mollusca and annelida. relationship of mollusca and annelida similar patterns of embryology true...
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Mollusca and Annelida
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Relationship of Mollusca and Annelida
Similar patterns of embryologyTrue coelomFluid filled cavity within the mesoderm
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Advantages of a Coelom
Digestion independent of locomotionCirculatory system functions without interference from other organsHydrostatic skeleton against which the muscles can contract
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Trochophore Larvae
Larval Similarities between Annelids and MollusksPear shaped ciliated larvaeCilia on both ends and middleDispersal of offspringAdvantage for shell burdened adultsTerrestrial forms develop within the egg; Not free-living
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Importance of Mollusks
100,000 speciesSome are filter feedersOthers are active predatorsSome are alternative parasitic hostsSome cause considerable economic damage to cropsOthers are valued by collectors
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CharacteristicsTrue coelomDistinct partsMuscular footHeadVisceral massBilateral symmetryOne or more shells in most species
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Organ Systems
RespiratoryDigestiveCirculatoryExcretoryNervousReproduction
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Body Plan
Muscular foot for locomotionHead contains mouth, sense organs and central gangliaVisceral mass containing heart, digestive, excretory, and reproductive organs
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Mantle
Epidermal layer secretes shell Covers visceral mass
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Shell
Calcium carbonate shells evolved earlyAdaptation to protect soft body from predationReduces surface area for gas exchange
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Gill
Large surface areaRich supply of bloodSpecialized for gas exchangeProtected within mantle cavityBetween mantle and visceral mass
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Class Polyplacophora
ChitonsShells divided into 8 overlapping plates
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Class Gastropoda
75,000 speciesSnails, slugs, abalones, nudibranchs, and conchesSingle shell (valve) or none
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Body Plan
Based on ancestral mollusksTorsion results from twisting during larval developmentVisceral mass twists 180 degrees in relation to the headMantle cavity in front of animalAllows head to be drawn into mantle cavity
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Snails
Wide variety of habitatsTerrestrialAquatic (freshwater and marine)
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Respiration - Aquatic Snails
Respire through gills in mantle cavity
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Respiration - Terrestrial Snails
Mantle cavity acts as modified lungAllows gas exchange with airMembrane must be keep moist to allow gas exchangeRetreats into shell and seals opening with mucus in dry environment
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Circulation
Open circulatory systemBlood does not circulate entirely within vesselsCollected from gills (lungs) and pumped through heartReleased directly into spaces in the tissue (hemocoel)Returned via gills or lungs to heart
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Digestion
Saw like radula with flexible tongue like strip covered with chitinous teethScrape up algae (aquatic)Saw off leaves
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Reproduction
Land snails hermaphroditicMost aquatic species have distinct sexesInternal fertilization
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Movement
Muscular contractions of the footGlands in foot secrete layer of mucus
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Sensory
2 eyes on retractable tentacles
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Other Gastropods
Slugs Lack shellsMoist environment
Oyster DrillsDrill like radula
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Bivalvia
Clams, oysters, scallops, and shipwormsSessileFilter feeders2 valves (shells)Muscular footLack distinct head regionAnterior ganglia
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Shell
3 layers secreted by mantle
Thin outer layerProtects shell against acidic conditions
Thick middle layerComposed of calcium carbonate crystalsStrengthens shell
Smooth inner layerProtects animals soft body
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Mantle secretes mother of pearlHinge connects the two valvesPowerful adductor muscles
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Clams
Adapted for filter feedingBeating cilia on gills setup currentWater enters incurrent siphonPropelled over gillsExit through excurrent siphon
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Digestion
Food trapped in sticky mucusCilia moves mucus into mouth
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Respiration
Water passes over gillsOxygen diffuses into bloodCarbon dioxide diffuses out
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Nervous
Cells along edge of mantle respond to light and touchGanglia located above mouth, in the digestive system, and footConnected by 2 pairs of long nerve cords
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Reproduction
Separate sexesShed sperm and eggs into waterExternal fertilizationZygote -> trochophore larvaeLarvae settles to bottom -> adult
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Other BivalvesOystersPermanently attached to substrateScallops move by jet propulsionTeredo (shipworm) bores into driftwoodSymbiotic protozoa that digests cellulose
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Cephalopoda
Octopuses, squids, cuttlefish, and chambered nautilusesWell developed headProminent foot divided into tentaclesFree swimmingPredatoryStrong suckersRadula and sharp beak
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Closed circulatory systemBlood circulates entirely within blood vesselsAllows for high metabolic rate
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SquidsMarineLargest invertebrate (20 M; 3,360 kg)Large complex brainHighly developed nervous systemLarge pair of vertebrate like eyes
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Ten tentaclesLargest pair used to capture preySmaller pairs force prey into mouthMuscular mantel propels by pumping water through siphonExcrete inky substance when threatened
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Chromatophores - change colorInternal fertilizationLay mass of gelatinous encased eggsMaternal protection until hatching
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Octopus
Eight tentaclesSimilar to squidsScrawl along bottom looking for prey
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Chambered Nautilus
Retain exterior shellLives in outer chamber of its shellSecretes gas into other chamberRegulates buoyancy
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Annelida
Segmented wormsFeather worms, earthworms, and leechesTrue coelomBody divided into segments (metamerism)
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Some segments fusedWell developed organ systemMost have external bristles (setae)
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Classification
Based on number of setae and presence or absence of parapodia (fleshy appendages)
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Class Oligochaeta No parapodia and few setae
Class HirudineaNo setae or parapodia
Class PolychaetaMany setae and parapodia
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Class OligochaetaEarthworms
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Structure
More than 100 segmentsSegments separated by partitions that divide coelomSegments identical except when specialized and fussedAnterior and posterior ends
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Cephalization specialized for burrowingHead contains sense organsCircular and longitudinal muscles
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Movement
Setae anchored during movementCircular muscles contractHydrostatic pressure increases in anterior coelomic cavities
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Body elongates pushing head forwardAnterior setae grip groundLongitudinal muscles contractPosterior pulled alongCoelomic divisions allow simultaneous contraction and expansion of segments
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Feeding
Feed on organic matter in soilDigest organic matterWastes and undigested soil eliminated as castingLoosens and aerates soil
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Digestion
Soil sucked in by pharynx -> Esophagus ->Crop (storage) ->Gizzard (grinds soil releasing organic matter) -> Intestines (nutrients absorbed) Intestines (nutrients absorbed)
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Circulation
Transport oxygen, nutrients, and wastesFlow of blood toward posterior via ventral blood vesselFlow of blood toward anterior via dorsal blood vessel
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5 pair of aortic arches link vessels near anteriorSmaller vessels branch into segmentsContraction of ventral vessel and aortic arches forces blood through body
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Respiration
Lack respiratory organsOxygen and carbon dioxide diffuse across skinRequires moist skinLimited habitatSecrete mucusThin cuticle
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Excretion
Elimination of nitrogen wastesLong tubules (nephridia) excrete wastesCoelomic fluid enters nephridium through ciliated funnel openingSome water reabsorbed by bloodRemaining fluid excreted through ventral pores
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Nervous
Sensitive to touch, light, moisture, chemicals, temperature, and vibrationsLight receptors on head and tailSense direction
Anterior Ganglia Ventral Nerve cord
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Most other sense organs and nerves that control muscle contractions found in individual segmentsPair of ganglia in each segment coordinate movement with adjacent segments Cerebral ganglion in headControls total body Ventral nerve cord connects brain with ganglia
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Reproduction
HermaphroditesCannot self fertilizeWorms join head to tailForm mucus coat around bodiesEach inject sperm into mucus
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Sperm move to seminal receptacleEggs move through oviducts to female genital poreClitellum secretes mucus and chitinous sheathWorm wiggles to slip off sheathEggs and sperm join
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Class Polychaeta
PolychaetesMarineSome free swimming predators with strong jawsSome burrowing
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Numerous setae projecting from parapodia used for movementSome parapodia function in respirationHave antennae and specialized mouth parts
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Hirudinea
LeechesNo setaeAnterior and posterior sucker (walking motion)Fresh water