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Phylum.Phylum. Mollusca MolluscaClass.Class. Aplacophora Aplacophora √√
Class.Class. Polyplacophora Polyplacophora √√
Class.Class. Monoplacophora Monoplacophora
Class.Class. Gastropoda Gastropoda
Class.Class. Cephalopoda Cephalopoda
Class.Class. Bivalvia Bivalvia
Class.Class. Scaphopoda Scaphopoda
Phylum.Phylum. Mollusca MolluscaClass.Class. Aplacophora Aplacophora √√
Class.Class. Polyplacophora Polyplacophora √√
Class.Class. Monoplacophora Monoplacophora
Class.Class. Gastropoda Gastropoda
Class.Class. Cephalopoda Cephalopoda
Class.Class. Bivalvia Bivalvia
Class.Class. Scaphopoda Scaphopoda
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Aplacophora Polyplacophora Mon
opla
coph
ora
Gas
tropo
daCep
halo
poda
Bivalvia Scaphopoda
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Monoplacophora
NephridiumNephridiumNephridiumNephridium
CtenidiumCtenidiumCtenidiumCtenidium
GonadsGonadsGonadsGonads
Pedal retractorPedal retractormusclemusclePedal retractorPedal retractormusclemuscle
MouthMouthMouthMouth
Nerve cordNerve cordNerve cordNerve cord
Heart atriaHeart atriaHeart atriaHeart atria
AnusAnusAnusAnus
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Monoplacophora
• Extant spp discovered in 1952• Only 20 spp, all marine, deep water zones (1800-
7000m). • Poorly studied• Likely ancestor of gastropods, cephalopods and bivalves,
and the bivalvia and scaphopods• Monoplacophorans and Polyplacophorans evolved shells
independently from a shell-less ancestor. Evidence: shells differ in internal layer structure
• Superficially similar to gastropod limpets
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GastropodaGastropoda
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Gastropoda
• Most diverse taxon of mollusca
• Estimates range from 40,000-100,000 spp (probably 60,000 extant, 15,000 extinct spp)
• Three major groups:– Prosobranchs – benthic marine spp– Opisthobranchs – secondary loss of the shell– Pulmonates – air breathers
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Torsion is unique to gastropods
Most gastropods are dextral
Pretorsion Post torsion
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Prosobranch Opisthobranch Pulmonata
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Prosobranchs
• Mantle cavity anterior, due to torsion
• Most common, typical “snail”
• Mostly marine, some freshwater, terrestrial
• Most primitive group of gastropods
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Opisthobranchs
• Mantle cavity lateral or posterior, due to detorsion or loss of shell
• ca 2000 spp. e.g. nudibranchs (sea hares, sea slugs)
• Ctendia often lost. Gas exchange via cerata
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Pulmonata
• Highly vascularized mantle for
gas exchange (lung)
• 17,000 spp: slugs, pond snails
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Gastropoda
• More active than mono and polyplacophorans– Highly cephalized: tentacles, eyes
• Gonochoristic (dioecious)
• Veliger larva (an advanced version of the trochophore larva)
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Veliger larva
VelumVelumVelumVelum
StomachStomachStomachStomach
DigestiveDigestivececumcecum
DigestiveDigestivececumcecum
FootFootFootFoot
ShellShellShellShell
EsophagusEsophagusEsophagusEsophagus
MetanephridiumMetanephridiumMetanephridiumMetanephridium
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Cephalopoda
• Swift, agile carnivores • Closed circulatory system, 2 hearts• Separate sexes• Foot modified to form arms, tentacles, siphon• Brain, cranium, complex image-forming eye• 700 extant spp, 10,000 extinct spp• Arose from limpet-like monoplacophorans• Ergo, ventral became functional anterior, etc
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CephalopodaCephalopoda
DorsalDorsalVentralVentral
Posterior surfacePosterior surface
RightRight
LeftLeft
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Cephalopod eyeCephalopod eye
IrisIrisIrisIris
LensLensLensLens
CorneaCorneaCorneaCornea
RetinaRetinaRetinaRetina
Optic nervesOptic nervesOptic nervesOptic nerves
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eye
Optic lobe
statocyst
Cerebral ganglion
Brain is surrounded by a cranium
Brachial nerves
Buccal ganglia
esophagus
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Cephalopoda
• Ectocochleate cephalopods– Have external shell with internally subdivisions
used for buoyancy control– This ancestral group is almost completely extinct– E.g. Nautilus
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• Endocochleate cephalopds– Reduced internal shell, or shell absent
– Squids, cuttlefish, octopi
Cephalopoda
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FinFinFinFin
ArmArmArmArm
Funnel (siphon)Funnel (siphon)Funnel (siphon)Funnel (siphon)
EyeEyeEyeEye
TentacleTentacleTentacleTentacle
CollarCollarCollarCollar
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Shell (Pen)Shell (Pen)Shell (Pen)Shell (Pen)
CtenidiumCtenidiumCtenidiumCtenidium
FunnelFunnelFunnelFunnel
SystemicSystemicheartheartSystemicSystemicheartheart
Branchial heartBranchial heartBranchial heartBranchial heart
Hectocotylus (sperm-bearing arm in males)
Reproduction: trochophore and veliger are bypassed and hatch into planktonic juveniles
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Nautilus is the only cephalopod with an external shell and lacking chromatophores
Chromatophores (color cells)
Iridocytes (reflective cells)
- Millions of these allow rapid changes in color, polarized signals
- Also have photophores for bioluminescence
Cephalopods except Nautilus have ink sac
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Bivalvia (Pelecypoda)
• 8000 extant spp (1300 fw, 6700 marine)
• Specialized for infaunal habitat
• Sessile, little cephalization
• Filter feeders, using gills– 3 major groups of bivalves based on gill shape– Protobranchs (deposit feeders, most primitive)– Lammelibranchs (suspension feeders, most common)– Septibranchs (carnivores, most derived)
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Protobranchs
• Gills for gas exchange only
• Tend to live in deeper waters (>1000m)
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Lamellibranchs• Gills: gas exchange + filter feeding
• Incurrent siphon, excurrent siphon
IncurrentIncurrentsiphonsiphonIncurrentIncurrentsiphonsiphon
CtenidiumCtenidiumCtenidiumCtenidium
ExcurrentExcurrentsiphonsiphonExcurrentExcurrentsiphonsiphon
HingeHingeHingeHinge
FootFootFootFoot
mouth
Cut-away of gill structure
Blood vessel
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LocomotionLocomotion
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glochidium
Glochidia
Glochidia on gills
Freshwater mussels
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Septibranch
• Ctenidia lack filaments
• Feed on polychaetes, crustaceans
• Weird side group
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ScaphopodaScaphopoda
• Shared (extinct)
common ancestor
with bivalves
• 300-400 spp
• Lack ctenidia, heart
• Burrowers
• Have 100-200
captacula (tentacles)
with which to catch food