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Annelids: Segmented Worms
Invertebrate Zoology
Phylum Annelida means
“Little Rings”
Test Objectives for Annelids
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Objective 5: Describe the form and function of the
Annelida
Objective 6: Differentiate between the classes of
Annelida (Polychaeta, Oligochaeta, Hirudinea, )
Objective 7: Describe the Internal form and function of
the following classes: Polychaeta, Oligochaeta,
Hirudinea.
Objective 8: Describe the life cycle and reproductive
strategies for Molluscs and Annelids
Taxonomy
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Phylum Annelida
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Class
Polychaeta
Class
Oligochaeta
Class
Hirudinida
Phylum Echiura
 Phylum
Sipuncula
Phylum Annelida

Annelids are
 protostome coelomates

Nervous system more
centralized & circulatory
system more complex than
in previous phyla (due to
evolutionary milestone).
Phylum Annelida
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Annelids are segmented worms.
Segments marked with circular rings called
annuli or metameres
Examples:
 earthworms
 leeches
 clam worms
Phylum Annelida
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The evolutionary milestone shown by
annelids is
segmentation (metamerism).
 Segmentation evolved separately in
annelids, arthropods, and chordates.
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The body is divided into a series of
segments, each having similar
components of all major organ systems.
Built in fail-safe (if parts of body fail!).
 Allows for specialization.
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Phylum Annelida
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Many annelids have
chitinous bristles
called setae.
 Help in locomotion
 Anchor worm in
place
 Deter predators
Phylum Annelida
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Diverse Phylum
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15,000 species
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2/3rd are polychaetes (marine worms)
Annelids can be found worldwide in
marine, freshwater, and terrestrial
habitats.
Phylum Annelida – Body Plan

3 part body plan
 Prostomium –
anterior part
 followed by
Segmented Body.
 Pygidium – terminal
portion.
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Segments added on
at the pygidium
Phylum Annelida – Body Plan
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Peritonium (layer of
mesodermal
epithelium) of adjacent
segments meet to form
septa (dividing walls).
 Fluid-filled coelom acts
as a hydrostatic
skeleton.
 Coelomic lining holds
in the organs
Phylogeny
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Traditionally, annelids are divided among
3 classes:
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Class Polychaeta (marine worms)
Class Oligochaeta (segmented worms)
Class Hirudinida (leeches)
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Oligochaeta and Hirudinida form a
monophyletic group called Clitellata.
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Characterized by reproductive structure
called a clitellum.
Class Polychaeta
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Polychaeta is
the largest,
most diverse
class.
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May be
brightly
colored,
variable
shape.
10,000
species in the
ocean
Class Polychaeta
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Polychaetes have
some features other
annelids do not:
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A well developed
head.
Paired appendages,
parapodia, that
function as gills and
aid in locomotion.
No clitellum!
Many setae
Class Polychaeta
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Polychaetes are
mostly marine and
mostly benthic.
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May live under
rocks, burrow into
sediment, or build
their own tubes.
Some are
planktonic.
Euryhaline: can
tolerate salt
Class Polychaeta
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Sedentary and
errant (freemoving) forms.
 Sedentary forms
often have
elaborate devices
for feeding and
respiration.
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Filter or
deposit
feeders.
Class Polychaeta
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Errant forms
include pelagic
and benthic
types and are
often predators
or scavengers.
Class Polychaeta - Reproduction
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Gonads are temporary structures in polychaetes.
Sexes usually separate.
Fertilization is external.
Early larva is a trochophore.
Circulation and Respiration
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Most have parapodia and “gills” for
gaseous exchange.
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Circulation varies.
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Others use the body surface.
In Nereis a dorsal vessel carries blood forward and
a ventral vessel carries blood posteriorly.
Blood flows across between these major vessels in
networks around the parapodia and intestine.
In some, septa are incomplete and coelom
fluid serves circulatory function.
Excretion
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Excretory organs vary, from protonephridia to
metanephridia, and mixed forms.
 One pair per metamere.
Nervous System and Sense Organs
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Double ventral nerve cord runs length of the
worm with ganglia in each metamere.
 Sense organs include:
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Eyes, nuchal organs and statocysts (balance).
Eyes vary from simple eyespots to well-developed
image-resolving eyes similar to mollusc eyes.
Nuchal organs are ciliated sensory pits that are
probably chemoreceptive.
Some burrowing and tube-building polychaetes use
statocysts to orient their body.
Representative Polychaetes
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Clam Worms: Nereis
 Errant(free moving) polychaetes
 Live in mucus-lined burrows near low
tide level.
 Come out of hiding places at night to
search for food.
 Prostomium bears a pair of palps
sensitive to touch and taste, a pair of
short sensory tentacles, and two
small dorsal eyes sensitive to light.
 Peristomium has a ventral mouth, a
pair of jaws, and four pairs of
sensory tentacles.
Representative Polychaetes
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Scale worms
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Flattened bodies
are covered with
broad scales.
Some are large, all
are carnivores and
some are
commensals in
burrows of other
organisms.
Representative Polychaetes
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Fireworms
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Have hollow, brittle
setae that contain
poisonous
secretions.
Feed on cnidarians.
Representative Polychaetes
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Tubeworms
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Tube-dwellers
May line their
burrows with mucus
Use cilia or mucus
to obtain food
Representative Polychaetes
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Fanworms or
Featherduster
worms
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Unfurl tentacular
crowns to feed.
Food moved from
radioles (feathery
arms) to mouth by
ciliary action.
Representative Polychaetes
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Parchment Worms
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Lives in a U-shaped
tube.
Modified segments
pump water through
tube.
Feeds off particles
that settle into the
sand from water.
Class Hirudinea
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Class Hirudinea
includes the leeches.
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Primarily
freshwater, a few
marine & terrestrial.
More common in
tropical climates.
Smallest class
Class Hirudinea
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Many leeches live as carnivores on small
invertebrates.
 Some are temporary parasites.
 Some are permanent parasites – they never
leave their host.
Class Hirudinea
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Leeches are hermaphroditic
and have a clitellum (only
appears during breeding
season), like oligochaetes.
Leeches do not have setae.
They’ve developed 2
suckers for attachment and
a specialized gut for storing
large amounts of blood.
Class Hirudinea - Respiration and
Excretion
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Some fish leeches have
gills.
All other leeches
exchange gases
across epidermis
(skin).
10 to 17 pairs of
nephridia.
Coelomocytes and other
special cells may assist
in excretion.
Class Hirudinea - Nervous and
Sensory Systems
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Two “brains”
Anterior fused ganglia form a ring around
the pharynx.
 Seven pairs of posterior fused ganglia.
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21 pairs of segmental ganglia in between
along a double nerve cord.
 Epidermis contains free sensory nerve
endings and photoreceptor cells.
 Pigment-cup ocelli are present.
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Class Hirudinea - Circulation
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Coelom reduced by invasion of connective
tissue..
 Some have a typical oligochaete circulatory
system.
 Some lack blood vessels.
Class Hirudinea
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Leeches are highly sensitive to stimuli
associated with the presence of prey.
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Those that feed on mammals are attracted
by warmth.