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Phylum. Mollusca Class. Aplacophora √ Class. Polyplacophora √ Class. Monoplacophora

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Phylum. Mollusca Class. Aplacophora √ Class. Polyplacophora √ Class. Monoplacophora Class. Gastropoda Class. Cephalopoda Class. Bivalvia Class. Scaphopoda. Monoplacophora. Cephalopoda. Gastropoda. Bivalvia. Scaphopoda. Polyplacophora. Aplacophora. Monoplacophora. Mouth.

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Presentation Transcript
slide1
Phylum. Mollusca

Class. Aplacophora √

Class. Polyplacophora √

Class. Monoplacophora

Class. Gastropoda

Class. Cephalopoda

Class. Bivalvia

Class. Scaphopoda

slide2
Monoplacophora

Cephalopoda

Gastropoda

Bivalvia

Scaphopoda

Polyplacophora

Aplacophora

monoplacophora
Monoplacophora

Mouth

Nephridium

Nerve cord

Ctenidium

Gonads

Heart atria

Pedal retractor

muscle

Anus

monoplacophora1
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
gastropoda1
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
slide7
Torsion is unique to gastropods

Most gastropods are dextral

Pretorsion

Post torsion

slide8
Prosobranch

Opisthobranch

Pulmonata

prosobranchs
Prosobranchs
  • Mantle cavity anterior, due to torsion
  • Most common, typical “snail”
  • Mostly marine, some freshwater, terrestrial
  • Most primitive group of gastropods
opisthobranchs
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
Pulmonata
  • Highly vascularized mantle for

gas exchange (lung)

  • 17,000 spp: slugs, pond snails
gastropoda2
Gastropoda
  • More active than mono and polyplacophorans
    • Highly cephalized: tentacles, eyes
  • Gonochoristic (dioecious)
  • Veliger larva (an advanced version of the trochophore larva)
veliger larva
Veliger larva

Metanephridium

Velum

Shell

Stomach

Digestive

cecum

Foot

Esophagus

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
cephalopoda1
Cephalopoda

Posterior surface

Right

Ventral

Dorsal

Left

cephalopod eye
Cephalopod eye

Iris

Lens

Retina

Cornea

Optic nerves

slide20
eye

Optic lobe

Buccal ganglia

Cerebral ganglion

statocyst

esophagus

Brachial nerves

Brain is surrounded by a cranium

cephalopoda2
Cephalopoda
  • Ectocochleate cephalopods
    • Have external shell with internally subdivisions used for buoyancy control
    • This ancestral group is almost completely extinct
    • E.g. Nautilus
slide22
Cephalopoda
  • Endocochleate cephalopds
    • Reduced internal shell, or shell absent
    • Squids, cuttlefish, octopi
slide23
Tentacle

Arm

Funnel (siphon)

Fin

Collar

Eye

slide24
Shell (Pen)

Systemic

heart

Branchial heart

Ctenidium

Funnel

Hectocotylus (sperm-bearing arm in males)

Reproduction: trochophore and veliger are bypassed and hatch into planktonic juveniles

slide25
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

bivalvia pelecypoda
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
Protobranchs
  • Gills for gas exchange only
  • Tend to live in deeper waters (>1000m)
lamellibranchs
Lamellibranchs
  • Gills: gas exchange + filter feeding
  • Incurrent siphon, excurrent siphon

Cut-away of

gill structure

Hinge

Blood vessel

mouth

Ctenidium

Excurrent

siphon

Foot

Incurrent

siphon

slide31
Glochidia

glochidium

Glochidia on gills

Freshwater mussels

septibranch
Septibranch
  • Ctenidia lack filaments
  • Feed on polychaetes, crustaceans
  • Weird side group
scaphopoda
Scaphopoda
  • Shared (extinct)

common ancestor

with bivalves

  • 300-400 spp
  • Lack ctenidia, heart
  • Burrowers
  • Have 100-200

captacula (tentacles)

with which to catch food

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