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Biology
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27-3 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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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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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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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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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. -Leeches
Annelids that feed on decaying vegetation have a
pharynx covered with sticky mucus. -Earthworms
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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.
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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
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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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
Circular muscles
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Form and Function in Annelids
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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Form and Function in Annelids
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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In the earthworm, food is absorbed in an organ
called the
a. intestine.
b. crop.
c. gizzard.
d. clitellum.
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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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How many major blood vessels does an
earthworm have?
a. one
b. two
c. three
d. none
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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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In an earthworm, the thickened bank of
specialized segments is the
a. nephridia.
b. gizzard.
c. clitellum.
d. seta.
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