BIOLOGY Volume 2 - A Guide to General Biology - 2004

18. THE ANIMAL SKELETAL AND MUSCULAR SYSTEMS

18.5. Locomotion in Selected Invertebrates

18.5.1. Locomotion in the Earthworm (Lumbricus terrestris)

Muscular locomotion is only possible in the presence of a Skeleton. In Annelids, such as the earthworm, this is a hydrostatic skeleton, meaning it is formed by coelomic fluid whose pressure maintains and alters the body shape (Sec. 18.1.2). The body cavity (coelom) (Sec. 2.10.5) of the earthworm is enclosed by walls consisting of two antagonistic Muscle layers: an outer circular layer and an inner longitudinal layer. Intersegmental partitions, or septa, divide the animal's circular musculature into independent units, whereas the longitudinal Muscles span several segments. Locomotion is achieved through the coordinated activity of both muscle layers, as well as the muscles that move the setae located on the ventral surface of the worm's body.

During forward movement in the earthworm, the contraction of the circular muscles begins at the anterior end of the body and, encompassing segment after segment, propagates as a wave down the entire length of the body. The contracted muscles in each segment exert pressure on the coelomic fluid, which stretches the relaxed longitudinal muscles and alters the shape of the segment, making it longer and thinner. As a result, the anterior end of the worm advances forward. The setae, present on all body segments except the first and last, are retracted during circular Muscle contraction and thus do not impede forward progression (Fig. 18.25).

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Fig. 18.25. Locomotion of the earthworm.

As the anterior end moves forward, the longitudinal muscles at the posterior end contract, and this part of the body thickens and presses against the surrounding soil. The setae here are extended to help anchor this section in place. This is of particular importance during burrowing, as it enables the worm to exert considerable force against the soil as it pushes forward.

Following the circular muscles, the longitudinal muscles rapidly begin to contract, and this contraction wave also travels the entire length of the body. Thus, at any given time, different PARTS OF THE body may move forward (when the circular muscles contract) or remain stationary (when the longitudinal muscles contract). The net effect is a smooth peristaltic wave of contractions moving along the entire body of the worm as it crawls forward. By reversing The sequence of muscle contractions (i.e., the direction of the wave), the worm is capable of crawling backward.

Muscle activity is controlled by a complex network of intra- and intersegmental Neurons. The ventral nerve cord runs through the segments, with each segment containing its own segmental nerves. This allows for the control of both individual segments and the Organism as a whole (Fig. 18.26).

Fig. 18.26. Segmental Innervation of the longitudinal musculature of the earthworm (dorsal view). The circular muscles are innervated in a similar manner.

Running through the ventral nerve cord is a giant axon that extends the entire length of the animal's body and conducts impulses from the anterior end to the posterior end. When sensory receptors at the HEAD are stimulated, impulses travel along the giant axon to the longitudinal muscles, causing them to contract and thereby withdrawing the anterior end of the body away from the stimulus. The ventral cord also contains two longitudinal lateral fibers that conduct impulses from the posterior end to the head. If a stimulus acts near the posterior end, impulses are transmitted via the lateral nerves to the head, prompting the rear part of the body to be pulled forward. This forms The basis of the escape reactions in the earthworm.