1. Class. Polyplacophora √ Class. Monoplacophora Class. Gastropoda
Mollusca
4/23/2017
Phylum. Mollusca
Class. Aplacophora √
Class. Polyplacophora √
Class. Monoplacophora
Class. Gastropoda
Class. Cephalopoda
Class. Bivalvia
Class. Scaphopoda

2. Monoplacophora
Cephalopoda
Gastropoda
Bivalvia
Scaphopoda
Aplacophora
Polyplacophora

3. Monoplacophora Mouth Nephridium Nerve cord Ctenidium Gonads
Mollusca
4/23/2017
Monoplacophora
Mouth
Nephridium
Nerve cord
Ctenidium
Gonads
Heart atria
Pedal retractor
muscle
Anus
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4. Monoplacophora Extant spp discovered in 1952
Only 20 spp, all marine, deep water zones ( m).
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

5. Gastropoda
Mollusca
4/23/2017

6. Gastropoda Most diverse taxon of mollusca
Estimates range from 40, ,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

7. Torsion is unique to gastropods Most gastropods are dextral
Pretorsion
Post torsion

8. Prosobranch
Opisthobranch
Pulmonata

9. Prosobranchs Mantle cavity anterior, due to torsion
Most common, typical “snail”
Mostly marine, some freshwater, terrestrial
Most primitive group of gastropods

10. 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

11. Pulmonata Highly vascularized mantle for gas exchange (lung)
17,000 spp: slugs, pond snails

12. Gastropoda More active than mono and polyplacophorans
Highly cephalized: tentacles, eyes
Gonochoristic (dioecious)
Veliger larva (an advanced version of the trochophore larva)

13. Veliger larva Metanephridium Velum Shell Stomach Digestive cecum Foot
Mollusca
4/23/2017
Veliger larva
Metanephridium
Velum
Shell
Stomach
Digestive
cecum
Foot
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Esophagus

16. Mollusca
4/23/2017
Cephalopoda

17. 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

18. Cephalopoda Posterior surface Right Ventral Dorsal Left Mollusca
4/23/2017
Cephalopoda
Posterior surface
Right
Ventral
Dorsal
Left
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19. Cephalopod eye Iris Retina Lens Cornea Optic nerves Mollusca 4/23/2017
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20. eye
Optic lobe
Buccal ganglia
Cerebral ganglion
statocyst
esophagus
Brachial nerves
Brain is surrounded by a cranium

21. Cephalopoda Ectocochleate cephalopods
Have external shell with internally subdivisions used for buoyancy control
This ancestral group is almost completely extinct
E.g. Nautilus

22. Cephalopoda Endocochleate cephalopds
Reduced internal shell, or shell absent
Squids, cuttlefish, octopi

23. Tentacle Arm Funnel (siphon) Fin Collar Eye Mollusca 4/23/2017
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24. Shell (Pen) Systemic heart Branchial heart Ctenidium Funnel Mollusca
4/23/2017
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
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25. 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

26. Bivalvia
Mollusca
4/23/2017

27. 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)

28. Protobranchs Gills for gas exchange only
Tend to live in deeper waters (>1000m)

29. 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

30. Locomotion Mollusca 4/23/2017
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31. Mollusca Glochidia glochidium Glochidia on gills Freshwater mussels
4/23/2017
Glochidia
glochidium
Glochidia on gills
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Freshwater mussels

32. Septibranch Ctenidia lack filaments Feed on polychaetes, crustaceans
Weird side group

33. Scaphopoda Shared (extinct) common ancestor with bivalves 300-400 spp
Mollusca
4/23/2017
Scaphopoda
Shared (extinct)
common ancestor
with bivalves
spp
Lack ctenidia, heart
Burrowers
Have
captacula (tentacles)
with which to catch food
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