Phylum Annelida
Learning Outcomes
Students should be able to:
• Describe the unifying characteristics of
members of phylum annelida
• Describe how annelids carry out their life
functions
• Describe the ecological roles of annelids
Phylum Annelida
Leeches
General Information
• Annelids are ringed, segmented worms
• Examples include earthworms and
leeches
• Habitat: terrestrial, fresh water, marine
• Have bilateral symmetry with 3 body
layers (ectoderm, mesoderm, &
endoderm)
3 Unique Features
• Have a true coelom (body cavity) lined by
mesoderm
• Have externally visible body segmentation
• Have a closed circulatory system
Coelom
• Located between the body wall and the
intestine
• Lined by mesoderm
• Filled with fluid
• Acts as a buffer between body muscles
and the intestine (allows worm to move
without putting pressure on the intestine)
Body Segmentation
• The body is divided into segments
• Each segment contains a portion of the
coelomic cavity
Class Polychaeta
•
•
•
•
Polychaete worms
Marine
Flattened bodies
Body segments with
paired paddle-like
appendages
(parapodia)
Class Oligochaeta
•
•
•
•
Earthworms
Have a few hairs
Small head for burrowing
May have a saddle-like swelling (clitellum)
Class Hirudinea
•
•
•
•
Leeches
Often aquatic
Bodies with suckers
Many are
ectoparasites feeding
on blood
Earthworm Ingestion
• Earthworms have a complete digestive
system (one way digestive tract)
• Food (organic material in the soil) enters
through the mouth
• The pharynx draws food into the mouth
(suck in and swallows food)
• Food moves through the esophagus to the
crop where it is stored temporarily
Earthworm Digestion
• Food moves from the crop to the gizzard
• Gizzard contains sand grains which help
grind up food by muscular contractions
• Food moves into the intestine where
enzymes chemically digest (break down)
food into particles small enough to be
absorbed into blood
• Intestine has a typhlosole which increases
the surface area for nutrient absorption
Earthworm Elimination
• Undigested food waste moves to the end
of the intestine and exits through the anus
Earthworm Respiration
• Gas exchange (O2 & CO2) happens via
diffusion through the moist skin
• Mucus glands secrete mucus to keep skin
moist
• O2 diffuses across the moist skin and into
the capillaries and blood
• O2 is carried by hemoglobin (Hb) in the
blood
Earthworm Circulation
• Earthworms have a closed circulatory
system (blood is always contained in
vessels)
• Closed circulatory systems allow for more
effective delivery of nutrients which allows
for increased activity and size
• Earthworms have 5 pairs of hearts which
pump blood through the vessels
Earthworm Excretion
• Excretory organs called nephridia filter
excess H2O and nitrogenous wastes from
the coelomic cavity of one segment
• This waste passes through a long tube
surrounded by blood vessels (much like
our kidneys) and eventually exits through
an excretory pore in the next segment
• Excess water is reabsorbed into the blood
vessels
Earthworm Sexual Reproduction
• Earthworms are hermaphrodites (male &
female on same individual)
• 2 earthworms cross-fertilize (exchange
sperm)
• Earthworms have a swelling called a
clitellum which secretes a mucus cocoon
into which fertilized eggs are deposited
Earthworm Asexual Reproduction
• Earthworms are able to regenerate
missing body parts
Earthworm Nervous System
• Earthworms have a paired ventral nerve
cord which swells into ganglia (a mass of
nerve cell bodies acting as a primitive
brain) at the head
Earthworm Locomotion
• Uses longitudinal and circular muscles
combined with the fluid pressure in the
coelom and the setae (bristles) to help
burrow and move
• Contraction of the circular muscles of
anterior (front) cause worm to lengthen
• Paired setae (bristles) anchor worm to soil
• Longitudinal muscles contract causing
worm to shorten pulling it’s posterior (tail
end forward)