Download 27-3 Annelids

27-3 AnnelidsSegmented Worms
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27-3 Annelids
What Is an Annelid?
What are the defining features of
annelids?
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27-3 Annelids
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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27-3 Annelids
What Is an Annelid?
Three Germ Layers of an Annelid
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27-3 Annelids
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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27-3 Annelids
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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27-3 Annelids
Form and Function in Annelids
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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27-3 Annelids
Form and Function in Annelids
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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27-3 Annelids
Form and Function in Annelids
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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27-3 Annelids
Form and Function in Annelids
Circulation
Annelids typically have a closed circulatory
system, in which blood is contained within a
network of blood vessels.
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Form and Function in Annelids
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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27-3 Annelids
Form and Function in Annelids
Blood in the ventral (bottom) vessel runs from head
to tail.
Ventral blood
vessel
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27-3 Annelids
Form and Function in Annelids
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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Form and Function in Annelids
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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27-3 Annelids
Form and Function in Annelids
Excretion
Digestive waste passes out through the anus.
Cellular waste containing nitrogen is eliminated by
nephridia.
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27-3 Annelids
Form and Function in Annelids
Nephridia are excretory organs that filter fluid in the
coelom.
Nephridia
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Form and Function in Annelids
Response
Most annelids have a well-developed nervous
system consisting of a brain and several nerve
cords.
Brain
Ganglia
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Form and Function in Annelids
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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27-3 Annelids
Form and Function in Annelids
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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Form and Function in Annelids
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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Form and Function in Annelids
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
What are the characteristics of the three
classes of annelids?
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Groups of Annelids
Groups of Annelids
Annelids are divided into three classes
• oligochaetes
• leeches
• polychaetes
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27-3 Annelids
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
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.
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