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Chapter 12
Annelida: The Metameric
Body Form
Evolutionary Perspective
1. Body metameric, bilaterally
symmetrical, and worm-like
2. Spiral cleavage, trochophore
larvae, and schizocoelous coelom
formation
3. Paired epidermal setae
4. Closed circulatory system
5. Dorsal suprapharyngeal ganglia and
ventral nerve cord(s) with ganglia
6. Metanephridia or protonephridia
Relationships to Other
Animals
• Lophotrochozoans (Figure 12.1)
• Shared characteristics with
Arthropoda are probably convergent
– Metamerism
– Schizocoelous coelom formation
• Traditional classification into two
classes probably incorrect (Table
12.1)
– Annelid evolution poorly understood
Figure 12.1 Evolutionary relationships of Annelida to other
animals.
Table 2.4
Metamerism and
Tagmatization
• Metamerism
– Segmental arrangement of body parts
including many internal organ systems
– Coelomic compartments develop
segmentally (figure 12.3).
– Advantages
• Hydrostatic compartments function in
locomotion.
• Lessened impact of injury
• Tagmatization possible
– Specialization of body regions for specific
functions
Figure 12.3 Development of
metameric, coelomic spaces
in annelids.
Class Polychaeta
• Mostly marine
• Largest annelid class
• External structure and locomotion
– Parapodia
• Paired lateral extensions of body wall in each
segment
• Setae
– Prostomium
• Lobe projecting dorsally and anteriorly from the
mouth
– Peristomium
• First body segment surrounds the mouth
– Crawling or swimming locomotion using
parapodia
Figure 12.5 Polychaete locomotion involves
antagonism of longitudinal muscles and locomotor
waves.
Feeding and the Digestive
System
• Straight tube suspended within
body cavity by mesenteries and
septa
• Anterior proboscis may be
everted.
• Predators, herbivores,
scavengers, deposit feeders, filter
feeders
• Crop, gizzard, intestine
Gas Exchange and
Circulation
• Gas exchange
– Diffusion across body wall
• Circulation
– Closed circulatory system (figure 12.6)
– Blood with respiratory pigments
– Dorsal aorta and ventral aorta are
contractile.
– Segmental vessels and capillaries serve
intestine, body wall, and parapodia.
Figure 12.6 Circulatory system of a polychaete.
Nervous Functions
• Suprapharyngeal and subpharyngeal
ganglia
• Ventral nerve cord (double) with
segmental ganglia
• Segmental ganglia coordinate
movements in isolated segments.
• Subpharyngeal ganglia coordinate
distant segments.
• Suprapharyngeal ganglia control
sensory and motor functions involved
with feeding and locomotion.
• Giant fibers mediate escape responses.
Figure 12.7 Nervous system of a polychaete.
Sensory Functions
• Two to four pairs of eyes on
prostomium
• Nuchal organs in head
– Chemoreceptors
• Statocysts in head
– Equilibrium and balance
• Tactile receptors cover body.
Excretion
• Ammonia
– Nitrogenous waste
– Freely diffusible
• Water and ion regulation
– Protonephridia present in some
primitive polychaetes
• Flagella drives fluids to nephridiopore.
– Metanephridia present in most
polychaetes
• Nephrostome, bladder, tubule
• Active transport of ions between blood and
nephridium
Regeneration, Reproduction,
and Development
• Regeneration abilities well developed
• Asexual reproduction
– Budding or transverse fission
• Sexual reproduction
– Most dioecious
– Gametes shed into coelom to mature.
– Released via nephridium and nephridiopore or
rupture of body wall, or gonoduct in a few species
– External fertilization
– Epitoky common
• Development
– Spiral cleavage with trochophore larva that buds
segments posteriorly (figure 12.9)
– Direct development in some
Figure 12.9 Polychaete development usually involves
the formation of a (a) trochophore larva that (b) buds
segments posteriorly. (c) A juvenile worm.
Class Clitellata
• Earthworms and leeches
Subclass Oligochaeta
• 3,000 species of earthworms
• Freshwater and terrestrial
• External structure
– Few, small setae
– Lack parapodia
– Clitellum
• Girdlelike structure secretes mucus
during copulation and cocoon formation.
Figure 12.10 Earthworm structure.
Subclass Oligochaeta
• Locomotion
– Antagonism of circular and
longitudinal muscles (figure 12.11)
– Burrowing
• Coelomic hydrostatic pressure
transmitted toward prostomium
Figure 12.11 Earthworm
locomotion. Arrows
designate activity in
specific segments of the
body. Broken lines
indicate regions of contact
with the substrate.
Subclass Oligochaeta
• Feeding and the digestive system
– Scavengers
– Fallen and decaying vegetation
– Digestive system structure (figures
12.12 and 12.13)
Figure 12.12 Earthworm structure.
Figure 12.13 Earthworm cross section.
Subclass Oligochaeta
• Gas exchange
– Diffusion across body wall
• Circulation
– Closed circulatory system (figure 12.12)
– Blood with respiratory pigments
– Dorsal aorta and ventral aorta are
contractile.
– Segmental vessels and capillaries serve
intestine and body wall.
• Some contractile and propel blood between
dorsal and ventral blood vessels
• Earthworm “hearts”
Subclass Oligochaeta
• Nervous functions
– Similar to polychaetes
– Fusion of double ventral nerve cords
• Sensory functions
– Lack well-developed eyes
• Dermal light sense
• Negative phototaxis
– Well developed chemoreceptors and
mechanical receptors
Subclass Oligochaeta
• Excretion
– Metanephridia
• Excretion
• Water and ion regulation
– Chloragogen tissue
• Liverlike functions
– Amino acid metabolism
– Glycogen and fat synthesis from excess
carbohydrates
Subclass Oligochaeta
• Reproduction and development
– Monoecious
– Mutual sperm exchange between
copulating individuals (figure 12.14)
• Seminal vesicles store sperm prior to
copulation.
• Seminal receptacles receive sperm during
copulation.
– Fertilization within mucous sheath
secreted by clitellum
– Forms cocoon that is deposited in soil
– Spiral cleavage and direct development
Figure 12.14 Earthworm reproduction.
Subclass Hirudinea
• 500 species of leeches
• Most freshwater
• External structure (figure 12.15)
– Lack parapodia
– Setae absent in most
– Body subdivided into annuli
– Anterior and posterior suckers
Figure 12.15 Leech structure.
Subclass Hirudinea
• Locomotion (figure 12.16)
– Complex body wall musculature
– Loss of septa results in interconnecting
coelomic sinuses.
• Coelom acts as single hydrostatic compartment.
• Looping movements and undulating swimming
movements
• Feeding and the digestive system
– Predators on invertebrates and blood of
vertebrates
– Protrusible proboscis armed with 3 jaws
– Salivary anticoagulant
– Pharynx pumps blood and other body fluids.
Figure 12.16 Leech locomotion.
Subclass Hirudinea
• Gas exchange and circulation
– Gas exchange by diffusion through
body wall
– Circulation
• Similar to oligochaetes
• Coelomic sinuses replace vessels.
• Coelomic fluid takes over function of
blood.
• Respiratory pigments lacking in most
Subclass Hirudinea
• Nervous and sensory functions
– Similar to other annelids
– Ventral nerve cords unfused
– Pigment cup photoreceptors
• Negatively phototactic
– Temperature sense in mammalian
predatory leeches
• Excretion
– One pair of metanephridia per segment
– Chloragogen tissue throughout body
cavity
Subclass Hirudinea
• Reproduction and Development
– Monoecious
– Sexual reproduction only
– Clitellum present during spring
breeding
– Copulation as in oligochaetes
• Penis aids in sperm transfer.
Further Phylogenetic
Considerations
• Undergone intense taxonomic revision
– Monophyletic phylum
– Polychaeta is paraphyletic and should be
abandoned as a class name (figure 12.17).
– Other “groups” included (figures 12.1812.20)
• Echiura
• Siboglinidae
• Sipuncula
– Clitellata evolved from ancestral
polychaete that invaded freshwater.
• Invaded land and evolved with flowering plants
• Hirudinea derived from ancient aquatic
oligochaete
Figure 12.17 Annelid phylogeny. Polychaeta is a polyphyletic
grouping. Clitellata, Echiura, Siboglinidae, and Sipuncula are
nested within the polychaetes.
Echiura
• Echiurans (spoon
worms)
– Formerly phylum
Echiura
– Burrowers in sand
and mud
– Proboscis used to
sweep organic
material from
substrate.
– Unsegmented
Figure 12.18 Echiurans
Siboglinids
• Siboglinidae (beard
worms)
– Formerly phylum
Pognophora
– Now polychaete
family
– Deep marine
sediments
– No digestive tract
– Nutrient uptake
from water and via
symbiotic bacteria
Figure 12.19. Siboglinids
Sipunculans
• Sipunculans
(peanut worms)
– Formerly phylum
Sipuncula
– Marine mud and
sand
– Introvert
comprised of
tentacles used in
feeding
– Retracts into
“peanut” shape
when disturbed
– Unsegmented
Figure 12.20 Sipunculans