Phylum Annelida
Leech
Earthworm
The Segmented Worms
Nereis
Are There Many Kinds of
Annelids??
• 15,000 species of annelids can be divided into
three major groups
Polychaeta
Oligocheata
Hirudinea
What Are Annelids?
General Characteristics
• Bilateral symmetry
• Definate anterior /
posterior ends
• Distinguishable dorsal /
ventral side
• True tissue / organ level of
organization
• Segmented bodies
• The distinguishing feature
of all annelids
» Annelida = “little
rings”
• Range in size
• < 1mm up to 3 meters!!
» Eg. giant Australian
earthworm
The Internal
Structure of
Annelids
• Complete digestive
tract
– Several specialized
regions
• Circulatory system
– Closed vessels
– Blood containing
hemoglobin
• Nervous system
– Brain-like structure
• Reproductive structures
– Ovaries and testes
All Annelids Are Coelomates . . .
• They have a true
body cavity
– Coelom is lined with
mesoderm cell layer
• “Tube-within-a-tube”
body plan
– Outside tube
• Body wall
– Inside tube
• Digestive tract
Annelids are the simplest animals
to have a true coelom!
What happens to the “stuff”
annelids eat?!?
• Specialized
regions aid in
digesting the
organic matter
in soils ingested
– CROP
• Temporarily
stores food
– GIZZARD
• Thick walled
organ which
grinds food
Nereis . . . The Hunter
• Marine Sandworm
– Found at tide level
• Carnivorous
– Feeds on small animals
• Distinct anterior segment
– Prostomium
• 2 short tentacles + 2 palps
• 2 pairs of small eyes
– Peristomium
• 4 pairs of tentacles
surrounding mouth
Figure 32-14,
Pg. 698
Tentacles
Palps
How does the pharynx work??
• The pharynx is
extended out of the
mouth
• Pointed claw-like jaws
capture prey
Pharynx
Jaw
• Pharynx pulls food back
through mouth
• Food moves into
intestine where
digestion begins
These Guys Can Move!
Parapodia
• Nereis have
structures allowing
them to swim AND
creep over sand
– Parapodia
• Paddle-like extensions
– Setae
Setae
• Bristle-like structures
Reproduction in Annelids
• Oligochaeta &
Hurudinea
– Hermaphrodites
• Cross-fertilization
– Internal fertilization
• Polycheata
– Separate sexes
– External
fertilization
» Gametes are
released
through
excretory
organs
» Free-swimming
larva
Earthworms Create a Cocoon!
Clitellum : sperm are stored temporarily until a mucus cocoon is
created to protect the fertilized eggs!
Leeches: the ecto-parasite!
• Feed on the blood of
their prey
• Hirudin
» Enzyme which
prevents blood
clotting
• The jaws break through
the host’s skin
• Blade-like jaws make
incision
Two Evolutionary Innovations
• The presence of a true coelom
– Provides support
» Acts as a hydrostatic skeleton
– Room for organ development
– Advancements in locomotion
» Not the dance!!!! . . . Motility!!!
• Segmentation of the body
– Specialization of body regions
» Feeding and sensory input, motility
– Modification of various segments for different
functions
Worms today. . . Mollusks
tomorrow!
Until then . . .
Be kind to your
neighborly
annelid . . .
You may depend
on one someday!