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Annelida: The Metameric Body Form
Chapter 12 Annelida: The Metameric Body Form
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Evolutionary Perspective
Body metameric, bilaterally symmetrical, and worm-like Spiral cleavage, trochophore larvae, and schizocoelous coelom formation Paired epidermal setae Closed circulatory system Dorsal suprapharyngeal ganglia and ventral nerve cord(s) with ganglia Metanephridia or protonephridia
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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
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Figure 12.1 Evolutionary relationships of Annelida to other animals.
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Table 2.4
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Metamerism and Tagmatization
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
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Figure 12.3 Development of metameric, coelomic spaces in annelids.
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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
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Figure 12.5 Polychaete locomotion involves antagonism of longitudinal muscles and locomotor waves.
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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
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Gas Exchange and Circulation
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.
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Figure 12.6 Circulatory system of a polychaete.
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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.
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Figure 12.7 Nervous system of a polychaete.
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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.
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Excretion Ammonia Water and ion regulation 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
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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
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Figure Polychaete development usually involves the formation of a (a) trochophore larva that (b) buds segments posteriorly. (c) A juvenile worm.
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Class Clitellata Earthworms and leeches
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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.
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Figure 12.10 Earthworm structure.
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Subclass Oligochaeta Locomotion
Antagonism of circular and longitudinal muscles (figure 12.11) Burrowing Coelomic hydrostatic pressure transmitted toward prostomium
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Figure 12. 11 Earthworm locomotion
Figure Earthworm locomotion. Arrows designate activity in specific segments of the body. Broken lines indicate regions of contact with the substrate.
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Subclass Oligochaeta Feeding and the digestive system Scavengers
Fallen and decaying vegetation Digestive system structure (figures and 12.13)
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Figure 12.12 Earthworm structure.
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Figure 12.13 Earthworm cross section.
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Subclass Oligochaeta Gas exchange Circulation
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”
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Subclass Oligochaeta Nervous functions Sensory 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
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Subclass Oligochaeta Excretion Metanephridia Chloragogen tissue
Water and ion regulation Chloragogen tissue Liverlike functions Amino acid metabolism Glycogen and fat synthesis from excess carbohydrates
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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
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Figure 12.14 Earthworm reproduction.
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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
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Figure 12.15 Leech structure.
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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.
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Figure 12.16 Leech locomotion.
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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
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Subclass Hirudinea Nervous and sensory functions Excretion
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
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Subclass Hirudinea Reproduction and Development Monoecious
Sexual reproduction only Clitellum present during spring breeding Copulation as in oligochaetes Penis aids in sperm transfer.
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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 ) Echiura Siboglinidae Sipuncula Clitellata evolved from ancestral polychaete that invaded freshwater. Invaded land and evolved with flowering plants Hirudinea derived from ancient aquatic oligochaete
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Figure 12. 17 Annelid phylogeny. Polychaeta is a polyphyletic grouping
Figure Annelid phylogeny. Polychaeta is a polyphyletic grouping. Clitellata, Echiura, Siboglinidae, and Sipuncula are nested within the polychaetes.
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Echiura Echiurans (spoon worms) Formerly phylum Echiura
Figure Echiurans Echiurans (spoon worms) Formerly phylum Echiura Burrowers in sand and mud Proboscis used to sweep organic material from substrate. Unsegmented
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Siboglinids Siboglinidae (beard worms) Formerly phylum Pognophora
Figure Siboglinids Siboglinidae (beard worms) Formerly phylum Pognophora Now polychaete family Deep marine sediments No digestive tract Nutrient uptake from water and via symbiotic bacteria
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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 Sipunculans
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