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Biology
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27-3 Annelids
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What Is an Annelid?
What are the defining features of 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
Annelids are among the simplest animals to have a true coelom that is lined with mesoderm. Annelids are also called segmented worms because the body is divided into many similar segments.
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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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8. 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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9. 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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10. 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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11. 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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12. 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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13. Form and Function in Annelids
Blood in the ventral (bottom) vessel runs from head to tail.
Ventral blood vessel
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14. 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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15. 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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16. 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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17. Form and Function in Annelids
Nephridia are excretory organs that filter fluid in the coelom.
Nephridia
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18. 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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19. 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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20. 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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21. 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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22. 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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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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27-3
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27-3
In the earthworm, food is absorbed in an organ called the
intestine.
crop.
gizzard.
clitellum.
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27-3
Annelids differ from other worm phyla because they have
segmented bodies and a true coelom.
unsegmented bodies and a pseudocoelom.
mesoderm and one opening in the digestive system.
segmented bodies and one opening in the digestive system.
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27-3
How many major blood vessels does an earthworm have?
one
two
three
none
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27-3
Ecologically, one of the most beneficial functions of an earthworm is to
provide food for tropical fish.
suck the blood of dead animals.
aerate the soil.
kill pest insects that inhabit the soil.
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27-3
In an earthworm, the thickened bank of specialized segments is the
nephridia.
gizzard.
clitellum.
seta.
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37. END OF SECTION