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Annelids
Segmented Worms
Key Terms
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Septa – internal walls between segments
Seta – bristles attached to each segment
Crop – organ where food is stored
Gizzard – organ where food is ground into
small pieces
Closed circulatory system – blood is
contained within a network of blood vessels
that run from head to tail
Key Terms
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Gill – an organ specialized for breathing
underwater
Nephridium – excretory organs that filter fluid
in the coelem
Clitellum – band of thickened, specialized
segments
Annelids
Body Diagram
Feeding
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Could be filter feeders or predators
Get their food through the pharynx
Earthworms gets food through the pharynx
which pumps food into the crop and gizzard
for grinding and digestion.
Food is absorbed in the digestive tract in the
organ called the intestine.
Respiration, Circulation and Excretion
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Respiration occurs across their skins
Aquatic annelids may breathe through the
gills
Annelids have a closed circulatory system in
which blood is contained in a series of blood
vessels.
Excretion includes digestive wastes that are
passed through the anus. Cellular wastes are
processed through the nephridia that filter
fluid in the coelem.
Defense and Response
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Most annelids have little defense
mechanisms.
Most annelids have a well developed nervous
system that includes a brain and several
nerve cords running through the body from
head to tail.
Reproduction
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Most annelids reproduce sexually.
Some use external fertilization and have
separate sexes.
Most are hermaphrodites but do not fertilize
themselves.
Worms will attach to each other and
exchange sperm into special sacs. During
fertilization, the clitellum secretes a mucus
ring into which the sperm and eggs are
released. Fertilization takes place in this ring.
Worms
Classes of Annelids
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Oligiochaetes – such as an earthworm
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Have relatively few setae compared to other
classes.
Most live in fresh water or soil.
Classes of Annelids
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Polychaetes – marine annelids that have
paired, paddle-like appendages tipped with
setae.
Includes sandworms, bloodworms and their
relatives.
Classes of Annelids
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Hirudina – external parasites that suck the
blood and body fluids of their host.
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Includes leeches which have suckers at both ends
of their bodies.
Secrete anti-coagulants to prevent the blood from
clotting so they can feed.
If they feed well, they may crawl into a cool place
and not feed again for months.
Ecology of annelids
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Worms are important for soils. By burrowing
through the soils, they aerate it and mix it
thoroughly to about 2 meters.
These tunnels allow oxygen into the soil
which the growth of beneficial bacterial and
fungus in the soils.
Aquatic worms are an important food source
in the food web.
Review Question 1
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In earthworms, the clitellum is used for
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A. digestion
B. excretion
C. reproduction
D. respiration
C. Reproduction
Review Question 2
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In the earthworm, waste created by cellular
metabolism is eliminated by the
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A. Crop
B. Nephridia
C. Gizzard
D. Flame cell
B. Nephridia
Conclusion
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Annelids are worms with segmented bodies.
They have a true coelom that is completely
lined with mesoderm.
Oligochaetes are annelids that typically have
streamlined bodies and relatively few setae
ocmpared to ploychaetes. Most oligochaetes
live in soil or water.
Leeches are typically external parasites that
suck the blood and body fluids of their host.
References
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Biology textbook - dragonfly