section_path: Classification
section_name: Classification
paragraph_text: Classification
  
  Opinions vary about the number of classes of molluscs; for example, the table below shows seven living classes, and two extinct ones. Although they are unlikely to form a clade, some older works combine the Caudofoveata and Solenogasters into one class, the Aplacophora. Two of the commonly recognized "classes" are known only from fossils.
  
  Classification into higher taxa for these groups has been and remains problematic. A phylogenetic study suggests the Polyplacophora form a clade with a monophyletic Aplacophora. Additionally, it suggests a sister taxon relationship exists between Bivalvia and Gastropoda, uniting them in the new clade Pleistomollusca. Tentaculita may also be in Mollusca (see Tentaculites).