Chapter 11, Phylum Annelida
(Segmented Worms)
Phylum Annelida
Phylum Annelida
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Annelids exhibit metamerism.
Metamerism is a characteristic that features
regularly repeating body segments.
This phylum includes earthworms, leeches, and
marine worms.
Annelids are sometimes called “Bristle worms”,
because of the tiny chitonous bristles that they
bear (except leeches) called setae.
Important Characteristics of
Phylum Annelida
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Metamerism or
segmentation.
Epidermal,
chitonous setae
(except leeches).
Fleshy appendages
called parapodia
in some annelids.
Coelomate body
cavity.
Annelid Ecology
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Annelids are found in the ocean, freshwater,
and in terrestrial soil.
Often live in burrows in the ground and feeds
on organic matter in soil (earthworms).
Others (polychaetes) feed on suspended
particles that are trapped on parapodia.
Some are predators, and hunt by hiding in
rocks and corals, ambushing their prey.
Others are bloodsucking parasites (leeches).
Annelid Ecology
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Earthworms are extremely important for
terrestrial ecosystems.
Without earthworms, plants would not be
able to grow, and the entire food chain
would be affected.
Earthworms help aerate, fertilize, mix, and
provide proper drainage to soils.
Medical Purposes for Leeches
Leeches are sometimes used in the medical field
when fingers or toes are cut off. Leeches can help
stop the bleeding at the site of the wound while
veins and arteries are still healing.
Annelid Anatomy and Locomotion
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The body of an annelid usually consists of a twopart head (prostomium and peristomium), a
series of body segments (metameres), and a
terminal segment called the pygidium.
Annelids have both longitudinal and circular
muscles, which create waves of expansion and
contraction called peristalsis.
Class Polychaeta
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Name polychaete means “many + long hairs”.
Many setae per body segment.
Mostly marine and dioecious.
Most segments with parapodia used for crawling,
swimming, feeding and respiration.
Examples include the clam worms, scale worms,
fireworms, and fanworms (or featherdusters).
Class Oligochaeta
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Name means “few long
hairs”.
Few setae per body
segment.
Terrestrial and freshwater.
No head or parapodia.
Hermaphroditic
Example, earthworms
Earthworms
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Earthworms (nightcrawlers), burrow in
moist soil and emerge at night to feed on
detritus and vegetation and to breed.
Usually about 5-12 inches long.
Some giant tropical earthworms can get 3
– 4 meters.
Earthworm Anatomy
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Setae project through small pores in the
cuticle to provide anchoring points when
the earthworm moves or burrows.
Food is brought in by a muscular pharynx.
The digestive tract is unsegmented and
runs continuously the length of the body.
The intestine has a u-shaped fold called a
typhlosole that increases surface area.
Earthworm Anatomy
Earthworm Anatomy
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Earthworms also have a crop which
stores food, and a gizzard, which
mechanically breaks down food.
Annelids have closed circulatory systems.
Earthworms have five (aortic arches)
hearts. The dorsal blood vessel is the
main blood vessel.
The excretory system consists of a pair of
nephridia (similar to our kidneys) in
each body segment (except the first
three and the last one).
Earthworm Anatomy
Earthworm Anatomy
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The nervous system in annelids consist of a pair
of cerebral ganglia.
Posterior to the cerebral ganglia, ventral nerve
cords run the length of the body and each body
segment has its own pair of smaller ganglia.
Earthworms are hermaphrodites and exchange
sperm with each other during copulation.
When earthworms mate, they come out of their
burrows at night and touch ventral surfaces.
Earthworm Anatomy
Earthworm Reproduction
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Mating earthworms are held together by mucus
secreted by an organ called the clitellum, and
they are also held together by ventral setae.
Sperm are discharged and travel to an opening
called the seminal receptacle.
After sperm exchange, the worms separate.
Each worm then secretes a protective covering
over the clitellum that forms a cocoon.
Earthworm Reproduction
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The cocoon slides forward along the body.
As it slides forward, it passes over the oviducts.
Eggs from the oviduct and sperm from the
seminal receptacles transfer into the cocoon.
After fertilization the cocoon slides off the worm.
The embryos develop in the cocoon and
eventually emerge as juvenile worms.
Earthworm Reproduction
Class Hirudinea
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Name means leeches
Mostly freshwater, some marine and
some terrestrial.
Body usually with posterior and anterior
suckers that secrete anticoagulants and
consume blood.
No parapodia or setae
Clitellum is present
Hermaphroditic