Ch. 10 Phylum Annelida

Segmented worms

Segmentation

• Divisions of body sections.

• Earthworm has about 100 segments

External earthworm

• Anterior end is darker and more pointed than the posterior.

• Clitellum (enlarged segments that form a band around segments 32-37) is closer to the anterior.

• The mouth and prostomium (upper lip) is at the anterior end.

• Anus is the opening at the posterior end where waste exits.

Setae

• Tiny bristles on the sides of the earthworm used for movement and to cling to the sides of its burrow or underground tunnel.

• Each segment has 4 pairs of setae except for the first and last segments.

• The setae and muscles of the worm work in coordination to push and pull the worm by contracting and extending the body.

Earthworm setae

Internal EarthwormDigestive System

1. Pharynx- muscular organ for sucking in soil that contains food particles.2. Esophagus- long tube that connects the pharynx with the crop.3. Crop-round structure that temporarily stores food.4. Gizzard-round, muscular structure used to grind food. Contractions along with sand particles help physically grind

up the food.5. Intestine- Stretches from segments 19 to anus. This organ chemically breaks down food and allows it to be

absorbed into the bloodstream. Undigested food is released as castings.

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Earthworm castings help to fertilize the soil. The tunnels help

create holes to let oxygen and water to plant roots.

Circulatory System

• Blood carries food and oxygen to all cells.

• The earthworm has a closed circulatory system. Blood is contained inside of tubes.

• Dorsal and ventral blood vessels are main two tubes that carry blood to anterior and posterior regions and back.

• Aortic arches are 5 branching tubes that pump the blood. Not a true heart.

Respiratory System

• Earthworms have a thin skin that is able to absorb oxygen and give off carbon dioxide. The skin is protected by a thin cuticle. The skin must be kept moist by mucus in order for the gases to be diffused.

Excretory system

• Nephridia are tiny tubes located in each segment of the body. They remove cellular waste and clean the blood. (Similar to the function of the human kidney.) Openings on the ventral side called nephridiopores allow the waste to exit the body.

Nervous system

• The brain is found in segment 3. It is attached to a ventral nerve cord. Ganglia, enlarged nerve centers, are found in each segment.

• Earthworms have specialized cells in their skin to respond to light and sound.

Reproductive system

• Earthworms are hermaphroditic. They have both male and female organs in the same body. However, earthworms must cross-fertilize and exchange sperm.

• Sperm is produced in the testes and is stored in the seminal vesicals. Eggs are produced in a pair of ovaries. They leave the ovaries and mature in the body cavity. Earthworms line up side by side and exchange sperm. Sperm from each worm is stored in the seminal receptacles until the eggs are fertilized when laid.

Eggs are released into a slime ring secreted by the clitellum.• Fertilization occurs and the slime ring

slips off the body and forms a cocoon, where the young worms develop.

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Giant earthworm found in Africa and Australia.