Phylum Annelida
the segmented worms
Phylum Annelida
• Annelida from the Latin word
annelus meaning “little ring”
• All are segmented worms
• Approximately 15,000 species
including earthworms, freshwater
worms, leeches, and marine worms
Annelida Characteristics
Triploblastic- 3 cell layers
Highest level of organization: organs
Bilateral Symmetry
Cephalization
Eucoelomate
Body Cavity
Eucoelomate
Have a “true” body cavity that is completely
surrounded by mesoderm
ectoderm
coelom
mesoderm
Gut
endoderm
Annelid Characteristics
the coelom
ectoderm
• is a closed, fluid filled cavity
that
surrounds the gut
• the fluid within acts as a circulatory
system (blood)
• tissue membranes (mesenteries)
suspend organs in the coelom
Annelida Characteristics
Metamerism
The body is made up of a series of
repeating, coordinated segments called
metameres that are separated from one
another by septa.
•Each metamere contains sets of repeating
organs
e.g. gut, blood vessels, nerve cord,
excretory organs
Metamerism
septa
Metamerism
Annelid Characteristics
• Two part head consisting of
o Prostomium: fleshy lobe that overhangs the
mouth
o Peristomium: first body segment
• Annuli:
circular rings
• Pygidium: the
last segment;
where the anus is
• Clitellum:
reproductive
structure
annuli
pygidium
Annelida Characteristics
Nervous system
• 2 cerebral ganglia
• a ventral nerve cord with 2 ganglia per metamere.
• In some species, sensory organs
such as eyes, palps, and tentacles
have arisen
(palps are appendages that may or
may not be covered in cilia that help
the worm find and pull in food)
Annelida Characteristics
Locomotion
• both longitudinal and circular muscles which
allow them to “wiggle”
• most have setae (chitonous bristles secreted by
the epidermis) that aid in locomotion and
burrowing
Skeletal System
• fluid in coelom acts as a hydrostatic skeleton
Annelid Characteristics
Gas exchange
• mainly by diffusion through skin
• Class Polychaeta often has specialized
structures for gas exchange
(e.g. parapodia, gills)
Annelid Characteristics
Digestive System
• unidirectional
• regional specialization (digestive organs)
Circulatory System
• closed circulatory system composed of blood
vessels (some of which are contractile and act
as “hearts”)
• some circulation is also accomplished by the
coelomic fluid
Annelid Characteristics
Excretion
• excretion is accomplished by organs called
nephridia (singular nephridium)
Reproduction
• sexual
Phylum Annelida
Class Polychaeta
Class Oligochaeta
Class Hirudinea
Class Polychaeta
Class Polychaeta
• All marine
• This class contains 2/3 of all known Annelids
• Common species are clamworms,
sandworms, and lugworms
• have a well developed head with specialized
sense organs
Class Polychaeta
• Commonly called “bristle worms”
• Have many setae (chitonous bristles secreted
by the epidermis)
(Poly = many, chaeta= setae)
• These setae are arranged in bundles on
paddle-like appendages called parapodia
setae
Class Polychaeta
Parapodium
setae
The parapodia function in gas
exchange, locomotion, and feeding.
Class Polychaeta
Tagmatization (tagmosis)
• the
fusion and specialization of
formerly metameric segments
Class Polychaeta
Many are filter-feeders
with specialized structures
Class Polychaeta
Many are predatory with
specialized structures
Class Polychaeta
Many construct their own
homes out of CaCO3 or
sand debris and mucous
Class Polychaeta
Reproduction
• Usually dioecious (separate male and female)
• No permanent sex organs; gametes are shed into
coelom where they stay until time for fertilization
• Fertilization is usually external in water
• Indirect development  trocophore larvae swims
around until it grows into
adult form by adding
segments
Ecology
• Polychaetes often have effective
defense strategies:
• some have tubes to hide in
• some have vicious jaws
• some have modified “stinging” setae
a fireworm
Ecology
• Some Polychaetes have a
mutualistic relationship with their
host
• for example, many scaleworms
are found near, or in the mouth,
of brittlestars, starfish, and sea
urchins.
• The scaleworm eats its host’s
leftovers and with its vicious
jaws, it will attack any predator
trying to eat it’s host.
Class Oligochaeta
Class Oligochaeta
• Habitats: terrestrial, freshwater, and marine
• Have few setae (Oligo = few, chaeta = setae)
Class Oligochaeta
What do earthworms eat?
• Usually feed on detritus
(decaying organic matter)
• Depends on habitat, but they can feed on…
 Dead leaves and plant roots
 Living things such as other worms,
bacteria, and fungi
 Decomposing remains of other animals
Class Oligochaeta
Do earthworms eat dirt?
• Yes and No.
• Earthworms consume dirt as they burrow,
so yes they do swallow it.
• HOWEVER, they do not get nutrients from
the dirt. Their nutrients come from decaying
plant and animal remains that are in the dirt.
Class Oligochaeta
• Earthworms have specialized digestive
system to obtain the maximum amount of
nutrients out of the detritus.
• Example: pharynx, crop, gizzard, etc.
Class Oligochaeta
Locomotion
Circular
muscle
contraction
Longitudinal
muscle
contraction
Class Oligochaeta
Reproduction
• Usually monoecious
• Still have to have a partner.
• Cross-fertilize by
exchanging sperm
clitellum
testis
Ecology
• Earthworms are essential soil aerators, meaning
they allow air to enter the soil.
• Mix the soil with their tunnels.
• Worm feces are great plant food = fertilizer!
Class Hirudinea
Class Hirudinea
• Includes leeches
•usually freshwater but there are some
marine and terrestrial species
• no septa between metameres
• no setae or parapodia
• have 2 suckers
Class Hirudinea
• have an extendable proboscis for
feeding
Class Hirudinea
• usually have a fixed number of
segments (34)
• each metamere consists of several
annuli (think accordion)
1 metamere
annuli
Class Hirudinea
Locomotion
Lack septa between
metameres, so they are
incapable of moving like
Oligochaetes.
Instead, they use their
anterior and posterior
suckers to move.
Class Hirudinea
Reproduction
• usually monoecious
• cross-fertilize by
exchanging sperm
Ecology
• Although some leeches are parasitic blood
suckers (can be temporary or permanent), many
are predators.
Ecology
• Leeches have been used
medicinally since the 19th
century.
• Currently they are used to
increase blood flow following
reconstructive surgery
• Hirudin is a powerful
anticoagulant that is found in
the salivary glands of leeches