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Lecture 4
Marine Annelids
the Polychaetes
polychaeta General characters
 (pol'e-ke'ta) (Gr. polys, many, + chaiti?,
long hair).
 About 5300 species.
 Segmented inside and out; parapodia
used in locomotion with many large
number of chitinous bristles setae
 Distinct head with eyes, palps, and
tentacles
 No clitellum
 Separate sexes
 Trochophore larva usually present
 Mostly marine.
 Examples: Nereis
Jaws
Teeth
Postomial tentachels
Peristomial palps
Peristomial tentachels
Introduction marine worms of
polychaeta
• Almost all marine annelids are polychaetes (class
Polychaeta), which are also known as “bristle worms”
for their stiff and often sharp bristles stemming from
each of their repeated segments
– Bristles replaced by gills in many species
– Planktonic larvae stage; named?????
 Most free-living (living in substrate or freeswimming), but some live in mucus-supported tubes
that are surrounded by substrate
 Many are active predators, while others are filter
feeders
Habitat
 Many crawl on or burrow in the
seafloor.
 A few drift and swim in the plankton.
 Polychaetes include carnivores,
scavengers, and planktivores.
 Tube-dwelling polychaetes
are suspension feeders,
using feathery tentacles to
trap and capture suspended
material
 The brightly colored fanworms trap
planktonon by feathery tentacles.
External features
Hyperlink to external
features..\..\Documents\Nereis, polychaete
clamworm.mp4
Parapodium
Body wall
• It consists of an outer cuticle which is secreted by The cells of the
hypodermis just beneath it .
• Several muscular layers under the hypodermis (could you mention
these muscles?
• The coelom (cavity between …….and ……..), it is divided into
chambers by segments septa which is perforated beneath the
intestine to allow the fluid to pass between chambers.
Polychaeta
systems
Nervous
System
Digestive
system
Circulatory
System
Respiratory
system
Excretory
System
Reproduction
8
Nervous System
 dorsal brain: (brain is fond above the pharynx in the head which is
consists of a pair of supra-pharyngeal ganglia that connected with a pair of
sub pharyngeal ganglia by acircum pharyngeal commissure on either side
which from a ring around the pharynx).
 Ventral nerve cord with a pair of ganglia in each segment.
 The brain gives off:A- An optic nerve to each eye
B- Palpal nerve to each palp
C- Atentacular nerve to each group of tentacles.
 Three pairs of nerves arise from each of ganglia of the ventral nerve
chain
- One pair to
A- the parapodia
B- the anterior of the segment
C- the muscles of the segment
Nervous System
10
Digestive system
• The mouth opens into the
pharynx which forms assort
of proboscis which is
accomplished by protractor
and retractor muscles this
enable the muscular pharynx
to everted through the mouth.
• The pharynx leads into a
slender esophagus which
have digestive gland .
• Straight stomach (intestine)
following the esophagus it
extending to the anus .
Feeding
Clamworms (Nereis)
crawl about on marine
surfaces hunting for food
using well-developed
sensory structures. Once
located, prey are seized
with chitinous jaws on a
rapidly everted,
muscular pharynx
• ..\..\Documents\Polyc
haete worm.mp4
• ..\..\Documents\Nerei
s virens feeding.mp4
Larval polychaete feeds on algae!!!!!!
Circulatory System
•The blood is contained in
contractile tube ( the blood
vessels ).
•There is a dorsal vessel , that
lies between the two dorsal
longitudinal muscles bands (
which carried blood anteriorly ).
• A ventral vessel below the
intestine which carries blood
posteriorly.
• In each segment , the
longitudinal vessels are
connected on either side by right
and left transverse vessels to
form networks of capillaries in
parapodia.
Respiratory system
• Although many of the
smaller polychaetes lack
respiratory structures, the
larger one possess gills
(the parapodia)
• Gills are usually
modifications of the
parapodia
14
Excretory System
• Every segment contains a pair of
nephridia except the
peristomium and the anal
segment .
• Each nephridium opens into the
coelomic cavity by means of a
ciliated funnel.
• Each nephridium passes
posteriorly through the septum
into the following segment.
Where it forms coiled tube .
• It opens at the base of
aparapodium on the ventral
surface through nephridiopore .
Reproductive system and reproduction
 Sexes are separate.
 No well defined gonads are present but during breeding
season ova or spermatozoa arise from the wall of the coelom
in each segment except near the anterior end.
 At this time the worm is differentiated into 2 regions:
 An anterior atoke.
 A posterior epitoke which contains the gonads
 The specialized worm is called aheteronereid.
(Could you summarize the morphological changes that takes
place in heteroneries?
 Fertilization occurs (external ) in the open water.
 Trochophore larvae develop from the fertilized eggs.

..\..\Documents\Spawoing Sandworms (Nereis
virens) with Tidepool Tim of Gulf of Maine, Inc..flv