Mollusca and Annelida

Relationship of Mollusca and Annelida

Similar patterns of embryologyTrue coelomFluid filled cavity within the mesoderm

Advantages of a Coelom

Digestion independent of locomotionCirculatory system functions without interference from other organsHydrostatic skeleton against which the muscles can contract

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

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

CharacteristicsTrue coelomDistinct partsMuscular footHeadVisceral massBilateral symmetryOne or more shells in most species

Organ Systems

RespiratoryDigestiveCirculatoryExcretoryNervousReproduction

Body Plan

Muscular foot for locomotionHead contains mouth, sense organs and central gangliaVisceral mass containing heart, digestive, excretory, and reproductive organs

Mantle

Epidermal layer secretes shell Covers visceral mass

Shell

Calcium carbonate shells evolved earlyAdaptation to protect soft body from predationReduces surface area for gas exchange

Gill

Large surface areaRich supply of bloodSpecialized for gas exchangeProtected within mantle cavityBetween mantle and visceral mass

Class Polyplacophora

ChitonsShells divided into 8 overlapping plates

Class Gastropoda

75,000 speciesSnails, slugs, abalones, nudibranchs, and conchesSingle shell (valve) or none

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

Snails

Wide variety of habitatsTerrestrialAquatic (freshwater and marine)

Respiration - Aquatic Snails

Respire through gills in mantle cavity

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

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

Digestion

Saw like radula with flexible tongue like strip covered with chitinous teethScrape up algae (aquatic)Saw off leaves

Reproduction

Land snails hermaphroditicMost aquatic species have distinct sexesInternal fertilization

Movement

Muscular contractions of the footGlands in foot secrete layer of mucus

Sensory

2 eyes on retractable tentacles

Other Gastropods

Slugs Lack shellsMoist environment

Oyster DrillsDrill like radula

Bivalvia

Clams, oysters, scallops, and shipwormsSessileFilter feeders2 valves (shells)Muscular footLack distinct head regionAnterior ganglia

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

Mantle secretes mother of pearlHinge connects the two valvesPowerful adductor muscles

Clams

Adapted for filter feedingBeating cilia on gills setup currentWater enters incurrent siphonPropelled over gillsExit through excurrent siphon

Digestion

Food trapped in sticky mucusCilia moves mucus into mouth

Respiration

Water passes over gillsOxygen diffuses into bloodCarbon dioxide diffuses out

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

Reproduction

Separate sexesShed sperm and eggs into waterExternal fertilizationZygote -> trochophore larvaeLarvae settles to bottom -> adult

Other BivalvesOystersPermanently attached to substrateScallops move by jet propulsionTeredo (shipworm) bores into driftwoodSymbiotic protozoa that digests cellulose

Cephalopoda

Octopuses, squids, cuttlefish, and chambered nautilusesWell developed headProminent foot divided into tentaclesFree swimmingPredatoryStrong suckersRadula and sharp beak

Closed circulatory systemBlood circulates entirely within blood vesselsAllows for high metabolic rate

SquidsMarineLargest invertebrate (20 M; 3,360 kg)Large complex brainHighly developed nervous systemLarge pair of vertebrate like eyes

Ten tentaclesLargest pair used to capture preySmaller pairs force prey into mouthMuscular mantel propels by pumping water through siphonExcrete inky substance when threatened

Chromatophores - change colorInternal fertilizationLay mass of gelatinous encased eggsMaternal protection until hatching

Octopus

Eight tentaclesSimilar to squidsScrawl along bottom looking for prey

Chambered Nautilus

Retain exterior shellLives in outer chamber of its shellSecretes gas into other chamberRegulates buoyancy

Annelida

Segmented wormsFeather worms, earthworms, and leechesTrue coelomBody divided into segments (metamerism)

Some segments fusedWell developed organ systemMost have external bristles (setae)

Classification

Based on number of setae and presence or absence of parapodia (fleshy appendages)

Class Oligochaeta No parapodia and few setae

Class HirudineaNo setae or parapodia

Class PolychaetaMany setae and parapodia

Class OligochaetaEarthworms

Structure

More than 100 segmentsSegments separated by partitions that divide coelomSegments identical except when specialized and fussedAnterior and posterior ends

Cephalization specialized for burrowingHead contains sense organsCircular and longitudinal muscles

Movement

Setae anchored during movementCircular muscles contractHydrostatic pressure increases in anterior coelomic cavities

Body elongates pushing head forwardAnterior setae grip groundLongitudinal muscles contractPosterior pulled alongCoelomic divisions allow simultaneous contraction and expansion of segments  

Feeding

Feed on organic matter in soilDigest organic matterWastes and undigested soil eliminated as castingLoosens and aerates soil

Digestion

Soil sucked in by pharynx -> Esophagus ->Crop (storage) ->Gizzard (grinds soil releasing organic matter) -> Intestines (nutrients absorbed) Intestines (nutrients absorbed)

Circulation

Transport oxygen, nutrients, and wastesFlow of blood toward posterior via ventral blood vesselFlow of blood toward anterior via dorsal blood vessel

5 pair of aortic arches link vessels near anteriorSmaller vessels branch into segmentsContraction of ventral vessel and aortic arches forces blood through body

Respiration

Lack respiratory organsOxygen and carbon dioxide diffuse across skinRequires moist skinLimited habitatSecrete mucusThin cuticle

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

Nervous

Sensitive to touch, light, moisture, chemicals, temperature, and vibrationsLight receptors on head and tailSense direction

Anterior Ganglia Ventral Nerve cord

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

Reproduction

HermaphroditesCannot self fertilizeWorms join head to tailForm mucus coat around bodiesEach inject sperm into mucus

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

Class Polychaeta

PolychaetesMarineSome free swimming predators with strong jawsSome burrowing

Numerous setae projecting from parapodia used for movementSome parapodia function in respirationHave antennae and specialized mouth parts

Hirudinea

LeechesNo setaeAnterior and posterior sucker (walking motion)Fresh water