Animals perform a variety of movements in order to survive and reproduce in a particular environment. The movement of the body as a whole from one place to the other is called locomotion. This may take the form of walking, running, flying, etc. Movements enable the animal to search out and procure food, escape from predators of unfavourable environmental conditions and also to find its mate for reproduction.
Animals also perform another variety of movements in which a part of the body is only moved. The vital processes such as the contraction of the heart, constriction of the blood vessels, breathing, peristalsis of the digestive tract are all movements of this type.
All the movements of the body are brought about by the extensive system called the muscular system. Beside movements, the muscular system also helps the animals to maintain the particular posture against the effects of gravity. The muscular system consists of different types of muscles in the body and the muscles are composed of thousands of elongated fibres or cells organised in a variety of ways and bound together by connective tissue.
Simple multicellular animals such as Hydra do not have an extensive muscular system. However, certain cells in the body wall are specialised to perform the contraction and relaxation with the help of which the animals perform various movements. These cells are called musculo-epithelial cells. In multicellular animals like the earthworm and leech there are circular and longitudinal muscles in the body wall, the contractions and relaxations of which enable the animals to move about.
In higher multicellular animals the movements of the muscles are supported by a rigid skeletal system to which the muscles are attached. In arthropods, such as insects a tough chitinous exoskeleton covers the entire body and its appendages. Muscles are attached to the inside of this exoskeleton so that it can be moved. In vertebrates the muscular system is in close association with the internal skeletal system. The skeletal system of vertebrates is composed of a number of different bones which are specialised, rigid connective tissue.