5.6. Phylum Annelids

Annelids are complex segmented worms. Earthworms, leeches, and many worms of the ocean make up the Annelid phylum. Annelids differ from the other worms in having a true coelom, giving them a “tube- within – a- tube” body plan. Most annelids are free-living. The leeches live as parasites. They attach themselves to animals in lakes and ponds and suck blood and other tissues from them.

The bodies of Annelids are arranged in segments, which can specialize in different tasks. Between segments 35 and 37 lies a swelling called the clitellum, which plays an important role in reproduction. The tissues of these complex worms are organized into many highly developed organ systems. Earthworms have pairs of bristles called setae.

Earthworms

Earthworms, which belong to the class Oligochaeta, live in soils all over the world. Earthworms vary in size from a few centimetres to 3.3 m long. A common North American species has a dark dorsal surface and light ventral surface. Most of the worm’s 100 to 150 segments are identical, except for the pointed anterior and posterior ends. Earthworms have pairs of bristles, called setae on each body segment except the first and last. An earthworm moves by anchoring the setae on its posterior segments and then contracting the circular muscles in front of the anchored segments. These contractions extend the body forward.

Digestion. An earthworm feeds by taking in soil with its muscular pharynx. The soil moves down a tube called the esophagus and then enters a storage chamber called the crop. From the crop, the soil moves to another chamber called the gizzard. Here, the grinding together of soil particles swallowed by the earthworm crushes pieces of organic matter. The food next moves into the intestine which extends to the posterior end of the worm. Folds in the wall of the intestine increase the surface area where absorption of digested food into the bloodstream takes place. Solid wastes pass out of the body through the anus.

Earthworms are very valuable to gardeners and farmers. As the worms eat their way through the soil, they break up soil clumps, aerate the soil, and add nutrients. Earthworms break down organic material faster than normal bacterial decomposition does.

Circulation. Unlike flatworms and roundworms, earthworms have a closed circulatory system. In a closed circulatory system, the blood circulates through a series of vessels. Dorsal and ventral blood vessels run through the length of the worm’s body. The blood absorbs molecules and carries them through the dorsal vessel to five pairs of muscular pumping tubes, or “heats”. These “heats” pump blood into the main ventral blood vessel. Smaller blood vessels carry the blood to all parts of the body.

Respiration and Excretion. Earthworms take in oxygen and give off carbon dioxide by diffusion through the skin. Because diffusion can occur only across a moist membrane, an earthworm must remain in an environment that is neither too wet nor too dry. Earthworms can drown in soil that is saturated with water because there is not enough oxygen. During dry periods, earthworms dig deep into the soil in search of moisture.

Earthworms eliminate liquid wastes through ciliated tubes called nephridia. The beating of cilia draws fluid from the coelom into a funnel-shaped opening in the nephridia. As the fluid passes through the tubules, needed water is reabsorbed by tiny blood vessels. Waste materials then pass out of the body through a pore in the skin. Each body segment has a pair of nephridia.

Nervous Control. An earthworms can respond rapidly to changes in its environment because of a concentration of nerve cells called a cerebral ganglion, near the worm’s anterior end. The cerebral ganglion is connected to the rest of the body by a ventral nerve cord that extends the entire length of the animal. A ganglion connects each body segment to the ventral nerve cord.

The earthworm has no external eyes or ears, but receptors in the skin which enable the worm to react to light, sound and chemicals.

Earthworms are active mainly at night and will move away from bright light. However, the light-sensitive cells of earthworms do not respond to red light. For this reason, earthworms to be used as finishing bait may be most easily dug up at night if red light is used for illumination.

Reproduction

Like planarians, earthworms are hermaphrodites. The female structures are located towards the anterior portion of the earthworm, and the male structures towards the posterior. Fertilization occurs when two worms exchange sperm. A mucous secretion from the clitellum holds the two earthworms together while they mate. The sperm each worm receives is stored in a seminal receptacle until just before the eggs are laid.

Two or three days after mating, the earthworm produces an external mucous case formed of sticky secretions from the clitellum. Muscular contractions push the case along the body. Mature eggs and sperm held in the seminal receptacle enter the case as it passes over the body of the worm. The case then seals forming a coat that protects the fertilised eggs until they hatch.