Biology - Discover Math and Science Now

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Copyright Pearson Prentice Hall

Biology

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27-3 Annelids

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What Is an Annelid?

What Is an Annelid?

Annelids are worms with segmented

bodies. They have a true coelom that is

lined with tissue derived from

mesoderm.

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What Is an Annelid?

Three Germ Layers of an Annelid

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What Is an Annelid?

The body of an annelid is divided into segments.

Each segment is separated by septum, which are internal walls between each segment.

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What Is an Annelid?

Body segments may carry eyes, antennae, other

sense organs, or be specialized for functions such as

respiration.

Bristles called setae may be attached to each

segment.

Annelids have a tube-within-a-tube digestive tract

that food passes through from the mouth to the anus.

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Form and Function in Annelids


Annelids have complex organ systems.

Many of these systems are unique because of the

segmented body plan of this group.

Resources/Movies/vaearthw.mov

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Feeding and Digestion

In carnivorous species, the pharynx usually holds

two or more sharp jaws that are used to attack

prey. -Leeches

Annelids that feed on decaying vegetation have a

pharynx covered with sticky mucus. -Earthworms

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In earthworms, the pharynx pumps food and soil into

the esophagus.

The food then moves through the crop, where it can

be stored.

It then moves through the gizzard, where it is ground

into smaller pieces.

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Circulation

Annelids typically have a closed circulatory

system, in which blood is contained within a

network of blood vessels.

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Respiration

Aquatic annelids often breathe through gills.

Land-dwelling annelids take in oxygen and give off

carbon dioxide through their moist skin. Diffusion

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Nephridia are excretory organs that filter fluid in the

coelom.

Nephridia

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Response

Most annelids have a well-developed nervous

system consisting of a brain and several nerve

cords.

Ganglia

Brain

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Movement

Annelids have two groups of body muscles that

function as part of a hydrostatic skeleton.

Longitudinal muscles

Circular muscles

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Reproduction

Most annelids reproduce sexually.

Earthworms are hermaphrodites. Two worms

attach to each other, exchange sperm, and then

store the sperm in special sacs.

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A clitellum is a band of thickened, specialized

segments.

After eggs are fertilized in the ring, the ring slips off

the worm's body and forms a protective cocoon.

Young worms hatch weeks later.

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Groups of Annelids

Groups of Annelids

Annelids are divided into three classes

• oligochaetes

• leeches

• polychaetes

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Groups of Annelids

Oligochaetes

Oligochaetes contains earthworms and their

relatives.

Oligochaetes typically have streamlined

bodies and relatively few setae compared

to polychaetes. Most oligochaetes live in

soil or fresh water.

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Groups of Annelids

Leeches

The class Hirudinea contains the leeches.

Leeches are typically external parasites

that suck the blood and body fluids of their

host.

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Groups of Annelids

Polychaetes

Polychaetes are marine annelids that have

paired, paddlelike appendages tipped with

setae.

The setae are brushlike structures on the

worm.

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27-3

Resources/Movies/vaearthw.mov

Resources/ch27_sectn03_quiz.qtb

Resources/ch27_sectn03_quiz.qtb

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27-3

In the earthworm, food is absorbed in an organ

called the

a. intestine.

b. crop.

c. gizzard.

d. clitellum.

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27-3

Annelids differ from other worm phyla because they have

a. segmented bodies and a true coelom.

b. unsegmented bodies and a pseudocoelom.

c. mesoderm and one opening in the digestive system.

d. segmented bodies and one opening in the digestive system.

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27-3

How many major blood vessels does an

earthworm have?

a. one

b. two

c. three

d. none

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27-3

Ecologically, one of the most beneficial functions

of an earthworm is to

a. provide food for tropical fish.

b. suck the blood of dead animals.

c. aerate the soil.

d. kill pest insects that inhabit the soil.

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27-3

In an earthworm, the thickened bank of

specialized segments is the

a. nephridia.

b. gizzard.

c. clitellum.

d. seta.

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Biology - Discover Math and Science Now

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