Phylum Annelida: summary of
characteristics
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Name from Latin annulus meaning a ring.
Vermiform. Possess tissues and organs.
Muscular gut with mouth and anus.
Body divided into segments.
Outer epithelium with clumps of bristles (except in forms
with suckers).
Body wall muscular with both circular and longitudinal
muscles.
Closed circulatory system.
Nervous system with supraoesophageal ganglion,
circum-oesophageal ring and ventral nerve cord.
Nephridia responsible for most excretion
Phylum Annelida

The annelids (L. annelus: a little ring) are the
segmented worms.
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Annelids are coelomate, protostomes and the
body is metameric being composed of serially
repeated segments or metameres.
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Each segment is separate from the next
segments being divided by partitions or septa.
Segmentation
 Within
each segment are components of
most organ systems such as the
circulatory, nervous and excretory
systems.
 Thus,
there is a degree of redundancy in
annelids so that if a segment is damaged it
need not be fatal.
Segmentation
 The
evolution of segmentation is the great
evolutionary innovation of the annelids.
 Segmentation
allows annelids to make
more precise body movements than
organisms that have a hydrostatic
skeleton, but lack segmentation e.g. the
nematodes.
Segmentation
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Because the coelom is divided by septa the
force of muscle contraction in a segment is not
transmitted throughout the body, but instead is
confined to the single segment.
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Thus, one segment may elongate while the
adjacent one contracts and this allows the
animal to make fine, controlled movements.
Movement
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With the exception of the leeches, the coelom is
filled with fluid and acts as a hydrostatic
skeleton.
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Annelids possess circular and longitudinal
muscles and this enables individual segment to
be elongated or contracted.
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Crawling is achieved by alternating waves of
contraction by circular and longitudinal muscles
passing down the body (peristalsis).
Movement
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Because they have fine control of movement
annelids have evolved a relatively sophisticated
nervous system.
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Most annelids are burrowing forms and as an
adaptation to this lifestyle bear short chitinous
bristles called setae on each segment. The
setae enable the annelid to gain traction against
the side of the burrow.
Movement
 In
other annelids longer hair-like setae
assist the animal in swimming.
 For
the annelids that live in burrows or in
tubes the setae help to prevent the animal
from being pulled out.
Annelids
 Annelids
occur worldwide being found in
the sea, freshwater, and in the soil.
 They
feed on organic matter in the mud or
soil, by filtering suspended particles from
the water, act as predators, or suck blood.
Annelids
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The typical annelid body has a two part head
made up of a prostomium and a peristomium, a
series of segments, and a terminal pygidium
which contains the anus.
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Neither the head nor the pygidium are
considered true segments. In growth, new
segments form anterior to the pygidium. If an
annelid is cut in two the posterior segments can
be regrown.
Figure 17.03a
11.1
Figure 17.03c
Annelid Classification
 There
are approximately 12,000-15,000
species of annelids divided into 4 classes:
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Polychaeta: polychaete worms
Oligochaeta: earthworms
Hirundinea: leeches
Siboglinidae: pogonophorans
Class Polychaeta
 The
polychaetes are the largest of the
annelid classes and include more than
10,000 described species, most of which
are marine. Morphologically very diverse.
 The
name “poly” “chaete” refers to the
numerous chaetae or bristles they
possess.
Polychaetes
 Polychaetes
have a well differentiated
head that has sense organs including eyes
and cirri (short tentacles), jaws (in
predatory forms), or a fan for filter feeding.
 Most
segments bear parapodia, which are
lobed structures used in swimming,
crawling, or for anchorage in tubes.
Parapodia also serve as gills.
Figure 17.03e
Figure 17.03c
Class Polychaeta
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Polychaetes follow one of two basic lifestyles being
either sedentary/sediment burrowing (“sedentary”) or
active hunting (“errant”) species.
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Sedentary polychaetes usually exhibit variation in the
structure of segments. All are filter-feeders or deposit
feeders.
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Sedentary polychaetes burrow in mud and soil or build
their own tubes from which they filter feed.
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Tubes may be made from calcium carbonate, a secreted
paper-like material, or sand grains.
Figure 17.02b
11.3B
Fanworms
 Most
of the sedentary polychaetes, which
inhabit burrows or build tubes, are filter
feeders and consume plankton or detritus.
 Forms
such as fanworms extend long,
modified feathery crowns of stiff prostomial
tentacles to feed. Ciliary action draws in
food, which is trapped in mucus and
delivered down grooves to the mouth.
Figure 17.02a
11.3A
Figure 17.10
11.7
Burrowing polychaetes
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A number of families of sedentary polychaetes
burrow in soft sediments either swallowing
sediment or scraping it of bacteria, algae, fungi
and other live material.
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Many functionally resemble oligochaetes and
have reduced parapodia, lack prominent sense
organs and have well developed circular muscles
and septa.
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Some have soft prehensile tentacles they use
collect food particles.
http://www.nw1design.com/clients/afen/images/pics/Polychaete.jpg
Burrowing Polychaetes
 Burowing
polychaetes such as lugworms
are very common on estuaries.
 They
make burrows in the sand and
consume large quantities of sand. After
they’ve extracted the digestible material
the remaining material is defecated and
forms a characteristic pile outside the
burrow.
Lugworm (two images above) from
http://marinebio.org/species.asp?id=57
Above right Lugworm casts.
http://upload.wikimedia.org/wikipedia/commons/7/7b/Lugworm_cast.jpg
Burrowing Polychaetes
 Lugworms
are an important source of food
for wading birds.
http://cache2.asset-cache.net/xc/88392324.jpg?v=1&c=IWSAsset&k=2&d=
EDF6F2F4F969CEBD9BAF6D58632300DBDC286D88A6C0AC12DB7DFB94F05A5839
Predatory polychaetes
 Predatory
forms of polychaetes such as
Nereis have a muscular pharynx equipped
with jaws that can be quickly everted to
grab prey.
Figure 17.03a
11.1
http://weblog.greenpeace.org/
defendingourmediterranean/images/
180polychaete_worm_hermodice_carunculata
__whole_worm_for_hibsy.jpg
Bobbit worm a predatory polychaete.
http://www.tonywublog.com/20090319/fright-night.html
Predatory polychaetes
 Predatory
polychaetes typically can crawl
rapidly using their parapodia.
 They
are active hunters that can sqeeze
through small spaces (e.g. in coral,
crevices, etc.) is search of prey.
 They
consume any other invertebrates
that they can catch and dismember.
Figure 17.09
11.6
Class Oligochaeta
 There
are over 3000 species of
oligochaetes, the most familiar of which
are the earthworms.
 Lumbricus
terrestris, the common
earthworm, grows from 4-12 inches, but
tropical forms may reach 12 feet in length.
Class Oligochaeta
 Earthworms
burrow in rich, damp soil and
leave their burrows at night to eat
vegetation and to breed.
 Earthworms
play a significant role in soil
fertility by aerating the soil with their
burrows, adding vegetable material, and
mixing subsoil and topsoil.
http://www.cheshirewildlifetrust.co.uk/IMAGES/watch_earthworm.jpg
Class Oligochaeta

Darwin studied earthworms and published a book on
their effects on soil.
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He estimated that an earthworm eats its own weight in
soil daily and that in an acre of land 10-18 tons of dry soil
passed through their guts annually.
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Earthworms consume dead organic material and partially
digest it, the waste passing out of them containing
nutrients valuable to plants and supplemented with
nitrogenous wastes from the worm.
Class Oligochaeta
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In addition to the earthworms there are many
freshwater species, most of which burrow in silt
and mud or creep along the bottom, although
some live among submerged vegetation.
 Freshwater forms usually are smaller than
terrestrial and have more conspicuous setae.
 Most respire through their skins, but some have
gills. Most are algae or detritus feeders.
Figure 17.18
11.16
Freshwater oligochaetes
Class Oligochaeta
 Oligochaetes,
like all annelids, have a
double circulatory system as both the
coelomic fluid and circulatory system are
used to carry food, wastes and gases.
 The
blood system is closed, with the
dorsal blood vessel being the main
pumping organ.
http://z.hubpages.com/u/94165_f520.jpg
Class Oligochaeta
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The excretory organs are called nephridia and
there is a pair in each segment, each of which
occupies parts of two successive segments.
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A ciliated funnel (the nephrostome) opens just
anterior of an intersegmental septum and from
this a tubule leads into the posterior segment
and forms a series of loops that are closely
surrounded by blood vessels.
Excretory organs
 The
tubule eventually opens to the outside
via an aperture called a nephridiopore.
 The
system works by cilia drawing
coelomic fluid into the nephrostome and
selective reabsorbtion of salts and water
occurs in the loops leaving only a dilute
urine to be excreted to the outside.
Figure 17.14
11.14
Reproduction in earthworms
 Earthworms
are hermaphroditic and mate
by aligning their ventral surfaces together.
 Each worm’s clitellum (thickened section
of some midbody segments) secretes
mucus, which holds the two worms
together.
 Sperm is exchanged and stored in a
seminal receptacle.
Reproduction in earthworms
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After sperm has been exchanged the worms
separate and each secretes a cocoon around its
clitelleum. The cocoon slides along the body
and picks up eggs and sperm.
 Fertilization occurs within the cocoon as does
later embryonic development.
 As the cocoon slides off the worm its ends seal.
Young worms emerge several weeks later.
Figure 17.17
11.15
Class Hirudinea
 There
are more than 500 species of
leeches, most of which are freshwater
inhabitants.
 Leeches
have anterior and posterior
suckers which they use in locomotion.
With the exception of one group, leeches
lack septae and their coelom is largely
filled with connective tissue and muscle.
Class Hirudinea
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Many leeches are carnivorous, but leeches are
best known as blood-sucking ectoparasites.
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The leech penetrates its host using its jaws or
proboscis and sucks blood with its powerful
pharynx.
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To ensure blood continues to flow the leech
secretes a powerful anticoagulant (hirudin) in its
saliva.
Figure 17.20
11.18
Figure 17.19
World’s largest leech
Haementeria ghilianii
11.17
Class Hirudinea
 For
hundreds of years leeches were used
for blood letting, in the belief that too much
blood caused a variety of medical
conditions
 After
being discarded as a medical tool
leeches are again being used by
surgeons.
Class Hirudinea
 In
reattachments of severed digits and in
the case of skin grafts, because the blood
vessels are damaged, pooling of blood
often threatens to kill the attached tissue.
 Leeches,
however, can remove the
pooling blood safely allowing time for veins
to develop.
Figure 17.21
11.19
Medicinal leech feeding
Class Siboglinidae
(pogonophorans)
 The
pogonophorans (or beardworms)
were formerly considered to be a phylum,
but now are considered to be derived from
the polychaetes.
 These
were first discovered during deep
sea dredging in 1900 off Indonesia, but
since then about 80 species have been
identified in seas worldwide.
Class Siboglinidae
(pogonophorans)
 While
similar to tube-dwelling polychaetes,
the first pogonophorans were considered
to be a separate group because they lack
a complete gut (no mouth, gut or anus)
and appeared not to be segmented.
Class Siboglinidae
(pogonophorans)

The lack of segmentation proved to be illusory.
Pogonophorans live buried in the mud and their
lower ends were broken off when collected
during dredging.
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In 1964 complete pogonophorans were dredged
up and it was discovered that the posterior end
of pogonophorans (called the opisthosoma) is
segmented and bears setae.
Class Siboglinidae
(pogonophorans)
 Most
siboglinids live in the mud and silt of
the seafloor usually at depths > 200m.
 The
body is divided into a short forepart,
which bears tentacles, a long, slender
trunk, and the small segmented
opisthosoma. The body is covered with a
cuticle and has setae on the trunk and
opisthosoma.
Siboglinum fiordicum
11.11
Opisthosoma
Class Siboglinidae
(pogonophorans)
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Because siboglinids have no mouth or complete gut it’s
unclear how they obtain nutrition.
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They absorb some nutrients in the water through their
tentacles, but most energy apparently is derived from a
mutualistic association with chemoautotrophic bacteria.
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The bacteria oxidize hydrogen sulfide to produce energy
and live in an expanded section of the embryonic midgut
called a trophosome. There is no foregut or hindgut.