Chapter 12

Chapter 12Annelida: The Metameric Body FormEvolutionary PerspectiveBody metameric, bilaterally symmetrical, and worm-likeSpiral cleavage, trochophore larvae, and schizocoelous coelom formationPaired epidermal setaeClosed circulatory systemDorsal suprapharyngeal ganglia and ventral nerve cord(s) with gangliaMetanephridia or protonephridiaRelationships to Other AnimalsLophotrochozoans (Figure 12.1)Shared characteristics with Arthropoda are probably convergentMetamerismSchizocoelous coelom formationTraditional classification into two classes probably incorrect (Table 12.1)Annelid evolution poorly understood

Figure 12.1 Evolutionary relationships of Annelida to other animals.

Table 2.4Metamerism and TagmatizationMetamerismSegmental arrangement of body parts including many internal organ systemsCoelomic compartments develop segmentally (figure 12.3).AdvantagesHydrostatic compartments function in locomotion.Lessened impact of injuryTagmatization possibleSpecialization of body regions for specific functions

Figure 12.3 Development of metameric, coelomic spaces in annelids.Class PolychaetaMostly marineLargest annelid classExternal structure and locomotionParapodiaPaired lateral extensions of body wall in each segmentSetaeProstomiumLobe projecting dorsally and anteriorly from the mouthPeristomiumFirst body segment surrounds the mouthCrawling or swimming locomotion using parapodia

Figure 12.5 Polychaete locomotion involves antagonism of longitudinal muscles and locomotor waves.Feeding and the Digestive SystemStraight tube suspended within body cavity by mesenteries and septaAnterior proboscis may be everted.Predators, herbivores, scavengers, deposit feeders, filter feedersCrop, gizzard, intestineGas Exchange and CirculationGas exchangeDiffusion across body wallCirculationClosed circulatory system (figure 12.6)Blood with respiratory pigmentsDorsal 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 FunctionsSuprapharyngeal and subpharyngeal gangliaVentral nerve cord (double) with segmental gangliaSegmental 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 FunctionsTwo to four pairs of eyes on prostomiumNuchal organs in headChemoreceptorsStatocysts in headEquilibrium and balanceTactile receptors cover body.ExcretionAmmoniaNitrogenous wasteFreely diffusibleWater and ion regulationProtonephridia present in some primitive polychaetesFlagella drives fluids to nephridiopore.Metanephridia present in most polychaetesNephrostome, bladder, tubuleActive transport of ions between blood and nephridium

Regeneration, Reproduction, and DevelopmentRegeneration abilities well developedAsexual reproductionBudding or transverse fissionSexual reproductionMost dioeciousGametes shed into coelom to mature.Released via nephridium and nephridiopore or rupture of body wall, or gonoduct in a few speciesExternal fertilizationEpitoky commonDevelopmentSpiral 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 ClitellataEarthworms and leeches

Subclass Oligochaeta3,000 species of earthwormsFreshwater and terrestrialExternal structure Few, small setaeLack parapodiaClitellumGirdlelike structure secretes mucus during copulation and cocoon formation.

Figure 12.10 Earthworm structure.Subclass OligochaetaLocomotionAntagonism of circular and longitudinal muscles (figure 12.11)BurrowingCoelomic 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 OligochaetaFeeding and the digestive systemScavengersFallen and decaying vegetationDigestive system structure (figures 12.12 and 12.13)

Figure 12.12 Earthworm structure.

Figure 12.13 Earthworm cross section.Subclass OligochaetaGas exchangeDiffusion across body wallCirculationClosed circulatory system (figure 12.12)Blood with respiratory pigmentsDorsal 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 vesselsEarthworm hearts

Subclass OligochaetaNervous functionsSimilar to polychaetesFusion of double ventral nerve cordsSensory functionsLack well-developed eyesDermal light senseNegative phototaxisWell developed chemoreceptors and mechanical receptorsSubclass OligochaetaExcretionMetanephridiaExcretionWater and ion regulationChloragogen tissueLiverlike functionsAmino acid metabolismGlycogen and fat synthesis from excess carbohydratesSubclass OligochaetaReproduction and developmentMonoeciousMutual 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 clitellumForms cocoon that is deposited in soilSpiral cleavage and direct development

Figure 12.14 Earthworm reproduction.Subclass Hirudinea500 species of leechesMost freshwaterExternal structure (figure 12.15)Lack parapodiaSetae absent in mostBody subdivided into annuliAnterior and posterior suckers

Figure 12.15 Leech structure.Subclass HirudineaLocomotion (figure 12.16)Complex body wall musculatureLoss of septa results in interconnecting coelomic sinuses.Coelom acts as single hydrostatic compartment.Looping movements and undulating swimming movementsFeeding and the digestive systemPredators on invertebrates and blood of vertebratesProtrusible proboscis armed with 3 jawsSalivary anticoagulantPharynx pumps blood and other body fluids.

Figure 12.16 Leech locomotion.Subclass HirudineaGas exchange and circulationGas exchange by diffusion through body wallCirculationSimilar to oligochaetesCoelomic sinuses replace vessels.Coelomic fluid takes over function of blood.Respiratory pigments lacking in mostSubclass HirudineaNervous and sensory functionsSimilar to other annelidsVentral nerve cords unfusedPigment cup photoreceptorsNegatively phototacticTemperature sense in mammalian predatory leechesExcretionOne pair of metanephridia per segmentChloragogen tissue throughout body cavitySubclass HirudineaReproduction and DevelopmentMonoeciousSexual reproduction onlyClitellum present during spring breedingCopulation as in oligochaetesPenis aids in sperm transfer.Further Phylogenetic ConsiderationsUndergone intense taxonomic revisionMonophyletic phylumPolychaeta is paraphyletic and should be abandoned as a class name (figure 12.17).Other groups included (figures 12.18-12.20)EchiuraSiboglinidaeSipunculaClitellata evolved from ancestral polychaete that invaded freshwater.Invaded land and evolved with flowering plantsHirudinea 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.EchiuraEchiurans (spoon worms)Formerly phylum EchiuraBurrowers in sand and mudProboscis used to sweep organic material from substrate.Unsegmented

Figure 12.18 EchiuransSiboglinidsSiboglinidae (beard worms)Formerly phylum PognophoraNow polychaete familyDeep marine sedimentsNo digestive tractNutrient uptake from water and via symbiotic bacteria

Figure 12.19. SiboglinidsSipunculansSipunculans (peanut worms)Formerly phylum SipunculaMarine mud and sandIntrovert comprised of tentacles used in feedingRetracts into peanut shape when disturbedUnsegmented

Figure 12.20 Sipunculans