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

Aplacophora Polyplacophora Mon

opla

coph

ora

Gas

tropo

daCep

halo

poda

Bivalvia Scaphopoda

Monoplacophora

NephridiumNephridiumNephridiumNephridium

CtenidiumCtenidiumCtenidiumCtenidium

GonadsGonadsGonadsGonads

Pedal retractorPedal retractormusclemusclePedal retractorPedal retractormusclemuscle

MouthMouthMouthMouth

Nerve cordNerve cordNerve cordNerve cord

Heart atriaHeart atriaHeart atriaHeart atria

AnusAnusAnusAnus

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

GastropodaGastropoda

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

Torsion is unique to gastropods

Most gastropods are dextral

Pretorsion Post torsion

Prosobranch Opisthobranch Pulmonata

Prosobranchs

• Mantle cavity anterior, due to torsion

• Most common, typical “snail”

• Mostly marine, some freshwater, terrestrial

• Most primitive group of gastropods

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

Pulmonata

• Highly vascularized mantle for

gas exchange (lung)

• 17,000 spp: slugs, pond snails

Gastropoda

• More active than mono and polyplacophorans– Highly cephalized: tentacles, eyes

• Gonochoristic (dioecious)

• Veliger larva (an advanced version of the trochophore larva)

Veliger larva

VelumVelumVelumVelum

StomachStomachStomachStomach

DigestiveDigestivececumcecum

DigestiveDigestivececumcecum

FootFootFootFoot

ShellShellShellShell

EsophagusEsophagusEsophagusEsophagus

MetanephridiumMetanephridiumMetanephridiumMetanephridium

Cephalopoda

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

CephalopodaCephalopoda

DorsalDorsalVentralVentral

Posterior surfacePosterior surface

RightRight

LeftLeft

Cephalopod eyeCephalopod eye

IrisIrisIrisIris

LensLensLensLens

CorneaCorneaCorneaCornea

RetinaRetinaRetinaRetina

Optic nervesOptic nervesOptic nervesOptic nerves

eye

Optic lobe

statocyst

Cerebral ganglion

Brain is surrounded by a cranium

Brachial nerves

Buccal ganglia

esophagus

Cephalopoda

• Ectocochleate cephalopods– Have external shell with internally subdivisions

used for buoyancy control– This ancestral group is almost completely extinct– E.g. Nautilus

• Endocochleate cephalopds– Reduced internal shell, or shell absent

– Squids, cuttlefish, octopi

Cephalopoda

FinFinFinFin

ArmArmArmArm

Funnel (siphon)Funnel (siphon)Funnel (siphon)Funnel (siphon)

EyeEyeEyeEye

TentacleTentacleTentacleTentacle

CollarCollarCollarCollar

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

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

BivalviaBivalvia

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)

Protobranchs

• Gills for gas exchange only

• Tend to live in deeper waters (>1000m)

Lamellibranchs• Gills: gas exchange + filter feeding

• Incurrent siphon, excurrent siphon

IncurrentIncurrentsiphonsiphonIncurrentIncurrentsiphonsiphon

CtenidiumCtenidiumCtenidiumCtenidium

ExcurrentExcurrentsiphonsiphonExcurrentExcurrentsiphonsiphon

HingeHingeHingeHinge

FootFootFootFoot

mouth

Cut-away of gill structure

Blood vessel

LocomotionLocomotion

glochidium

Glochidia

Glochidia on gills

Freshwater mussels

Septibranch

• Ctenidia lack filaments

• Feed on polychaetes, crustaceans

• Weird side group

ScaphopodaScaphopoda

• Shared (extinct)

common ancestor

with bivalves

• 300-400 spp

• Lack ctenidia, heart

• Burrowers

• Have 100-200

captacula (tentacles)

with which to catch food