(A) Earthworm

(1) Anatomy of movement

Obtain a slide of a cross section of the common earthworm, Lumbricus, a member of the Class Oligochaeta. You'll notice the well-developed body wall musculature and the spacious coelom. You should also notice the centrally-located digestive tract and ventral nerve cord. Close inspection may reveal the setae projecting from the body wall, which are used by the worm when burrowing. The setae are able to move in and out from the body wall. Now obtain a live earthworm. Watch it for a minute as it moves over a rough surface like a moist paper towel. You can see the rings all along the worm's body. These are external manifestations of the internal segmentation mentioned above. Potentially each segment can be isolated from all the others by a septal membrane (septum, pl = septa) that can be closed around the gut tube via a sphincter muscle.

(2) Have setae, will travel

As the worm moves you will notice waves of peristaltic muscle contractions that move along its body. Which direction are they moving, anteriorly or posteriorly? Each segment's musculature is contracting sequentially, controlled by separate segmental nerves branching off the large ventral nerve cord. During locomotion the septal sphincter muscles remain closed and the septa act as bulkheads to keep the coelomic fluid within each segment constant. When the longitudinal muscles of a segment contract, the circular muscles relax, and because of the incompressibility of the coelomic fluid, the segment becomes shorter and fatter. At the same time the setae are protruded to anchor the worm. When the circular muscles contract, the longitudinal muscles relax and segments become long and thin. When the setae are withdrawn, the body extends, and the worm moves forward.

Notice also the worm will occasionally extend its anterior end forward in a probing motion. The worm uses this motion when burrowing to push its way through the substratum. This is accomplished by isolating the anterior segments from the remaining body coelom and using the contraction of the circular muscles to compress the coelomic fluid and stretch the longitudinal muscles. This pushes the worm's head pile-driver-like through the substratum while the posterior of the worm is anchored with protracted setae embedded in the burrow wall.

Dissect a preserved specimen of an earthworm and familiarize yourself with the main internal parts of the animal.

The earthworm represents the ultimate development of a hydrostatic skeleton. The compartmentalized coelom and segmental musculature allow the earthworm to burrow through the substratum very effectively. In fact most zoologists consider that the coelom evolved as an adaptation for burrowing. But what about locomotion above ground? What would you add? How about well-developed segmental appendages to grip the substrate surface, and while we're at it add a third set of muscles that run diagonally to the longitudinal and circular muscles? Throw in an anterior region well-endowed with sensory structures to assist in an above-ground existence and you have created an annelid known as a polychaete, a member of the annelid Class Polychaeta.

(B) Polychaeta

(1) Have parapodia, will travel

Obtain a cross section of a marine polychaete like Nereis or Neanthes. You'll note that each segment has a pair of lateral appendages called parapodia (para = like; pod = foot). Extending out from the parapodia are bristle-like bundles of setae, which, like those of the earthworm, the polychaete can extend and retract. You'll also notice the oblique muscles that connect diagonally to the body wall and the ventral mid-line. If available, obtain a live polychaete like Nereis. Place the animal on a wet paper towel and watch it move. Does the body undulate in a snake-like fashion? Watch the flexures of the body and the successive positions of the parapodia. Do parapodia on opposite sides of the body move synchronously in the same direction? Do all the parapodia on one side of the body move simultaneously? If not, how do they move? How are the movements of the parapodia related to twisting of the body? How is gripping the substratum achieved? Ask yourself what muscles are involved in these movements. It may help to know that this type of movement is known as parapodial stepping.

Also take a close look at the polychaete's head. You'll notice structures called palps, peristomal cirri, prostomial tentacles, and even eyes. This is an advanced stage of cephalization where the receptors for taste, smell, touch and light are all concentrated at the anterior end. Why so much more so in the polychaete than in the earthworm?

(2) Polychaetes and Arthropods, important evolutionary links?

Now that you have seen an animal that has an improved hydrostatic skeleton for movement above ground, ask yourself how you might improve on this body plan. What could you add to achieve more effective locomotion? The segmental body plan appears to be functional and successful as far as worms are concerned, but how could you improve on this? Perhaps by making the body wall firmer, maybe even strengthening it by some sort of rigid skeleton? How could you improve on the fleshy segmental appendages of the polychaete? Perhaps by making them more rigid also, and even adding joints so they would bend only in specific planes? We have just laid the ground work for the archetypical arthropod, a segmented group of animals with a rigid exoskeleton and jointed appendages, thought to have evolved from an annelid-like ancestor. They are also the most successful animal phylum, and we will study them in detail in a future lab.