Slugs, worms, and many other invertebrate animals have no skeleton, and thus movement is not produced by lever action. Even vertebrates have parts of the body that have muscles but no skeletal component (for example, the tongue). Many soft-bodied animals have muscle systems based on the principle illustrated by a simple wormlike animal. The longitudinal muscle fibres run lengthwise along the body, and the circular fibres encircle it. The body contents are liquids or tissues that can be deformed into different shapes, but they maintain a constant volume. If longitudinal muscles contract and the body shortens, it must widen to accommodate its volume; if the circular muscles contract and the body thins, it must lengthen. Thus, the longitudinal and circular muscles are antagonistic, and shortening of either extends the other. Further, if the length of a circular muscle remains constant while the longitudinal muscle of one side of the body shortens, the body bends, and the longitudinal muscle of the other side is stretched. Thus, the longitudinal muscles of the left and right sides can be antagonistic toward each other. In worms the body fluids render muscles antagonistic through hydrostatic forces. The principle involved is sometimes called the principle of the hydrostatic skeleton.
Tomopterid Worm
The Tomopterid worm is a marine worm.