Tree and most traits from (Nielson 1994 p. 111), and Brusca & Brusca (1990 p. 762).

       ======2*================= Aplacophora (shell-less, worm-like molluscs)
       |
<<=1*==|    ====4*============== Polyplacophora (chitons)
       |    |
       ==3*=|   ====6*========== Monoplacophora (primitive limpet-like molluscs)
            |   |
            =5*=|         ==10*= Gastropoda (snails, slugs, etc.)
                |    ==8*=|
                |    |    ==11*= Cephalopoda (nautiloids, cuttlefish, squids, octopus)
                ==7*=|
                     |    ==12*= Bivalvia (mussels, clams, scallops, etc.)
                     ==9*=|
                          ==13*= Scaphopoda (tusk shells)

1:a) muscular, creeping ciliated foot**, b) mantle with calcareous spicules produced by single cells, c) posterolateral mantle cavity with multiple bipectinate gills (ctenidia), d) radula, e) larval/adult body with 8 segments, f) trochophore larvae**, g) coelom via schizocoely**, h) 'open' circulatory system (hemocoel) with pumping heart, i) circumpharyngeal nerve-ring (brain) with ladder-like ventrolateral nerves**
2: a) small, elongate, worm-like body, b) reduction/loss of foot & head
3: a) large muscular foot, b) one or more dorsal, solid shell plates, c) multiple pedal 'retractor' muscles, d) mobile radular membrane & odontophore
4: a) 8 unique shell plates (perforated by aesthetes), b) mantle girdle, c) calcareous spicules from single cells**
5: a) 1 shell plate, b) 3-layered shell (periostracum, prismatic & nacreous calcareous layers), c) crystalline style
6: a) 8 pair pedal retractor muscles**, b) monopectinate ctenidia (5-6 pr.)
7: a) 2 pair pedal retractor muscles, b) 1 or 2 pr. ctenidia
8: a) dorsoventrally expanded body, b) reduced mantle cavity surrounding anus
9: a) burrowing foot, b) paired, lateral larval shell plates, c) 1 pr. ctenidia**
10: a) torsion, b) single pedal retractor muscle, c) 1 pr. ctenidia**
11: a) foot modified as siphon, b) shell with gas-filled chambers & siphuncle, c) 1 pair pedal retractor muscles, d) ink gland, e) beak-like jaws, f) large, circumoral tentacles/arms, g) 'closed' circulatory system (vessels lined by epithelium), h) image-forming eyes & large brain
12: a) loss of radula, b) loss of head, c) enlarged mantle cavity & ctenidia, d) byssal gland
13: a) larval shell plates fuse into tubular adult shell, b) ctenidia lost, c) captacula

** plesiomorphic (a primitive state, not unique to clade)

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