Locomotion is the means by which animals move from point to point – crawling or running over hard surfaces; burrowing in sand or soil; flying or swimming. Animals that can move are termed motile, contrasted with those that can't, which are said to be sessile. There are two anatomical features of the vast majority of animals that make locomotion possible: a skeletal system, and a muscle system. The skeletal system is frequently composed of chitin, cartilage, and bone, and provides mechanical levers which are operated by muscles. Soft-bodied animals employ a hydrostatic skeleton composed of water-filled cavities that are distorted by muscular walls to produce movement (see also amoeba).
Among vertebrates there are many variations of the basic locomotor organ, the limb. In birds and bats it has been modified to forma wing , while in various groups, notably snakes, the limbs are lost and the animal moves by undulations of the body. In aquatic animals, the tail is the most important organ of locomotion, the main function of the fins being steering and stabilization.