Worms are known for their soft, elongated bodies. Their appearance often leads to questions about how they maintain shape and move without a rigid framework. This article explores the anatomy that allows these animals to thrive.
The Straight Answer: No Bones
Worms do not possess bones or any traditional rigid skeletal system. This distinguishes them from vertebrates, which rely on an internal framework of bones for support and movement. The absence of a hard skeleton is a defining feature of many invertebrates. This structural difference often leads to questions about how these soft-bodied animals manage to hold their form and navigate their surroundings.
The Hydrostatic Skeleton: How Worms Are Structured
Instead of bones, many worms utilize a hydrostatic skeleton, a flexible system supported by fluid pressure. This internal support system consists of a fluid-filled cavity, known as a coelom, surrounded by layers of muscles. The fluid within this cavity is largely incompressible, meaning it resists changes in volume. When the muscles surrounding the coelom contract, they exert pressure on this fluid. This pressure provides internal support, allowing the worm to maintain its cylindrical shape and to stiffen its body.
The muscular layers typically include circular and longitudinal muscles. Circular muscles wrap around the body, while longitudinal muscles run along its length. These muscles work in opposition; when one set contracts, the other relaxes, manipulating the fluid pressure within the coelom. This arrangement enables the worm to change its body shape, becoming longer and thinner or shorter and fatter.
Movement and Function Without a Rigid Skeleton
Building on the principle of the hydrostatic skeleton, worms move through peristalsis, a process involving wave-like muscle contractions. To move forward, circular muscles in a segment contract, making that part thinner and longer and pushing it forward. Longitudinal muscles in a trailing segment then contract, making that part shorter and wider, which helps to anchor it and pull the rest of the body forward.
Many worms, such as earthworms, possess tiny bristle-like structures called setae on their segments. These setae extend and retract to grip the substrate, preventing backward slippage and providing traction. This coordinated action of muscle contractions and anchoring allows worms to burrow through soil, squeeze into tight spaces, and navigate their environment. This flexibility and ability to exert force through changes in body shape are also useful for functions like feeding and waste expulsion.