Phylum Annelida
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Phylum Annelida
• “little ring”
• Segmented worms
• 15,000 species
–
–
–
–
Marine worms
freshwater worms
terrestrial worms
Leeches
• worldwide distribution
• Soft body
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Size of Annelids
• <1mm  many meters
Megascolides australis up to 7.5 m!
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Fig. 9.9
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Fig. 9.9
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Trochophore larva
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Veligar larva
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Characteristics of Phylum
Annelida
• triploblastic
– schizocoelomate
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Fig. 9-13, p188
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Characteristics of Phylum
Annelida
• Coelom partitioned by walls (septa)
• Serial (repeating) segmentation= metamerism
– Each segment= metamere/ somite
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Development of Metameric, Coelomic Spaces
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Characteristics of Phylum
Annelida
• External evidence of septa= circular
grooves called annuli (rings)
Septa
annuli
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Advantages of metamerism
1. greater flexibility of movement, compare to nematode
2. Components organ systems repeated w/i
segments
– (repeated excretory, nervous, circulatory
structures)
– Built-in redundancy- increases survival
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Note:
• Not all organ systems are metameric
• ie, digestive system extends the length of
the organism and is differentiated along its
length
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Characteristics of Phylum
Annelida
• Greater flexibility
demands greater
fine motor skills
– Highly developed,
centralized
nervous system
• brain
• Ventral nerve
cord
• Ganglion in each
metamere
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Movement in Phylum Annelida
• Fluid-filled coelom (except Subclass Hirudinea)
= hydrostatic skeleton
• Longitudinal and circular muscles
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Characteristics of Phylum Annelida
• Closed circulatory system (except some leeches)
– Blood vessels and aortic arches (“hearts”)
– Branch to every metamere
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Excretory System of Annelids
• Consists of paired
metanephridia (in most)
•
Excretory tubes with
ciliated funnels that
remove waste from the
coelomic fluid
• open to the outside via
excretory pores.
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Phylum Annelida: Classes
• Class Polychaeta
• Class Clitellata
– Sublass Oligochaeta
– Subclass Hirudinea
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Class Polychaeta
• poly = many; chaeta = bristles
• Mostly marine
– Example: Neries video
• Mostly dioecious
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Class Polychaeta (cont’d)
• Parapodia
– fleshy segmented
appendages for
locomotion &
breathing
• have numerous
setae at ends of
parapodia
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Fireworm (Hermodice carunculata)
Class Polychaeta (cont’d)
• Gas exchange across parapodia
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Class Polychaeta (cont’d)
• parapodia have
numerous setae at
ends
– Tiny chitinous bristles
• Chitin= tough,
flexible
polysaccharide; not
soluble in water
– Provide anchorage
– Swimming
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Fireworm (Hermodice carunculata)
Class Polychaeta (cont’d)
• well differentiated
head (prostomium)
with specialized
organs (eyes, jaws)
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Class Polychaeta (cont’d)
• Many are euryhaline (able to tolerate a wide
range of salinity conditions).
• Live under rocks, coral crevices, abandoned
shells.
• Serve as the basis
for many marine
food chains.
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Class Clitellata
• Earthworms and leeches
• Few/no setae
• Possess clitellum
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Class Clitellata (cont’d)
• Clitellum
– secretes mucus for cocoon, copulation
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Class Clitellata (cont’d)
• monoecious, cross-fertilization
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Subclass Oligochaeta
• oligo = few; chaeta = bristles
• Terrestrial, freshwater, some marine
– Example: earthworms, night crawlers
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Subclass Oligochaeta (cont’d)
• Prostomium lacks sensory structures
•
Parapodia are absent
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bioweb.uwlax.edu/.../ earthworm_model_1a.htm
Subclass Oligochaeta (cont’d)
• Feeds on organic or
vegetable matter in
soil
food
– May be as many as
50,000/acre
• Earthworms very
beneficial in aerating
the soil.
• capable of recycling
up to 18 tons of
soil/acre annually
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Do these questions now…
1. The coelomic cavity of annelids is usually
divided by walls called ________
2. What is metamerism?
3. What are the advantages of metamerism?
4. Is any part of the annelid not metameric?
What part?
5. To what class do earthworms and leeches
belong?
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Subclass Oligochaeta (cont’d)
• Gas exchange across moist skin  to
circulatory system
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Subclass Hirudinea
• Leeches
• Mostly freshwater
• possess clitellum – apparent
only during reproduction
• have annelid characteristics
but generally lack setae
• Monoecious, cross-fert.
• Generally dorsoventrally
flattened
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Subclass Hirudinea (cont’d)
• Predatory
– Attach to host via 2
suckers
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Subclass Hirudinea (cont’d)
Locomotion
– More complex
muscle systemincludes oblique +
dorsoventral
muscles
– “inchworm”
movement via
suckers
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Subclass Hirudinea (cont’d)
• Have a fixed number of
metameres
• Internal septa are lacking
– coelom functions as a
single, large chamber with
connective tissue, muscle,
and spaces
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Subclass Hirudinea (cont’d)
• Gas exchange across skin (in most)
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• squeamish?
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Medicinal leech
Leeches cure
• Attach via suckers,
pierce skin with sharp
proboscis.
• Proboscis resembles 3
circular saw blades.
• Salivary glands
secrete local
anesthetic and an
anti-coagulant.
• remove hematomas
resulting from
surgery
Ch 14
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