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Simplified relationship diagram of mollusc's evolution:

                 Caudofoveata (?)
                 Aplacophora
hypothetical                     Polyplacophora
ancestral                Monoplacophora
mollusk                   Gastropoda
                    Cephalopoda
                    Bivalvia
                    Scaphopoda

Cladogram shows hypothetical mollusc's evolution.

Vertical cladogram (modified from: [1]):

        Polyplacophora   Gastropoda     Rostroconchia  
 Aplacophora  \Monoplacophora \  Cephalopoda  \  Scaphopoda  Bivalvia
       \       \       \       \      /        \       \      /
        \       \       \       \    /          \       \    /
         \       \       \       \  /            \       \  /
          \       \       \       \/              \       \/ 
           \       \       \       * Cyrtosoma     \      /
            \       \       \       \               \    /
             \       \       \       \               \  /
              \       \       \       \               \/
               \       \       \       \              * Diasoma
                \       \       \       \            /
                 \       \       \       \          /
                  \       \       \       \        /
                   \       \       \       \      /
                    \       \       \       \    /
                     \       \       \       \  /
                      \       \       \       \/
                       \       \       \      /
                        \       \       \    /
                         \       \       \  /
                          \       \       \/
                           \       \      * Conchifera 
                            \       \    /
                             \       \  /
                              \       \/
                               \      * Testaria
                                \    /
                                 \  /
                                  \/
                                  /
                    hypothetical ancestral mollusk

Horizontal cladogram (source: [2]):

Mollusca molluscs
  |
  +-- Amphineura
  |     |
  |     +-- Aplacophora
  |           |
  |           +-- Solenogastres
  |           +-- Caudofoeveata
  |
  +-- Testaria
        |
        +-- Polyplacophora - chitons
        |     |
        |     +-- Palaeoloricata
        |     +-- Neoloricata
        |
        +-- Conchifera
             |
             +-- Monoplacophora
             |
             +--+
                |
                +--Cyrtosoma
                |     |
                |     +-- Gastropoda - gastropods
                |     |
                |     +-- Cephalopoda - cephalopods
                +-- Diasoma
                      |
                      +-- Rostroconchia
                      |
                      +--+
                         |
                         +-- Scaphopoda - tusk shells
                         |
                         +-- Bivalvia - bivalves

References

  1. EES 227: Paleobiology (Spring 2004) http://ethomas.web.wesleyan.edu/wescourses/2004s/ees227/01/227lect18.htm
  2. Zoology 250 Phylogenetic Trees (2006) http://www.biology.ualberta.ca/courses.hp/zool250/Clades/clade08-Mollusca.htm

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