Phylum Mollusca

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Phylum Mollusca

• chitons, clams, snails, slugs, octopi,
  cuttlefish, squid, nautili
• second largest phylum (50,000 species)
• well known fossil record
• marine, fresh water, and terrestrial

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General Characteristics

• mostly bilateral symmetry
• unsegmented
• eucoelomates protostomes
• great size variation
• great diversity adaptive radiation

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Body Plan

• cephalization
• muscular foot modified as tentacles in
  cephalopods
• mantle- surrounding the viscera shell secretion
  jet propulsion
• calcareous shell protection support nacreous
  layer periostracum
• radula feeding in herbivorous species
• gills respiration filter feeding

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Radula
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Snail Radula
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Ctenidia- Gr. comb
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Physiology/Development

• open circulatory system except cephalopods
  hemocoel
• excretion metanephridia
• nervous system- ganglion
• external internal fertilization

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Class Polyplacophora chitons

• 800 species
• marine
• inhabit rocky intertidal zone
• dorsoventrally flattened
• overlapping dorsal shell plates
• herbivores w/ radula

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Polyplacophora chitons
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Chiton Anatomy
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Class Aplacophora
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Aplacophora- Diagnostic Features

• small, vermiform
• deep marine
• no shell calcareous spines
• many burrowing or on cnidarians

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Monoplacophora
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Monoplacophora- continued

• all deep marine
• single, unhinged, cap-shaped shell
• 3-6 ctenidia
• ancestor possibly gave rise to other molluscs

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Class Gastropoda

• largest taxonomic class
• 30,000 extant species
• snails, nudibranchs, sea hares, and slugs
• marine, freshwater, and terrestrial
• shell often asymmetrical spiral

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columella
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Cone Shells
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C. Gastropoda S.C. Pulmonata

• A. heart
• B. penis
• D. dart sac
• E. sperm duct
• F. oviduct
• H. digestive gland
• I. Stomach
• J. crop
• K. esophagus
• L. buccal bulb

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nudibranchs/sea hares
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Veliger Larvae
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Class Bivalvia

• clams, scallops, oysters, and mussels
• filter feeding
• mantle secretes a shell of CaCO3
• periostracum, prismatic, nacreous layers

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Clam Anatomy

• muscular foot
• intestine
• gonad
• hemocoel
• mantle
• gills
• excurrent/incurrent apertures

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Muscular Foot
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clam glochidia
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Class Scaphopoda

• Tusk Shells
• openings at both ends
• captacula- adhesive feeding tentacles

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Class Cephalopoda

• octopods, squid, cuttlefish, Nautilus
• 600 species entirely marine
• highly motile- jet propulsion
• highly developed nervous system and sensory
• shell reduced or entirely absent in octopods 5-6
  Nautilus possess shells

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Reproduction/Development

• dioecious
• internal fertilization spermatophore
• no larval phase
• female often broods the eggs before dying

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chromatophores
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Nautilus
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Giant Squid (Architeuthis)