Annelids – Part 1
AKA Segmented Roundworms
Annelids
• The word annelid is derived from
the word annulus which means
ring.
– In this way, annelids get their
name from their most defining
characteristic – their external and
internal segments, or rings.
Classes of Annelids
• Annelids have three main
classes based on habitat and
differences in body structure.
• They are:
– Polychaete (paul-ee-keet)
– Oligochaete (awl-ee-go-keet)
– Hirudinea (hir-oo-din-ee-uh)
Polychaete Worms
• AKA clamworms
• Size: millimeters to
meters long
• Change in body
structure: has many
bristles.
• Habitat: marine
environment
• Types of movement:
sedentary or mobile
Mobile Polychaete
• Mobile worms:
• Tend to swim around
and catch food
• Have parapodia (little
flaps of skin that act as
legs) for movement.
• Parapodia have setae
(bristles) and cirri
(needle-like projections)
on them.
• Have eyes, sensory
tentacles, and a
retractable pharynx with
jaws.
– Ex. Nereis virens
Sedentary Polychaete
• Sedentary Worms
• Tend to burrow in sand or
mud & filter feed.
• They suck up sand or mud
and selectively absorb
nutrients.
• Often lack eyes and have
much smaller parapodia.
• Some secrete their own
tube to burrow into.
• Ex. Arenicola
Oligochaete Worms
• Change in body structure: few
bristles.
• Habitat: damp terrestrial
environments
• Best known oligochaete is the
earthworm
Earthworm
• The earthworm is mostly
nocturnal and tends to live in
underground tunnels.
• They help to develop and add
nutrients to the soil.
• They are considered to be more
advanced than polychaete.
Hirudinea
• Change in body structure: flattened
body, with no parapodia or bristles.
• Habitat: freshwater or terrestrial
parasites or predators.
– They attach to their prey by suckers.
• Ex. Leeches
– After having it’s fill of a victims blood
they can go for weeks or months without
eating.
Annelid Body Structure
• Bilateral symmetry
• Coelomates: have a lined
coelom (body cavity).
– This is an advancement over
nematodes.
• Cuticle covered body to keep
the worm from drying out, while
still allowing movement.
Digestive System
• More advanced than the
nematodes.
• One way system, in addition to the
nematode parts they have:
– Calciferous glands
• Monitor acid-base levels of the
coelomic and blood fluids.
– Crop
• Used to store food.
– Gizzard
• Food is ground into digestible
pieces.
Mouth  buccal cavity  pharynx  esophagus
(attached to calciferous glands)  crop 
gizzard  intestine  anus
Nervous System
• Anterior brain with paired ventral nerve
cords.
– Has peripheral nerves that supply the
muscle layers and reach the sense
receptors of the outer wall.
• Also have ganglia that allow for
movement.
• Each segment has a nerve cord that
passes through a ganglia and attaches
to sense receptors.
– This nerve cord consists of giant nerve
fibers and is responsible for coordinating
movement of all the segments.
Muscular System
• Both longitudinal and
circular muscles –
another advancement
over nematodes.
• Seta retractors are
muscles that connect
from one set of setae
to another. This
allows for coordinated
movement.
– Remember, setae are
bundles of bristles.
Circulatory System
• We haven’t seen one of these
since the piggy!
• Two kinds are seen in annelids:
– Open: no vessels, blood empties
into sinuses, or cavitites. (in the
leech)
– Closed: has vessels
to hold and direct
blood. (in the clamworm & earthworm)
Reproduction
• Annelids are hermaphroditic.
• They reproduce sexually.
Excretory
System
The excretory system is called a
metanepridium.
– Each segment has a funnel like
nephrostome which is covered by tiny
hair-like structures (cilia) leading into
the coelom of that segment.
– The segments are connected together by
a convuluted tubule which leads from the
nephrostome into the bladder of another
segment.
– Wastes are excreted through the
nephridiopore.
A note on leeches
• Leeches are sometimes considered
to be parasitic organisms.
• However, their relationship with the
host is relatively brief, so it is more
correct to classify them as
predatory.
– #13 on page 99 in your resource book
incorrectly states that they are not freeliving, but rather are considered to be a
parasitic organism.
Annelid Importance
• Annelids are important to
humans for the following
reasons:
– Earthworms help to add nutrients
to our soil.
– Leeches are used medicinally to
help promote blood flow to areas.
– They make good bait for fishing!!