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Phylum Mollusca. Second largest animal phylum (> 100,000 species) Oysters, snails, clams, octopus, squid Two unifying characteristics External calcium carbonate shell Muscular foot for locomotion. Generalized Molluscan Shell. Outer periostracum (organic layer – conchiolin

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phylum mollusca
Phylum Mollusca
  • Second largest animal phylum (> 100,000 species)
  • Oysters, snails, clams, octopus, squid
  • Two unifying characteristics
    • External calcium carbonate shell
    • Muscular foot for locomotion
generalized molluscan shell
Generalized Molluscan Shell
  • Outer periostracum (organic layer – conchiolin
  • Inner layers largely calcium carbonate with some organic material in a matrix
class monoplacophora
Class Monoplacophora
  • Originally known from Cambrian to Devonian fossils
  • Living representatives found 1952
  • Since, 20 more species found (1800 – 7000 m deep)
  • Single bilaterally symmetrical shell
  • Probably share common ancestor with other Mollusca except Polyplacophora
  • Members probably actually gave rise to the other molluscan groups (bivalves, gastropods, cephalopods)
  • Small – 3 mm to 3 cm long
class polyplacophora
Class Polyplacophora
  • Chitons
  • Retain many features of generalized mollusc
  • Some adaptation for predominant lifestyle (intertidal zone)
  • Shell structuring (layering) different from other molluscs (Conchifera)
class gastropoda
Class Gastropoda
  • Most diverse group (~60,000 species)
  • >15,000 described fossil species
  • Most extensive adaptive radiation of any mollusc group
class gastropoda12
Class Gastropoda
  • Three “groups” – phylogeny revision
  • Prosobranchs – most common members when think of snails
    • Terrestrial, freshwater, and marine*
    • Common feature – operculum
  • Opisthobranchs
    • Sea slugs, sea hares
    • Many members lost shell
  • Pulmonates
    • Many terrestrial species, also freshwater, a few marine
major changes from generalized mollusc
Major Changes from Generalized Mollusc
  • Development of head
  • Dorsoventral elongation
  • Shell – from shield to retreat
  • Torsion
  • Conispiral coiling and asymmetry
slide14

Fig 10-15

Monoplacophoran

ancestor

torsion
Torsion
  • Weight of shell over head, mantle cavity posterior
  • Torsion – 180o counterclockwise rotation of visceral mass, shell, mantle, mantle cavity
  • Occurs in larvae not adult
  • First gastropods
  • Detorsion
costs of conispiral shell
Costs of Conispiral Shell
  • Loss of a gill, nephridium, atrium
  • Mantle cavity (anus and nephridiopore) now anterior and near mouth
  • Compensation - changes in water flow or shell structure
  • See Figure 12-20 (mantle cavity evolution) and 12-21A (abalone)
shell
Shell
  • Apex, whorl, columnella, aperature, siphonal canal
  • Spire, body whorl, outer lip, inner lip
  • operculum
locomotion
Locomotion
  • Most move using foot
  • Most have ciliated sole and secretory glands (mucus producing)
  • Hard-bottom dwelling and terrestrial, and large soft-bottom snails - undulating wave of muscle contractions (Figure 12-30)
feeding
Feeding
  • Most often thought of as algal scrapers (radula)
  • Deposit feeders
  • Suspension feeders
  • Scavengers
  • Predators
  • Parasites
class bivalvia
Class Bivalvia
  • Oysters, clams, mussels ~8,000 species (1,300 freshwater, rest marine)
  • Benthic filter-feeders (a few exceptions)
    • No radula
    • Enlarged gills
  • Compressed body
  • Shell
    • Two valves
    • Hinged dorsally
    • Completely encloses body
class bivalvia26
Class Bivalvia
  • Rostroconch ancestor
class cephalopoda
Class Cephalopoda
  • ~700 living species, 10,000 fossil species
  • Highly specialized
  • Pelagic (octopus secondary)
  • Shell – coiled, internalized, reduced, or lost
  • Closed circulatory system
  • Visual eye