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Phylum. Mollusca Class. Aplacophora √ Class. Polyplacophora √ Class. Monoplacophora - PowerPoint PPT Presentation


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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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Phylum. Mollusca

Class. Aplacophora √

Class. Polyplacophora √

Class. Monoplacophora

Class. Gastropoda

Class. Cephalopoda

Class. Bivalvia

Class. Scaphopoda


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


Torsion is unique to gastropods

Most gastropods are dextral

Pretorsion

Post torsion


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


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


Cephalopoda

  • Endocochleate cephalopds

    • Reduced internal shell, or shell absent

    • Squids, cuttlefish, octopi


Tentacle

Arm

Funnel (siphon)

Fin

Collar

Eye


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


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
Bivalvia lacking chromatophores


Bivalvia pelecypoda
Bivalvia (Pelecypoda) lacking chromatophores

  • 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 lacking chromatophores

  • Gills for gas exchange only

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


Lamellibranchs
Lamellibranchs lacking chromatophores

  • Gills: gas exchange + filter feeding

  • Incurrent siphon, excurrent siphon

Cut-away of

gill structure

Hinge

Blood vessel

mouth

Ctenidium

Excurrent

siphon

Foot

Incurrent

siphon


Locomotion
Locomotion lacking chromatophores


Glochidia lacking chromatophores

glochidium

Glochidia on gills

Freshwater mussels


Septibranch
Septibranch lacking chromatophores

  • Ctenidia lack filaments

  • Feed on polychaetes, crustaceans

  • Weird side group


Scaphopoda
Scaphopoda lacking chromatophores

  • 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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