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Phylum Annelida General
Characteristics
Bizarre gills!!!
What are they?
Round worm-like animals which are segmented.
The name comes from the Latin “annellus”
which means little ring.
• Appearance of the segments.
A ring in French is call anneau.
Can range in size from less than 3mm to more than
3m!
• Mostly worm-like in appearance.
The body segments are separated internally by
walls of tissue called septa.
• Singular ~ septum.
Most segments are similar, however some are
specialized.
General features of a
segment.
Annelids also have a true coelom like us,
meaning that the entire body cavity is lined with
mesoderm tissue.
Platyhelminthes
Nematoda
Annelida
This is of course a simplified diagram.
Digestive System
A one-way digestive tract, so they have an anus.
There are many specialized compartments.
This is a general plan although it varies between
species and classes.
1. Food enters the mouth.
2. Food enters the pharynx, a muscular tube.
• Can be protruded through the mouth.
• Carnivorous species have powerful jaws
attached to lunge and catch prey!!!
• Herbivorous worms also have jaws to bite off
bits of algae.
Nom nom nom!
• Pharynx can act as a sticky surface or a pump
for those annelids that are deposit feeders. (eat
dirt/mud/sand)
• Can also be a pump to suck up blood and
tissues in parasite species.
3. Food then enters the gut where it is moved by
peristalsis.
• Muscular waves of the gut wall.
4. Food leaves via the anus.
Respiratory System
Marine worms often breathe through gills called
parapodia or parapodium (singular).
• Thin extensions of the body wall.
Some marine worms can have large, fan-like
gills. ~ see slide #2.
Circulatory System
Possess a circulatory system!
It is a closed circulatory system with contractile
vessels.
• The system has two main blood vessels, a
dorsal and ventral blood vessel.
• Located on top and underneath of the gut.
• Supply blood to the internal organs.
Dorsal blood vessel
Ventral blood vessel
The earthworm has 5 contractile “hearts”.
• Pumping ring vessels.
5 “hearts”
Excretory System
Solid wastes from digestion pass through the
anus.
Metabolic wastes such as ammonia are released
via the metanephridia.
• More advanced versions of the
protonephridia we saw in Platyhelminthes.
• Metanephridia: A tube that collects wastes
from the body cavity and releases them
outside. (A pair per segment)
• Earthworms also release wastes via their skin.
Metanephridia
Metabolic wastes in.
Metabolic wastes
excreted.
Nervous System
Many annelids are active animals with well
developed nervous systems.
They have a brain.
The brain is at the front, (cephalization), and sits
on top of the digestive tract.
• Associated with a nerve ring that goes around the
gut with a pair of ganglia at the bottom.
A large ventral nerve cord then runs the length of
the body.
Each segment has nerves that branch off the
ventral nerve cord and a pair of ganglia.
• What does this remind you of?
Brain
Segmental nerves
Gut
Nerve Ring
Ventral Nerve Cord
Reproductive System
Some annelids can reproduce asexually by
budding, but not many.
Marine species are often gonochoristic and have
external fertilization.
Earthworms and leeches are often
hermaphroditic and have internal fertilization.