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Phylum Annelida
The Segmented Worms
Phylum Level Characteristics
• round
• segmented (repeated body units
containing similar structures)
• bilateral symmetry, hydrostatic skeleton
• complete digestive system
• Eg. earthworms, leeches
Coelom
• -coelem forms from
mesoderm
• Creates space for the
organs to grow and move
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Digestive System
• specialized areas/organs present:
– crop: stores food
– gizzard: grinds food
– pharynx: connects mouth with
digestive tract (gut)
– Intestine: where nutrients
are absorbed
Respiratory System
• Aquatic annelids often breathe through
gills
• Other annelids take in oxygen and expel
carbon dioxide through their skin
• Skin must be moist for gas exchange to be
possible
– If worms dry out they suffocate and DIE!!
Circulatory System
• closed system
– Typically, 2 blood vessels run length of body
• Dorsal: blood moves towards head
• Ventral: blood moves towards tail
– necessary to provide glucose and oxygen to
all cells of a fairly active animal
Excretory System
• Solid wastes pass out through anus
• Metabolic wastes excreted by nephridia
(like small kidneys) in each segment
Nervous System
• Brain sits at front of body
• Ventral nerve cord runs entire length of
worm
• Many different types of sense organs
depending on species
Movement
2 groups of muscles:
1. longitudinal-makes worm short and
then long
2. lateral, circular-makes worm fat then
thin
Setae- Hairlike anchors
Movement
http://www.biology.ualberta.ca/courses.hp/z
ool250/animations/Earthworm.swf
Reproduction
• A few reproduce asexually by budding
• Most reproduce sexually
• Some species have separate sexes and
external fertilization
– Coordinate the release of eggs and sperm to
increase the chances of successful
fertilization
– Done by aquatic annelids
Reproduction
Earthworms and leeches are
hermaphrodites:
1. the two worms attach
themselves to each other
2. worms exchange sperm
and it is stored
3. sperm fertilizes eggs
internally in clitellum (mucous
ring)
4. clitellum slips off worm’s
body into soil (holding
fertilized eggs) and becomes
a cocoon