Phylum Annelida
Segmented Worms
Phylum Annelida
This phylum includes earthworms, leeches, and
marine worms.
Characteristics of Phylum
Annelida
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Annelids exhibit metamerism - regularly
repeating body segments.
Bilateral symmetry
Coelomates
Organ-system Level of Organization
Characteristics of Phylum
Annelida
Closed circulatory system
 Complete digestive system
 Excretory system with nephridia
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Characteristics of Phylum
Annelida
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Cephalization: anterior ganglia
Characteristics of Phylum
Annelida
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Monoecious or dioecious
Trochophore larvae
Spiral cleavage
Mosaic development
Protostomes
Important Characteristics of
Phylum Annelida
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Epidermal,
chitonous setae
(except leeches).
Fleshy appendages
called parapodia
(in polychaetes).
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 Oligochaeta
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Name means “few long
hairs”.
Few setae per body
segment.
Terrestrial and freshwater.
Cephalization but no true
head
No parapodia.
Hermaphroditic/monoecious
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 long.
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.
Earthworms have five (aortic arches)
hearts. The dorsal blood vessel is the
main blood vessel. Blood is contained
within theses vessels (closed circulatory
system)
Earthworm Anatomy
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The excretory system consists of a pair
of nephridia (like 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.
A ventral nerve cords run the length of the
body; each segment has a pair of smaller
ganglia.
Earthworm Anatomy
Earthworm Reproduction
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Earthworms are hermaphrodites and
exchange sperm with each other during
copulation.
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
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).
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 could not 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.