An earthworm is a segmented worm that lives in soil. Through its tunneling and excretions, the worm aerates and adds nutrients to the soil as it feeds on organic soil matter. It has a structural adaptation that enables it to move into small spaces with the flexibility needed to smoothly coil and bend.
The earthworm is not comprised of a smooth tube with a single interior cavity, but is divided into small segments that give it a ridged or ringed appearance. These segments are cylinder-shaped compartments, separated by partitions, with walls made up of layered muscle fibers and connective tissues. Each segment is filled with a fixed volume of incompressible fluid. Because of this, while the shape of each segment can change, the volume remains constant.
Segment diameter and length are controlled by two types of muscles layered under the worm’s skin. Circular muscles are wrapped around the circumference of each segment and control the diameter. Longitudinal muscles extend down the length of the segment and control the length. Connective tissue fibers reinforce the muscular wall of the hydrostatic skeleton, and are organized in parallel sheets that are wound in both right and left-hand helices.
The circular and longitudinal muscles operate in opposition to each other. When the longitudinal muscles contract and shorten, they decrease the segment’s length; the fixed volume of fluid contained within the segment then moves around and causes the diameter to increase. The opposite happens when the circular muscles contract; the segment’s diameter decreases and the length increases. The angled helical fibers are the connective tissue mesh that enables smooth bending as the animal wriggles through soil by providing a supportive fibrous net that prevents kinking or collapsing. This type of skeleton, where muscle forces are applied to internal compartments of fluid under pressure, is called a hydrostatic skeleton. Sequentially contracting the muscles of each segment, which can work independently of each other, enables the earthworm to move into small spaces between soil particles.
This summary was contributed by Sue White.
“Flexible cylinders make body skeletons which have enormous advantages when it comes to moving around: a considerable volume of body can be passed through a small space — hence the earthworm burrowing through the ground… Provided the constructive material of a cylinder is flexible enough, the cylinder can be bent round corners…” (Foy and Oxford Scientific Films 1982:21)
The grand design: Form and colour in animalsNovember 20, 1983
“A wide range of animals and animal structures lack the rigid skeletal elements that characterize the skeletons of familiar animals such as the vertebrates and the arthropods. Instead, these animals rely on a ‘hydrostatic skeleton’ in which the force of muscle contraction is transmitted by internal pressure.” (Kier 2012:1247)
“The circular and longitudinal muscle fibers antagonize one another and, depending on the sequence of contraction, can be used to generate a diverse range of movements. The musculature can be used to produce alternating waves of elongation and shortening… used in crawling and locomotion…In annelids such as the earthworm, the coelom is divided into segments by muscular septa, which prevent movement of the hydrostatic fluid from one segment to another during normal locomotion (Newell, 1950; Seymour, 1969; Seymour, 1976). Such subdivision of the coelom [body cavity] allows individual segments to operate independently, thereby facilitating localized action and a more complex and variable pattern of movement.” (Kier 2012:1250)
“The septa [walls dividing segments] are thought to allow the pressure to be different in one location (or compartment) in the body compared with another… thus the high pressure of a localized maximal contraction of the longitudinal muscle (perhaps required to enlarge the burrow) will not be transmitted down the length of the worm and overwhelm the circular muscle in another part of the body… Subdivision of the coelom [body cavity] also provides protection from injury because rupture of the wall incapacitates an individual segment rather than the entire hydrostatic skeleton.” (Kier 2012:1251)