Key to Phylum Mollusca

Key written by Dave Cowles

Taken primarily from  Kozloff, 1987, 1996  (Copyright 1987, 1996, University of Washington Press.  Used in this web page by permission of University of Washington Press)
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1a Flattened body has 8 dorsal shell plates, usually visible dorsally.  Attach to rocksPolyplacophora: chiton (plates visible)Polyplacophora: chiton (gumboot--plates not visible) Class Polyplacophora,
1b Body does not have 8 dorsal shell plates, may or may not be flattened 2
2a Body laterally compressed, has two valves hinged together dorsally.  Head rudimentary but often have large foot and gillsBivalvia: Scallop Class Bivalvia,
2b Body does not have two valves hinged together dorsally 3
3a One shell, tubular, curved in less than 1/2 circle and opening on both ends, tapered so that one end is considerably smaller than the other Scaphopod: tusk shell Class Scaphopoda,
Tusk shells
3b Shell not as above 4
4a Have 8 to 10 tentacles with suckers instead of a creeping foot.  No external shell (in our area), but may have a reduced, internal shell.  Mouth has a beak.  Large, complex eyes not on stalks
Cephalopod: No shell (Octopus)Sepiolid squid: Reduced internal shell
Class Cephalopoda,
Squid, Octopus
4b No shell, or have a single shell which is curved into a spiral, a pyramid, or ear-shaped (in our area).  Crawl on muscular creeping foot.
Gastropoda: coiled (whelk)Gastropoda: coiled (olive shell)Gastropoda: Pyramid shaped (Limpet)Gastropoda: Pyramid-shaped (slipper shell)Gastropoda: ear-shaped (abalone)Gastropoda: No shell (nudibranch)Gastropoda: long spiraled shell (wentletrap)
Gastropoda:  5
5a Gastropods with shells Snails, limpets
(Class Gastropoda, 
with shells)
5b Gastropods without shells Nudibranchs and 
other Opisthobranchs

Taxonomic Levels Represented in This Key:
Class Polyplachophora (Chitons)
Class Bivalvia (Clams)
Class Cephalopoda (Squid, Octopus)
Class Gastropoda
  With shells (Snails, Limpets)
  Without Shells (Nudibranchs and other Opisthobranchs)

Page created by Dave Cowles, 5-2002
Edited by:
Robbie Wheeling
Anna Dyer
Hans Helmstetler 12-2002