Annelida: The Metameric Body Form Chapter 12 Annelida: The Metameric Body Form

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

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 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 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 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

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 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 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”

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

Subclass Oligochaeta Excretion Metanephridia Chloragogen tissue 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 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

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 12.18-12.20) 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 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 Figure 12.18 Echiurans Echiurans (spoon worms) Formerly phylum Echiura Burrowers in sand and mud Proboscis used to sweep organic material from substrate. Unsegmented

Siboglinids Siboglinidae (beard worms) Formerly phylum Pognophora Figure 12.19. Siboglinids Siboglinidae (beard worms) Formerly phylum Pognophora Now polychaete family Deep marine sediments No digestive tract Nutrient uptake from water and via symbiotic bacteria

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