Movement is one of the essential features of living things. Cellular movement is observed in one‐celled amoebas, ciliates, and flagellates. Flagella whip about to produce cellular motion, while cilia beat synchronously to propel a cell.
In animals, movement is essential for locating food and seeking mates. The movement process is centered in the muscle cell, which contracts and relaxes. The contraction yields great force, which is applied against a surface by means of a skeleton.
Skeletal systems provide structure or protection to a variety of organisms. A water-based skeleton provides the structure necessary for movement in worms. The hard external skeleton provides a protective mechanism for many organisms but also assists in movement of insects. The internal skeleton present in many animals provides the structural network for support, protection, and movement.
Many animals have a water-based skeleton, or hydrostatic skeleton. Hydrostatic skeletons do not contain hard structures, such as bone, for muscles to pull against. Rather, the muscles surround a fluid-filled body cavity. In a worm, for example, movement occurs when muscle cells contract, and the contractions squeeze internal fluid (the hydrostatic skeleton) against the skin, causing the worm to stiffen and the body to shorten and widen. The squirming motion of a worm also depends on a hydrostatic skeleton.
A second type of skeleton, the exoskeleton, exists in arthropods and mollusks. In mollusks, the exoskeleton is a hard, protective, outer covering; an example is a clamshell. When a clam's muscles contract, they close the shell rapidly, creating a spurt of water that propels the clam. In arthropods, the exoskeleton also provides protection and movement. Usually, wings are attached by muscles in the hard body surface, which provides a foundation for the muscle contractions. Muscle contractions raise and lower the wings, allowing flight.
1 ) that serves as a point of attachment for muscle. The endoskeleton thus transmits the force of muscle contractions. The endoskeleton also provides support for the body (for example, the legs) and protection (the skull).