Invertebrates 4Invertebrates 4

Phylum MolluscaPhylum Mollusca

Phylum Mollusca:Phylum Mollusca:The “soft-bodied” The “soft-bodied”

animalsanimals

Evolutionary relationshipsEvolutionary relationships

and HOX genes

Molluscan body plan Molluscan body plan Hypothetical ancestral mollusc Hypothetical ancestral mollusc

(HAM)(HAM)

ShellShell MantleMantle Mantle cavityMantle cavity CtenidiaCtenidia Visceral massVisceral mass Head and assocated Head and assocated

sensory structuressensory structures RadulaRadula Foot and epipodial Foot and epipodial

tentaclestentacles

Molluscan body plan (HAM)Molluscan body plan (HAM) Digestive systemDigestive system

completecomplete note position of anusnote position of anus

Circulation/RespirationCirculation/Respiration ctenidiumctenidium open circulatory systemopen circulatory system coelom=pericardial cavity. coelom=pericardial cavity.

True coelom, but reduced.True coelom, but reduced.

Water/ion balance and Water/ion balance and ammonia excretionammonia excretion Has paired nephridia: Has paired nephridia:

kidney-like in functionkidney-like in function

Molluscan body plan (HAM)Molluscan body plan (HAM)

Nervous systemNervous system BrainBrain Paired ventral nerve cords.Paired ventral nerve cords. Ganglia at key locationsGanglia at key locations Several sensory structures (see description Several sensory structures (see description

of HAM)of HAM)

Major Molluscan ClassesMajor Molluscan Classes LEARN!: How are the features of HAM LEARN!: How are the features of HAM

modified for each of the major classes, modified for each of the major classes, and how are these modifications and how are these modifications adaptive for each particular mollusc?adaptive for each particular mollusc?

??

Class Polyplacophora: Class Polyplacophora: “many plates”“many plates”

The chitonsThe chitons Differences from HAMDifferences from HAM

8 shell plates8 shell plates Mantle cavity expanded Mantle cavity expanded

around footaround foot Multiple ctenidiaMultiple ctenidia Mantle grows over shell, Mantle grows over shell,

thickened (new functions?)thickened (new functions?) Reduced head without Reduced head without

sensory structuressensory structures

Phylum Mollusca Phylum Mollusca Class PolyplacophoraClass Polyplacophora

Cryptochiton stelleriCryptochiton stelleri

Class Gastropoda: Class Gastropoda: “stomach-foot”“stomach-foot”

Representative members: Snails, Representative members: Snails, limpets, abalone, nudibranchs, sea limpets, abalone, nudibranchs, sea and land slugsand land slugs

Differences from HAMDifferences from HAM Undergo torsion (sketch)Undergo torsion (sketch)

What is torsion?What is torsion? Advantages of torsionAdvantages of torsion Disadvantages of torsionDisadvantages of torsion Be sure you understand how torsion makes Be sure you understand how torsion makes

Gastropods different from HAM!Gastropods different from HAM!

Phylum MolluscaPhylum MolluscaClass GastropodaClass Gastropoda

Limpet:Limpet: Lottia gigantea Lottia gigantea

Phylum MolluscaPhylum MolluscaClass GastropodaClass Gastropoda

DogwinkleDogwinkle: Nucella sp.: Nucella sp.

Phylum MolluscaPhylum MolluscaClass GastropodaClass Gastropoda

Opisthobranchs: “rear gill”Opisthobranchs: “rear gill” Discodoris (Dialula sandiegensis)Discodoris (Dialula sandiegensis)

Phylum MolluscaPhylum MolluscaClass GastropodaClass Gastropoda

Opisthobranchs: “rear gill”Opisthobranchs: “rear gill” Hermissenda (Phidiana) crassicornisHermissenda (Phidiana) crassicornis

Class Bivalvia:Class Bivalvia:“two valves”“two valves”

Representative members: Clams, mussels Representative members: Clams, mussels and scallopsand scallops

Differences from HAMDifferences from HAM Two shellsTwo shells Mantle/sensory structuresMantle/sensory structures Expanded mantle cavityExpanded mantle cavity Enlarged ctenidiaEnlarged ctenidia

Respiration AND feedingRespiration AND feeding Reduced headReduced head Foot more elongate; Foot more elongate;

Minimal area for suctionMinimal area for suction

Class Cephalopoda:Class Cephalopoda:“Head-footed”“Head-footed”

Representative members: Nautilus, squid, octopusRepresentative members: Nautilus, squid, octopus Differences from HAMDifferences from HAM

Shell reduced (pen in squid)Shell reduced (pen in squid) Foot gives rise to arms and funnelFoot gives rise to arms and funnel HeadHead

Modified cephalic tentaclesModified cephalic tentacles Well-developed eyesWell-developed eyes BeakBeak

MantleMantle ElongateElongate Thickened and protective body coveringThickened and protective body covering Functions in movementFunctions in movement ChromatophoresChromatophores

Mantle cavity also elongate, functions in movementMantle cavity also elongate, functions in movement Paired ctenidia as in HAMPaired ctenidia as in HAM

Circulatory system closed!Circulatory system closed! Nervous system highly developedNervous system highly developed