Slide 1 of 36 Copyright Pearson Prentice Hall 27-3 Annelids

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Copyright Pearson Prentice Hall

27-3 Annelids

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.

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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.

Annelids that feed on decaying vegetation have a pharynx covered with sticky mucus.

Other annelids obtain nutrients by filter feeding.

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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.

The food is absorbed farther along in the digestive tract in the intestine.

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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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Blood in the dorsal (top) vessel moves toward the head of the worm.

The dorsal blood vessel functions like a heart because it contracts rhythmically and helps pump blood.

Dorsal blood vessel

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Blood in the ventral (bottom) vessel runs from head to tail.

Ventral blood vessel

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In each body segment, a pair of smaller blood vessels connect the dorsal and ventral blood vessels and supply blood to the internal organs.

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Respiration

Aquatic annelids often breathe through gills.

A gill is an organ specialized for the exchange of gases underwater.

Land-dwelling annelids take in oxygen and give off carbon dioxide through their moist skin.

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Excretion

Digestive waste passes out through the anus.

Cellular waste containing nitrogen is eliminated by nephridia.

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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 run from the front of the worm to the rear and can contract to make the worm shorter and fatter.

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Circular muscles wrap around each body segment and can contract to make the worm longer and thinner.

The earthworm moves by alternately contracting these two sets of muscles.

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Reproduction

Most annelids reproduce sexually.

Some species use external fertilization and have separate sexes.

Other annelids are hermaphrodites. Two worms attach to each other, exchange sperm, and then store the sperm in special sacs.

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When eggs are ready for fertilization, a clitellum secretes a mucus ring into which eggs and sperm are released.

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

Leeches have powerful suckers at both ends of their bodies that help them cling to their hosts.

Some leeches force a muscular extension called a proboscis into the tissue of their host.

Others slice into the skin with a razor-sharp pair of jaws.

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Groups of Annelids

The leech uses its pharynx to suck blood from the wound.

Some leeches release a substance that anesthetizes the wound—keeping the host from knowing it has been bitten.

Leeches were once used to treat medical conditions.

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Groups of Annelids

Polychaetes

Polychaetes include sandworms, bloodworms, and their relatives.

Polychaetes are marine annelids that have paired, paddlelike appendages tipped with setae.

The setae are brushlike structures on the worm.

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Ecology of Annelids

Earthworms and many other annelids spend their lives burrowing through soil, aerating and mixing it.

Earthworms help plant matter decompose.

Earthworm castings are rich in nitrogen, phosphorus, potassium, micronutrients, and beneficial bacteria.

Ecology of Annelids

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Slide 1 of 36 Copyright Pearson Prentice Hall 27-3 Annelids