by muscle. In some cases, hydrostatic skeletons have become modified for the purposes of locomotion, e.g. annelid worms. Annelid worms are segmented and have their body cavity surrounded by both circular and longitudinal muscle layers. Circular muscle is arranged around the circumference of the animal, whereas longitudinal muscle is oriented along the length of the animal’s body. Contraction of discrete regions of the circular muscle results in some segments becoming narrower and more elongated, whilst contraction of the longitudinal layer results in some segments becoming shorter and wider. Movement in these animals is achieved by alternate contraction and relaxation of these muscle layers in different body segments (). In order to maximize movement, some of the segments of the worm have bristles called setae protruding from them. These bristles anchor certain regions of the worm to the surface across which it is moving and prevent it from moving backwards in the opposite direction.
Exoskeletons are skeletons on the exterior of the body and are found in molluscs and arthropods. In the case of molluscs, e.g. mussels, clams and other bivalve molluscs, the exoskeleton takes the form of a shell and is primarily used for defensive purposes ― the animal can retreat into the shell to escape predators, for example. In the arthropods, muscle is attached to the exoskeleton and, given that arthropods are segmented and jointed animals, contraction of the muscles allows the animal to move. The arthropod exoskeleton is a continuous structure, although there is tremendous variation in its flexibility; for example, flexibility is increased at joints, without which movement would be very limited. Overlying the exoskeleton, which is made of chitin (a complex polysaccharide), is the cuticle, a waxy substance which is an adaptation to minimize water loss. The biggest problem with such a skeleton is that it limits the animal’s growth. Therefore, in order for arthropods to grow they must periodically shed their exoskeleton and grow a larger, new one. This leaves the animal in a vulnerable position. First, the new exoskeleton provides little defence until it has completely hardened, so the animal is open to increased attack from predators. Second, because the new exoskeleton is soft, movement is limited until it hardens, as muscle needs a rigid structure against which to contract.
Endoskeletons are internal skeletons. These are best developed in the vertebrates, although they are present in some invertebrates, e.g. the echinoderms, where they are composed of calcium salts. In vertebrates, the skeleton serves much the same function as the exoskeleton seen in other phyla, providing protection and a rigid framework against which muscles can contract to produce movement. In most vertebrates the skeleton is made of bone, which is principally made of calcium phosphate. However, there are some animals (e.g. the sharks and rays) which have a skeleton made from cartilage, which is principally composed of collagen ― hence the term cartilaginous fishes by which these animals are known. The main advantage of the endoskeleton compared with the exoskeleton, is that endoskeletons grow continually with the animal, thus eliminating the need for moulting and the problems associated with this.