Class Oligochaeta

Annelids of the class Oligochaeta generally live in the soil or in fresh water and have no parapodia. Oligochaeta means "few bristles," and as the name suggests, these annelids have a few setae on each segment. The most familiar member of the class Oligochaeta is the earthworm. As you read about the earthworm, look for adaptations that enable this animal to lead a burrowing life.

objectives

• Identify the structures that provide the basis for dividing annelids into three classes.

• List the advantages of body segmentation.

• Describe the structural adaptations of earthworms.

• Compare the three classes of annelids.

vocabulary seta parapodium crop gizzard typhlosole aortic arch nephridium clitellum seminal receptacle chitin figure 35-10

Numerous setae help this bearded fireworm, Hermodice carunculata, move through its environment. The setae extend from fleshy flaps called parapodia. Also known as bristle worms, bearded fireworms are members of the class Polychaeta.

figure 35-10

Numerous setae help this bearded fireworm, Hermodice carunculata, move through its environment. The setae extend from fleshy flaps called parapodia. Also known as bristle worms, bearded fireworms are members of the class Polychaeta.

Parapodium Virus

Intestine' Nephridia Setae Gizzard Crop Esophagus Ganglia

Clitellum Circular muscle' Longitudinal muscle

Intestine' Nephridia Setae Gizzard Crop Esophagus Ganglia

Dorsal blood vessel Aortic arches

Pharynx

Cerebral ganglion

Mouth Ventral nerve cord Ventral blood vessel figure 35-11

The segmentation of annelids, such as this earthworm, is visible both externally and internally. Some of the internal structures, such as ganglia and nephridia, are repeated in each segment.

www.scilinks.org Topic: Annelids Keyword: HM60078

www.scilinks.org Topic: Annelids Keyword: HM60078

Structure and Movement

An earthworm's body is divided into more than 100 segments, most of which are virtually identical. Figure 35-11 shows that circular and longitudinal muscles line the interior body wall of an earthworm. To move, the worm anchors some of the middle segments by their setae and contracts the circular muscles in front of those segments. Contraction of the circular muscles increases the pressure of the coelomic fluid in those segments. This increased pressure elongates the animal and pushes the anterior end forward. Setae in the anterior segments then grip the ground as the longitudinal muscles contract, pulling the posterior segments forward. This method of locomotion in earthworms is an example of the kind of movement made possible by segmentation.

Feeding and Digestion

Earthworms ingest soil as they burrow through it. Soil is sucked into the mouth by the muscular pharynx. The soil then passes through a tube called the esophagus (ee-SAHF-uh-guhs) to a temporary storage area known as the crop. From the crop, the soil moves to a thick, muscular part of the gut called the gizzard. Find these parts of the digestive tract in Figure 35-11. The gizzard grinds the soil, releasing and breaking up organic matter. As the soil passes through the long intestine, digested organic compounds and nutrients in the soil are absorbed by the blood. An infolding of the intestinal wall called the typhlosole (TIF-luh-soHL) increases the surface area available for digestion and absorption. Undigested material is eliminated from the earthworm's body through the anus.

Earthworms play an important role in maintaining the fertility of soil. By decomposing dead leaves and other organic materials, earthworms help release nutrients into the soil. The burrows made by earthworms allow air to penetrate into the soil, bringing oxygen to plant roots and soil microorganisms. Earthworms also loosen the soil, making it easier for roots to grow and for water to seep into the soil.

Circulation

A closed circulatory system transports oxygen, carbon dioxide, nutrients, and wastes through the body of an earthworm. The blood travels toward the posterior end through a ventral blood vessel and then returns to the anterior end through a dorsal blood vessel. As you can see in Figure 35-11, five pairs of muscular tubes, the aortic (ay-AWR-tik) arches, link the dorsal and ventral blood vessels near the anterior end of the worm. Contractions of the dorsal blood vessel and the aortic arches force blood through the circulatory system.

Respiration and Excretion

Oxygen and carbon dioxide diffuse directly through the skin, which contains many small blood vessels. This exchange of gases can take place only if the skin is moist. Therefore, earthworms avoid dry ground and extreme heat. Secretions of mucus and the presence of a thin cuticle also help keep an earthworm's skin moist.

Earthworms eliminate cellular wastes and excess water through excretory tubules called nephridia (nee-FRID-ee-uh), some of which are shown in Figure 35-11. Each segment, except the first three and the last one, contains a pair of nephridia. As coelomic fluid passes through the nephridia, some of the water is reabsorbed by blood vessels. The remaining fluid and the wastes dissolved in it are released from the body through pores on the ventral surface.

Neural Control

The nervous system of an earthworm consists of a chain of ganglia connected by a ventral nerve cord. Most body segments contain a single ganglion. Nerves branching from each ganglion carry impulses to the muscles and from the sensory cells in that segment. In the most anterior segments, several ganglia are fused to form the cerebral ganglia, or brain, as you can see in Figure 35-11. One of the main functions of the cerebral ganglia is to process information from sensory structures that respond to light, touch, chemicals, moisture, temperature, and vibrations. Although these sensory structures are found in all segments, they are concentrated at the anterior end.

Reproduction

Earthworms are hermaphrodites, but an individual worm cannot fertilize its own eggs. Mating occurs when two earthworms press their ventral surfaces together with their anterior ends pointing in opposite directions. The worms are held together by their setae and by a film of mucus secreted by each worm's clitellum (klie-TEL-uhm). The clitellum, also shown in Figure 35-11, is a thickened section of the body. Each earthworm injects sperm into the mucus. The sperm from each worm move through the mucus to the seminal receptacle of the other, where the sperm are stored. The worms then separate. After several days, the clitellum of each worm secretes a tube of mucus and a tough carbohydrate known as chitin (KIE-tin).

Word Roots and Origins nephridium from the Greek nephros, meaning "kidney," and idion, meaning "small"

As this tube slides forward, it picks up the worm's eggs and the stored sperm from the other worm. Fertilization occurs inside the tube, which closes up to form a protective case. The young worms develop inside the case for 2-3 weeks before hatching.

figure 35-12

The leech Haemadipsa sp. is a parasite that sucks blood from animals, including humans. Other leeches are free-living carnivores.

figure 35-12

The leech Haemadipsa sp. is a parasite that sucks blood from animals, including humans. Other leeches are free-living carnivores.

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